Physiological State Of Phytoplankton Research Articles

Cellular uptake and biotransformation of arsenate by freshwater phytoplankton under salinity gradient revealed by single-cell ICP-MS and CT-HG-AAS

Environmental context Freshwater phytoplankton are involved in the biogeochemical cycling of arsenic within aquatic ecosystems via uptake processes. Rather than determining the mean arsenic content in a population of freshwater phytoplankton, we investigate the heterogeneity of arsenic uptake by single-cell ICP-MS. Our data show that arsenic distribution within a cell population may be highly heterogeneous, measured at the femtogram per cell level, and are affected by species and salinity. Rationale An advanced technique has been developed for analysing intracellular elements at the single-cell level using single-cell inductively coupled plasma mass spectrometry (SC-ICP-MS). Compared with conventional inductively coupled plasma mass spectrometry (ICP-MS) analysis, SC-ICP-MS provides uptake data with greater biological relevance. In this study, the use of SC-ICP-MS enabled the quantification of metal concentrations on an individual cell basis down to the femtogram (fg) per cell level. Methodology Three freshwater phytoplankton cells, namely Staurastrum paradoxum (S. paradoxum), Pediastrum duplex (P. duplex) and Scenedesmus acutus (S. acutus), were incubated in 0.1 µmol L−1 arsenate (AsV) solution for 14 days at varying salinity. Cold trap hydride generator atomic absorption spectrometry (CT-HG-AAS) was used to investigate the biotransformation of arsenate under varying salinity conditions. Results The results reveal that cellular arsenic levels decreased as salinity increased in P. duplex and S. paradoxum but increased in S. acutus. The SC-ICP-MS data, which show uptake of AsV by freshwater phytoplankton, were in good agreement with those produced using ICP-MS analysis. Various arsenic management strategies were seen in the phytoplankton species: P. duplex converted it to methylated forms; S. acutus produced organoarsenicals; and S. paradoxum reduced arsenate (AsV) to arsenite (AsIII) and excreted it. Our study also showed changes in the physiological status of phytoplankton following salt stress and arsenic exposure. Discussion Our results confirm the efficacy of SC-ICP-MS in precisely determining arsenic distribution at the single-cell level and reveal differences in intraspecies mechanisms for arsenic cycling in freshwater ecosystems.

Factors regulating the concentration of particulate iodine in coastal seawater

AbstractTo investigate the factors regulating the concentration of particulate iodine (PI) in seawater, it was measured in two Pacific coastal areas adjacent to Japan (offshore Aomori and offshore Fukushima) in two seasons (May and October) along with parameters such as particulate organic carbon (POC) and nitrogen (PON), dissolved iodine (DI), and phytoplankton pigments. The observed PI concentrations ranged from 0.0068 to 2.2 nmol L−1, with uncertain seasonal and regional differences. The atomic ratio of PI to POC (I/C) ranged from 4.4 × 10−7 to 1.1 × 10−4, in line with previous studies on marine phytoplankton, although the lower part of the range of the I/C in this study was found to be somewhat lower compared to the previous studies. The dataset was divided in three groups namely, lower (Group A), average (Group B), and higher (Group C) I/C because the I/C is an indicator of the accumulation efficiency of PI in particles in seawater and POC is the main carrier of PI. Based on the observed POC concentrations, the atomic ratio of POC to PON (C/N), and the ratio of carbon derived from chlorophyll a to POC, the three groups were characterized by phytoplanktonic physiological states as highly productive, steady, and senescent states for Groups A, B, and C, respectively. Based on the finding that PI production is associated with the phytoplankton physiological state, the seasonal and regional differences in PI concentration and I/C in the observation areas were consistently explained. Finally, we suggest that the phytoplanktonic physiological state is one of the vital factors regulating the PI concentration in seawater.

Short-term responses of phytoplankton size-fractionated structure and photosynthetic physiology to thermal effluent in a subtropical coastal bay

Elevated water temperature caused by the thermal discharge from power plants can exert multiple ecological impacts on the phytoplankton community in coastal ecosystems. Most recent studies have focused on the reshaping effects on the community structure; however, the short-term response of phytoplankton physiology to thermal discharge remains unclear. This study conducted research on the scope of thermal discharge from the nuclear power plant and the size-fractionated phytoplankton structure combined with photosynthetic physiology in Daya Bay, China. The thermal discharge significantly affected the surface temperature in the outlet regions, and the thermal plume mainly diffused along the northeast coast of the outfall site, resulting in a significant difference in the surface temperature between the inlet and outlet transects (p<0.05). Elevated surface temperatures decreased the total chlorophyll a concentrations by 33.19% at the outlet regions, with pico-phytoplankton decreasing the most. Chlorophyll a concentrations were higher at sites further away from the outlets, indicating that elevated water temperature might stimulate the rapid growth of phytoplankton, especially nano-phytoplankton which replaced pico-phytoplankton as the dominant group at stations away from the outlets. Significant negative correlations were observed between the photochemical quantum yield (Fv/Fm) and temperature (p<0.05), and the relative electron transport rate (rETR) and temperature (p<0.05). Phytoplankton showed a normal photosynthetic physiological state at most sites with a surface temperature<33°C but was severely affected at the outlet site with a 5°C rise, decreasing from ~0.5 on the inlet transect to 0.07. During the diurnal survey, the high temperatures near the outlet at midday had a compensatory effect on phytoplankton’s light suppression. The results indicated that the physiological state of phytoplankton was clearly influenced by the thermal discharge with diurnal variation, and different size-fractionated phytoplankton groups exhibited heterogeneous responses. The findings may provide further insights into the ecological impacts of thermal discharges as well as global warming in subtropical regions.

Physiological characteristics of phytoplankton in response to different light environments in the Philippine Sea, Northwestern Pacific Ocean

The physiological status of phytoplankton, used to determine the quantity and quality of basic food sources in marine ecosystems, can change rapidly due to ambient environmental conditions (e.g., light, temperature, and nutrients). To understand the physiological characteristics of phytoplankton, the phytoplankton community composition, pigment concentration, primary production, and pigment production rate were estimated at 100% and 1% light depths in the Philippine Sea during the summer of 2019. The predominant phytoplankton classes at both light depths were Prochlorococcus and Synechococcus during the study period. Pigment concentrations, except for photoprotective pigment concentrations (i.e., diadinoxanthin and zeaxanthin), were significantly higher (t-test, p<0.05) at 1% light depth to increase the light-harvesting efficiency. The production rates of these pigments had a weak correlation with primary production at 100% light depth, whereas they showed a strong positive relationship at 1% light depth. Moreover, all photosynthetic pigments had a significantly faster turnover rate at 100% light depth compared with 1% light depth to obtain light energy to repair PSII subunits damaged by strong light. This suggests that the phytoplankton community, especially cyanobacteria (Prochlorococcus and Synechococcus), could use light energy absorbed by newly produced photosynthetic pigments for repairing photoinhibition-damaged PSII as well as for production activity. A further study on photosynthetic pigments responding to light conditions must be conducted for a better understanding of the physiological conditions of phytoplankton.

Response of the Phytoplankton Sinking Rate to Community Structure and Environmental Factors in the Eastern Indian Ocean.

The phytoplankton sinking rate in the eastern Indian Ocean was measured during spring 2017 based on the SETCOL method. The range of phytoplankton sinking rates was −0.291 to 2.188 md−1, with an average of 0.420 ± 0.646 md−1. The phytoplankton sinking rate in the Equator (EQ) and the eastern boundary of the Indian Ocean near Sumatra (EB) was lower than that in the Bay of Bengal (BOB). The sinking rate above 100 m was low and increased rapidly below 100 m in all the three regions. The phytoplankton community composition had an important impact on the phytoplankton sinking rate in the east Indian Ocean. The strong stratification in BOB resulted in Trichodesmium spp. bloom and a lower phytoplankton diversity and evenness in BOB, while the phytoplankton in the deep layer are senescent cells that sink down from the upper layer and cannot actively regulate the state of the cells, resulting in a higher sinking rate. Depth and temperature have a great impact on the physiological state of phytoplankton. The sinking rate of phytoplankton depend on the dominant groups composing the phytoplankton community. For the eastern Indian Ocean, seawater stratification caused by temperature changes the distribution of nutrients in the upper layer, and phytoplankton are affected by temperature and nutrients, resulting in changes in community structure, and finally showing different subsidence characteristics.

Diel quenching of Southern Ocean phytoplankton fluorescence is related to iron limitation

Abstract. Evaluation of photosynthetic competency in time and space is critical for better estimates and models of oceanic primary productivity. This is especially true for areas where the lack of iron (Fe) limits phytoplankton productivity, such as the Southern Ocean. Assessment of photosynthetic competency on large scales remains challenging, but phytoplankton chlorophyll a fluorescence (ChlF) is a signal that holds promise in this respect as it is affected by, and consequently provides information about, the photosynthetic efficiency of the organism. A second process affecting the ChlF signal is heat dissipation of absorbed light energy, referred to as non-photochemical quenching (NPQ). NPQ is triggered when excess energy is absorbed, i.e. when more light is absorbed than can be used directly for photosynthetic carbon fixation. The effect of NPQ on the ChlF signal complicates its interpretation in terms of photosynthetic efficiency, and therefore most approaches relating ChlF parameters to photosynthetic efficiency seek to minimize the influence of NPQ by working under conditions of sub-saturating irradiance. Here, we propose that NPQ itself holds potential as an easily acquired optical signal indicative of phytoplankton physiological state with respect to Fe limitation. We present data from a research voyage to the Subantarctic Zone south of Australia. Incubation experiments confirmed that resident phytoplankton were Fe-limited, as the maximum quantum yield of primary photochemistry, Fv∕Fm, measured with a fast repetition rate fluorometer (FRRf), increased significantly with Fe addition. The NPQ “capacity” of the phytoplankton also showed sensitivity to Fe addition, decreasing with increased Fe availability, confirming previous work. The fortuitous presence of a remnant warm-core eddy in the vicinity of the study area allowed comparison of fluorescence behaviour between two distinct water masses, with the colder water showing significantly lower Fv∕Fm than the warmer eddy waters, suggesting a difference in Fe limitation status between the two water masses. Again, NPQ capacity measured with the FRRf mirrored the behaviour observed in Fv∕Fm, decreasing as Fv∕Fm increased in the warmer water mass. We also analysed the diel quenching of underway fluorescence measured with a standard fluorometer, such as is frequently used to monitor ambient chlorophyll a concentrations, and found a significant difference in behaviour between the two water masses. This difference was quantified by defining an NPQ parameter akin to the Stern–Volmer parameterization of NPQ, exploiting the fluorescence quenching induced by diel fluctuations in incident irradiance. We propose that monitoring of this novel NPQ parameter may enable assessment of phytoplankton physiological status (related to Fe availability) based on measurements made with standard fluorometers, as ubiquitously used on moorings, ships, floats and gliders.

Potential Implications of Changing Photosynthetic End-Products of Phytoplankton Caused by Sea Ice Conditions in the Northern Chukchi Sea

The recent dramatic decline in sea ice conditions in the Arctic Ocean has led to the ecophysiological changes in the phytoplankton community. Little is currently known about how the physiological status of phytoplankton has changed under rapidly changing environmental conditions in the Arctic Ocean. Using the 13C isotope tracer technique, the carbon allocation of phytoplankton into different photosynthetic end-products was determined in the northern Chukchi Sea on the basis of two Arctic expeditions conducted in 2011 and 2012 to identify the physiological status of phytoplankton. Lipids were the predominant photosynthetic biochemical fraction (42.5%) in 2011, whereas carbon allocation to proteins was most dominant under ice-free conditions in 2012 (47.7%). Based on a comparison of the photosynthetic carbon allocation of phytoplankton according to sea ice conditions, we found that photosynthetic carbon allocation to different macromolecular pools was significantly different depending on the sea ice conditions and that the light conditions caused by different sea ice conditions could be an important reason for the differences in carbon allocation to photosynthetic end-products. Different dominant phytoplankton groups related to size classes also could cause changes in the photosynthetic carbon allocation of phytoplankton related mainly to the lipid synthesis. Our results showed that the physiological status of Arctic phytoplankton could be changed by producing different photosynthetic end-products under current environmental changes. This change in photosynthetic end-products of phytoplankton as a basic food source could be further linked to higher trophic levels in regards to their nutritional and energetic aspects, which could have potential consequences for Arctic marine ecosystems.

A Review on the Macromolecular Compositions of Phytoplankton and the Implications for Aquatic Biogeochemistry

Biochemical composition of phytoplankton is a key indicator of the physiological and nutritional status of phytoplankton. A balanced biochemical pattern represents a healthy and productive metabolism in the autotrophic levels which can facilitate proper functioning of higher level organisms. The estimation of biochemical compositions was initiated in the early 1970’s. However, there has been a significant set of modifications in the extraction method and improvements in the sampling and analysis techniques since then. Similarly, the extent of biochemical measurements from various aquatic ecosystems around the globe has also increased. Recently, biochemical patterns are being used as a tool to track the changes in the physiological status of phytoplankton as a response to climate change. Such investigations are also forming part of research works on marine food webs and the nutritional status of ecosystems. This article is a brief review of research works carried out so far in an attempt to understand the biochemical compositions of phytoplankton in the global oceans and the implications with regard to changing environmental conditions.

Lipid-rich and protein-poor carbon allocation patterns of phytoplankton in the northern Chukchi Sea, 2011

The carbon allocations of phytoplankton into different photosynthetic end products (lipids, LMWM, polysaccharides, and proteins) were determined to understand physiological conditions of phytoplankton in the northern Chukchi Sea during the Korean Arctic expedition, 2011, using the 13C isotope tracer technique. The carbon allocation rates of lipids, LMWM, polysaccharides, and proteins were 0.00009–0.00062 h−1, 0.00001–0.00049 h−1, 0.00001–0.00025 h−1, and 0.00001–0.00062 h−1 within the euphotic depths from surface to 1% light depths during our cruise period, respectively. Significant relationships between protein production rates and chlorophyll a concentrations (large and total) were found in this study. Moreover, we found a significant negative relationship between lipid production rates and ammonium concentrations. These relationships match well with the previous results for environmental/physiological conditions for phytoplankton growth. Overall, phytoplankton allocated more photosynthetic carbon into lipids (42.5 ± 17.7%) whereas relatively lower to proteins (20.4 ± 15.5%) in this study. The lipid-rich and protein-poor allocation patterns in this study suggest that phytoplankton in the northern Chukchi Sea were in a stationary growth phase under nutrient deficient condition based on biological and environmental conditions observed during our study period. Based on comparison with the previous studies in the northern Bering Sea and southern Chukchi Sea, we found that the photosynthetic carbon allocation patterns depending on physiological status of phytoplankton under the different growth and/or nutrient conditions could be largely vary at different regions in the Arctic Ocean. More intensive research on the physiological status of phytoplankton is further required to determine how phytoplankton response to the changing environmental conditions and consequently how they impact on higher trophic levels in marine ecosystems in the Arctic Ocean.

Lake Caviahue: an extreme environment as a potential sentinel for nutrient deposition in Patagonia

The objective of this study was to determine if extreme acidic Lake Caviahue could be used as a sentinel of atmospheric deposition hypothesizing that the physiological state of algae will be the indicator parameter. The lake was sampled from 2000 to 2015 in order to determine chlorophyll concentration, algae abundance and phytoplankton in vivo fluorescence as a way to evaluate the physiological state of algae. Development and physiological state of phytoplankton in different seasons was related to concentration and dynamics of nutrients in the lake. Laboratory experiments of Keratococcus rhaphidiodes to nitrogen (N) enrichment confirmed that an increase in nutrient content enabled a better physiological state of algae (e.g. higher chlorophyll per cell). Therefore, under the projected scenarios of climate change, the increase of available N through the increase in deposition and the increase of dissolved inorganic carbon, as consequence of higher atmospheric CO2, will compensate the natural nutrient constraints observed in the phytoplankton of the lake. The effect that the atmospheric CO2 has on the DIC, and this on algal development, as well as the influence that N has on algal growth, make Lake Caviahue an interesting sentinel of nutrient deposition at regional level.

Novel Maximum Carbon Fixation Rate Algorithms for Remote Sensing of Oceanic Primary Productivity

Ocean remote sensing is the only technique capable of quantitatively detecting long-term and global variability of marine primary production. However, because the spectral curves of light derived from satellite sensors have a limited capability to identify the physiological state of phytoplankton, the direct estimation of photoadaptive variables, such as daily maximum carbon fixation in a water column $({P_{{{\rm opt}}}^{{\rm b}}})$ , from the spectra of satellite sensors is difficult. This difficulty results in most of the errors in ocean color-based primary productivity models. In this paper, algorithms were developed to estimate $P_{{{\rm opt}}}^{{\rm b}}$ using statistical models known as support vector machines (SVM). The algorithms were run using three different inputs: sea surface temperature (SST); both SST and sea surface chlorophyll a concentration; and SST, sea surface chlorophyll a concentration and photosynthetically active radiation (PAR) together. The results indicate that the algorithm using the three inputs (SST, sea surface chlorophyll a concentration, and PAR) had the best performance. The algorithms based on the SVM had better results than the general statistical regression method. Using the new $P_{{{\rm opt}}}^{{\rm b}}$ algorithm, the performances of the previous primary production models, including the vertically generalized production model and the statistical model developed in our earlier research, were improved. In contrast to the previous algorithm, the global primary productivity estimated using the new $P_{{{\rm opt}}}^{{\rm b}}$ algorithm showed regional changes, with higher primary productivity in most open ocean areas and lower primary productivity in coastal waters.

LA COMUNIDAD FITOPLANCTÓNICA DURANTE EVENTOS DE SURGENCIA Y NO SURGENCIA, EN LA ZONA COSTERA DEL DEPARTAMENTO DEL MAGDALENA, CARIBE COLOMBIANO

The phytoplankton community structural attributes were evaluated in Granate and Gaira inlets and Taganga Bay in order to establish the community´s response to changes in oceanographic dynamics modulated by upwelling and non-upwelling processes throughout the coastal waters of the Magdalena region, Colombian Caribbean, between February and June of 2008, during the main dry and short rainy seasons. Average wind speed divided in the research period in two seasons: February-April (upwelling: S) and June-July (non upwelling: NS) of 2008. In each period (upwelling and non-upwelling), 51-47 genera were identified distributed in 29-28 families, corresponding 78.23-86.19 % to centric diatoms, 16.68-9.47 % to pennate diatoms, 4.57-3.78 %) to cyanobacteria and about 0.50-0.55 %) to dinoflagellates, flagellates, and chlorophyta. The most abundant genera were Chaetoceros (colonial) and Skeletonema (colonial) with a relative abundance of 47-19.57 % and 8.13-60.93 %, respectively, and densities greater than 2000 cel/L. Chlorophyll a concentrations showed low values in this study from 0.72 ± 0.47 ng/m3 during S until 0.15 ± 0.15 mg/m3 during NS. The photosynthetic activity index (IAF1) of 6.34 ± 5.34 indicates either high pigment quality or low zooplankton grazing, representing an optimal phytoplankton physiological state during periods of S. In contrast, the IAF1 low average value of 0.52 ± 0.84 at BS ubducates a wide decline in the phytoplankton community's physiological state. A temporal segregation pattern was defined by cell densities from 22142.23 ± 15236.72 cel/L, for S and NS periods, respectively. Tempperature, salinity, and nitrate concentration (0-60 m depth), were the variables that represented the most seasonal changes, and affected the phytoplankton community structure.

Satellite‐detected fluorescence: Decoupling nonphotochemical quenching from iron stress signals in the South Atlantic and Southern Ocean

AbstractSatellite‐detected sunlight‐induced chlorophyll fluorescence could offer valuable information about the physiological status of phytoplankton on a global scale. Realization of this potential is confounded by the considerable uncertainty that exists in deconvolving the multiple ecophysiological processes that can influence the satellite signal. A dominant source of current uncertainty arises from the extent of reductions in chlorophyll fluorescence caused by the high light intensities phytoplankton are typically exposed to when satellite images are captured. In this study, results from over 200 nonphotochemical quenching (NPQ) experiments conducted on cruises spanning from subtropical gyre to Southern Ocean waters have confirmed that satellite fluorescence quantum yields have the potential to reveal broad regions of iron (Fe) stress. However, our results suggest significant variability in phytoplankton NPQ behavior between oceanic regimes. Dynamic NPQ must therefore be considered to achieve a reliable interpretation of satellite fluorescence in terms of Fe stress. Specifically, significantly lower NPQ was found in stratified subtropical gyre‐type waters than in well‐mixed Southern Ocean waters. Such variability is suggested to result from differences in incident irradiance fluctuation experienced by phytoplankton, with highly variable irradiance conditions likely driving phytoplankton to acclimate or adapt toward a higher dynamic NPQ capacity. Sea surface temperature empirically demonstrated the strongest correlation with NPQ parameters and is presented as a means of correcting the chlorophyll fluorescence signature for the region studied. With these corrections, a decadal composite of satellite austral summer observations is presented for the Southern Ocean, potentially reflecting spatial variability in the distribution and extent of Fe stress.

Factors controlling phytoplankton physiological state around the South Shetland Islands (Antarctica)

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 498:55-71 (2014) - DOI: https://doi.org/10.3354/meps10616 Factors controlling phytoplankton physiological state around the South Shetland Islands (Antarctica) Cristina García-Muñoz1,*, Cristina Sobrino2, Luis M. Lubián1, Carlos M. García3, Sandra Martínez-García2,5, Pablo Sangrà4 1Departamento de Ecología y Gestión costera, Instituto de Ciencias Marinas de Andalucía (ICMAN-CSIC), 11510 Puerto Real, Cádiz, Spain 2Departamento de Ecoloxía e Bioloxía Animal, Universidade de Vigo, 36200 Vigo, Pontevedra, Spain 3Departamento de Biología, Facultad de Ciencias del Mar y Ambientales, Universidad de Cádiz, 11510 Puerto Real, Cádiz, Spain 4Departamento de Física, Facultad de Ciencias del Mar, Universidad de Las Palmas de Gran Canaria, Campus de Tafira, 35017 Las Palmas de Gran Canaria, Spain 5Present address: C-MORE, University of Hawaii, C-MORE Hale, 1950 East West Road, Honolulu, Hawaii 96822, USA *Corresponding author: cristina.garcia@icman.csic.es ABSTRACT: To investigate factors controlling phytoplankton physiological state around the South Shetland Islands, phytoplankton abundance and structure, fluorescence properties, photoprotective pigment composition and physicochemical variables were studied. Nanophytoplanktonic cells (<20 µm) contributed 84% of total chlorophyll a (chl a), except for the station closest to the Antarctic Peninsula where microplanktonic cells (>20 µm) predominated (up to 85% of total chl a). Daily irradiance over the mixed layer depth (MLD) was inversely related with integrated nanoplanktonic chl a; however, its relative contribution to total chl a increased at mid-irradiance values. The average maximum quantum yield of photosystem II (Fv/Fm) below 20 m depth ranged from 0.17 to 0.53 and showed that cells were under suboptimal physiological conditions in the Drake region, but had higher performance around the South Shetland Islands and towards the Antarctic Peninsula. A reverse pattern in the fluorescence yield was detected. A deeper examination of the Fv/Fm vertical profiles according to the sampling time detected surface photoinhibition during the day and a spatial modulation of Fv/Fm related to irradiance and the silicic acid:nitrate ratio. It appears that cells’ photosynthetic performance was controlled by iron limitation in the Drake region, whereas irradiance regime controlled phytoplankton physiological state in the rest of the studied regions. Vertical mixing differences among stations, the relative position of the euphotic layer depth with respect to the MLD and the photoprotective pigment ratios revealed contrasting responses to light stress among the different phytoplanktonic groups, revealing a better adaptation of medium size cells, especially diatoms, to stratified waters receiving high irradiance. KEY WORDS: South Shetland Islands · Phytoplankton · Fv/Fm · Irradiance · Size · Mixed layer · Nutrients Full text in pdf format PreviousNextCite this article as: García-Muñoz C, Sobrino C, Lubián LM, García CM, Martínez-García S, Sangrà P (2014) Factors controlling phytoplankton physiological state around the South Shetland Islands (Antarctica). Mar Ecol Prog Ser 498:55-71. https://doi.org/10.3354/meps10616 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 498. Online publication date: February 17, 2014 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2014 Inter-Research.

A comparison of primary production models in an area of high mesoscale variability (South Shetland Islands, Antarctica)

Three types of primary productivity (PP) models were evaluated in a mesoscale area around the South Shetland Islands (Antarctica). Input variables were: phytoplankton carbon biomass, Chlorophyll a, sea water temperature, daily irradiance, among others, collected in situ during an oceanographic cruise (COUPLING, January 2010). Models of the first type were based on Chl a measurements: the widely used model VGPM (Behrenfeld and Falkowski, 1997) and a derived version developed for the Western Antarctic Peninsula (Dierssen et al., 2000). The second type included two models based on phytoplankton carbon biomass: one developed for the whole Southern Ocean (Arrigo et al., 2008) and one based on the Metabolic Theory of Ecology developed by López-Urrutia et al. (2006), being the first time that a model with these features is used for Antarctic waters. The third type was an updated version of the carbon-based model CbPM (first described by Behrenfeld et al. (2005)) based on the Chl a/carbon biomass ratio modulation. The degree of agreement among the results between the different types of models turned out to be low (>30% of difference), but high within models of the same type (<10% of difference). Biomass-based model predictions differed the most from those estimated by the other two types. The differences in PP estimates were primarily attributed to the different ways these models treat the phytoplankton assemblage, along with the difference in input variables. Among the five models evaluated, the output from the modified version of the CbPM showed the lowest bias (0.55) being the most realistic. It made a special attempt to detect the factors controlling phytoplankton physiological state, showing a nutrient limitation towards the Drake area similar to the one observed for the in situ PP values.

Nature and sources of suspended particulate organic matter in a tropical estuary during the monsoon and pre-monsoon: Insights from stable isotopes (δ13CPOC, δ15NTPN) and carbohydrate signature compounds

Mandovi estuary, on the west coast of India, behaves like a fresh water system during the monsoon (June to September), and as a marine system during the pre-monsoon (October to May). The effect of these contrasting conditions on the concentration and composition of suspended particulate carbohydrates was evaluated. Suspended particulate matter (SPM) was collected from 9 stations in the Mandovi estuary during the monsoon (August) and pre-monsoon (March), and was analyzed for δ13CPOC, δ15NTPN, particulate carbohydrates (PCHO), particulate uronic acid (PUA), and particulate neutral carbohydrate (PNCHO) concentration and composition. The concentrations of PCHO, PNCHO and PUA varied from 2 to 9μMC, 2 to 7μMC, and from 0.14 to 0.39μMC, respectively during the monsoon; and from 4 to 10μMC, 3 to 9μMC, and from 0.34 to 0.90μMC, respectively during the pre-monsoon. Both δ13CPOC and δ15NTPN were depleted during the monsoon and relatively enriched during the pre-monsoon. When expressed as % of SPM, particulate organic carbon (POC) and PCHO concentration decreased with increasing SPM content. The average PCHO yield (PCHO-C/POC%) was 13.9%±4.4% and ~10.7%±2.6% for the monsoon and pre-monsoon, respectively. POC and PCHO concentrations were influenced by cell numbers of bacteria and phytoplankton during the monsoon and pre-monsoon, respectively. Glucose was the most abundant constituent of PNCHO in both the monsoon and pre-monsoon. Rhamnose, fucose, ribose, galactose, arabinose, and xylose were relatively more abundant during the monsoon than the pre-monsoon. Preferential utilization of glucose results in accumulation of deoxy sugars (i.e. rhamnose plus fucose). The ratios of hexoses/pentoses and mannose/xylose and mol% glucose were relatively lower, and mol% deoxy sugars were higher during the monsoon than the pre-monsoon. Conversely, during the pre-monsoon, monosaccharide composition did not vary much and was mostly dominated by glucose. Monosaccharide abundance and ratios suggest that organic matter was subjected to extensive diagenetic alterations during the monsoon than the pre-monsoon. Carbohydrate carbon normalized to organic carbon was not useful to assess the degradation state of organic matter. POC containing 13.9% of PCHO is transported to coastal waters during the high discharge monsoon period. Dilution, differences in sources, biodegradation and physiological state of phytoplankton appear to affect the transport of particulate carbohydrates in the Mandovi estuary.

Seasonal variation in marine C:N:P stoichiometry: can the composition of seston explain stable Redfield ratios?

Abstract. Seston is suspended particulate organic matter, comprising a mixture of autotrophic, heterotrophic and detrital material. Despite variable proportions of these components, marine seston often exhibits relatively small deviations from the Redfield ratio (C:N:P = 106:16:1). Two time-series from the Norwegian shelf in Skagerrak are used to identify drivers of the seasonal variation in seston elemental ratios. An ordination identified water mass characteristics and bloom dynamics as the most important drivers for determining C:N, while changes in nutrient concentrations and biomass were most important for the C:P and N:P relationships. There is no standardized method for determining the functional composition of seston and the fractions of POC, PON and PP associated with phytoplankton, therefore any such information has to be obtained by indirect means. In this study, a generalized linear model was used to differentiate between the live autotrophic and non-autotrophic sestonic fractions, and for both stations the non-autotrophic fractions dominated with respective annual means of 76 and 55%. This regression model approach builds on assumptions (e.g. constant POC:Chl-a ratio) and the robustness of the estimates were explored with a bootstrap analysis. In addition the autotrophic percentage calculated from the statistical model was compared with estimated phytoplankton carbon, and the two independent estimates of autotrophic percentage were comparable with similar seasonal cycles. The estimated C:nutrient ratios of live autotrophs were, in general, lower than Redfield, while the non-autotrophic C:nutrient ratios were higher than the live autotrophic ratios and above, or close to, the Redfield ratio. This is due to preferential remineralization of nutrients, and the P content mainly governed the difference between the sestonic fractions. Despite the seasonal variability in seston composition and the generally low contribution of autotrophic biomass, the variation observed in the total seston ratios was low compared to the variation found in dissolved and particulate pools. Sestonic C:N:P ratios close to the Redfield ratios should not be used as an indicator of phytoplankton physiological state, but could instead reflect varying contributions of sestonic fractions that sum up to an elemental ratio close to Redfield.

Carbon fluxes within the epipelagic zone of the Humboldt Current System off Chile: The significance of euphausiids and diatoms as key functional groups for the biological pump

The information from 54 drifting sediment traps deployed between 1997 and 2006 along the Humboldt Current System off Chile (from 19.9°S to 42.2°S) was analyzed to contribute to unveiling the recurrent global-ocean issue of the lack of relationship between gross primary production (GPP) and particulate organic carbon (POC) export below 50 m depth. When the proportion of carbon that effectively sinks is relatively low compared to the carbon being fixed through GPP, a significant amount (average of 32%) of the sinking organic matter is composed of diatoms, regardless of GPP rates. Such a fraction seems to be affected by the physiological state of phytoplankton. In contrast, when the fraction of carbon sinking is high relative to GPP, most of sinking organic matter is composed of euphausid faecal strings. Such a situation occurs at relatively low values of GPP and chlorophyll-a. Most of these high sinking rates of pellets and low phytoplankton biomass occur during summer, when physical conditions favour the presence of phytoplankton blooms, and when the GPP/Biomass ratio indicates healthy phytoplankton physiological conditions. All this evidence supports the assessment of the relevance of euphausiids as key species in the Humboldt Current System pointing to (i) the top–down control that euphausiids are capable of exerting over primary producer biomass, and (ii) euphausiids‘ paramount role on total organic carbon flux over the Concepción continental shelf, regarding both POC export to the sediments and possibly the channelling of GPP directly to higher trophic levels.

Photoacoustics as a diagnostic tool for probing the physiological status of phytoplankton

In nature, conditions for the growth of phytoplankton rarely support their maximal potential doubling rates. The main rate-limiting factor determining the doubling rates of phytoplankters is photosynthesis. This key process is in turn controlled, and frequently limited, by the available light, by lack of essential nutrients, or by the presence of noxious pollutants. Any decrease in the rates of photosynthesis is associated with a reduction in the efficiency of light utilization in that process. We present here the decreased photosynthetic efficiency of phytoplankton upon depletion of nitrogen, phosphorus, and iron as measured by photoacoustics. Such effects have been documented extensively by other methods, such as fluorescence yield, growth rate, carbon assimilation, and oxygen evolution. However, photoacoustics measures directly the energy efficiency of the photosynthetic process. We determined that efficiency by this method in three phytoplankton species belonging to different taxa. Our results illustr...

Photosynthate allocation in a temperate sea over an annual cycle: the relationship between protein synthesis and phytoplankton physiological state

The seasonal and vertical variations in the patterns of photosynthate allocation into biomolecules by natural phytoplankton assemblages were determined, together with their species composition, in a coastal station of the central Cantabrian Sea (southern Bay of Biscay). Chlorophyll-a concentration ranged from values below 20 mg m −2 in winter to values above 80 mg m −2 during spring and during an upwelling event in summer. Low primary production rates (<300 mgC m −2 d -1) were measured during winter and during summer stratification periods. The rate of C fixation during summer upwelling conditions exceeded 3500 mgC m −2 d −1. In terms of photosynthate partitioning, proteins were the dominant fraction, as they typically accounted for >30% of total photo-assimilated C, with polysaccharides and low molecular weight metabolites showing incorporation percentages around 10–30%. Relative C incorporation into lipids was generally <15%. Recurrent patterns of vertical variability in photosynthate partitioning were observed: the relative synthesis of proteins increased toward the bottom of the euphotic zone, whereas the relative C incorporation into polysaccharides and lipids tended to be higher near the surface. When primary production decreased, the synthesis of proteins was maintained more than that of other molecules. Throughout the year, the relative synthesis of proteins was inversely correlated with phytoplankton biomass, production and growth rate. The conservation of protein synthesis under growth-limiting conditions and the enhancement of lipid and polysaccharide synthesis when irradiance is high seem to constitute general patterns of photosynthate partitioning in marine phytoplankton. In our study, these patterns represented metabolic strategies of phytoplankton in response to changing environmental factors, rather than the effect of variations in the species composition of the community.