Phytoplankton Organic Carbon Research Articles

Physiological control on carbon isotope fractionation in marine phytoplankton

Abstract. One of the great challenges in biogeochemical research over the past half a century has been to quantify and understand the mechanisms underlying stable carbon isotope fractionation (εp) in phytoplankton in response to changing CO2 concentrations. This interest is partly grounded in the use of fossil photosynthetic organism remains as a proxy for past atmospheric CO2 levels. Phytoplankton organic carbon is depleted in 13C compared to its source because of kinetic fractionation by the enzyme RubisCO during photosynthetic carbon fixation, as well as through physiological pathways upstream of RubisCO. Moreover, other factors such as nutrient limitation, variations in light regime as well as phytoplankton culturing systems and inorganic carbon manipulation approaches may confound the influence of aquatic CO2 concentrations [CO2] on εp. Here, based on experimental data compiled from the literature, we assess which underlying physiological processes cause the observed differences in εp for various phytoplankton groups in response to C-demand/C-supply, i.e., particulate organic carbon (POC) production / [CO2]) and test potential confounding factors. Culturing approaches and methods of carbonate chemistry manipulation were found to best explain the differences in εp between studies, although day length was an important predictor for εp in haptophytes. Extrapolating results from culturing experiments to natural environments and for proxy applications therefore require caution, and it should be carefully considered whether culture methods and experimental conditions are representative of natural environments.

Ice algae resource utilization by benthic macro- and megafaunal communities on the Pacific Arctic shelf determined through lipid biomarker analysis

We studied ice algae utilization by benthic fauna from the northern Bering and Chukchi Seas using highly branched isoprenoid (HBI) biomarkers. We assessed whether various food acquisition strategies influence the observed HBI signatures. The proportion of phytoplankton to ice algae-sourced HBIs was determined through the H-Print approach that is presumed to reflect the percentage of sea ice organic carbon (iPOC) incorporated into tissues, relative to phytoplankton organic carbon. Cluster analysis separated 3 groups based on location and feeding strategy that were significantly influenced by annual sea ice persistence. Ice algae utilization was most significant in the northeast Chukchi Sea, where seasonal sea ice was present the longest. General feeding strategy was determined to be a significant factor in the degree of ice algae utilization. Predominant deposit feeders (both surface and subsurface) used more ice algae relative to suspension feeders. Organic carbon incorporated by predominant suspension feeders was primarily phytoplankton-based. The vast majority of all organisms sampled (~90%) incorporated a measurable quantity of iPOC. Sipunculids and brittle stars had the highest relative dependence on ice algae, while other taxa displayed plastic dietary responses, including the suspension/surface deposit feeder Macoma calcarea. This study indicates that ice algae are widely utilized in Pacific Arctic benthic food webs, but most benthic organisms displayed flexibility in consuming the available food sources. The elevated utilization of ice algae by deposit feeders may prove to be a disadvantage for these organisms if they cannot adapt to the ongoing decline of iPOC as seasonal sea ice declines.

Viral lysis of Micromonas pusilla: impacts on dissolved organic matter production and composition

The viral mediated transformation of phytoplankton organic carbon to dissolved forms (“viral shunt”) has been suggested as a major source of dissolved organic carbon (DOC) in marine systems. Despite the potential implications of viral activity on the global carbon fluxes, studies investigating changes in the DOC composition from viral lysis is still lacking. Micromonas pusilla is an ecologically relevant picoeukaryotic phytoplankter, widely distributed in both coastal and oceanic marine waters. Viruses have been found to play a key role in regulating the population dynamics of this species. In this study we used axenic cultures of exponentially growing M. pusilla to determine the impact of viral lysis on the DOC concentration and composition, as estimated from lysate-derived production of transparent exopolymer particles (TEP) and two fractions of fluorescent dissolved organic matter (DOM): aromatic amino acids (excitation/emission; 280/320 nm; F(280/320)) and marine humic-like fluorescent DOM (320/410 nm; F(320/410)). DOC concentration increased 4.5 times faster and reached 2.6 times higher end concentration in the viral infected compared with the non-infected cultures. The production of F(280/320) and F(320/410) were 4.1 and 2.8 times higher in the infected cultures, and the elevated ratio between F(280/320) and F(320/410) in lysates suggested a higher contribution of labile (protein) components in viral produced DOM than in algal exudates. The TEP production was 1.8 times faster and reached a 1.5 times higher level in the viral infected M. pusilla culture compared with the non- infected cultures. The measured increase in both DOC and TEP concentrations suggests that viral lysis has multiple and opposite implications for the production and export processes in the pelagic ocean: (1) by releasing host biomass as DOC it decreases the organic matter sedimentation and promotes respiration and nutrient retention in the photic zone, whereas (2) the observed enhanced TEP production could stimulate particle aggregation and thus carbon export out of the photic zone.

Influence of the geostrophic transport of phosphates on primary production off Baja California (Mexico)

We estimated the net flux of inorganic phosphorus (kg P d–1) due to geostrophic transport in the upper 100 m of a control volume (CV) off Baja California (Mexico) during 2003 and 2004. The influence of this flux on integrated primary production (IPP) in the photic zone (less than 100 m depth) was estimated comparing the net P flux into the CV against the P requirements for IPP. The greatest P fluxes occurred during spring and summer 2004; the northern side of the CV was the main recipient of P. The most persistent P outflow occurred along the southern side of the CV, except in winter 2003, when there was an input of P fluxes related to cyclonic and anticyclonic eddies off Ensenada. The net geostrophic transport during 2003 and 2004 contributed enough P to support the IPP requirements during winter and spring, indicating that P was not limiting for phytoplankton organic carbon production during these seasons.

Importance of dissolved organic carbon for phytoplankton nutrition in a eutrophic reservoir

In July—September 2007 at monthly intervals, we performed four experiments on size-fractionated phytoplankton organic carbon uptake in the eutrophic Rimov Reservoir (Czech Republic). Along with primary production measurement, samples from surface and euphotic depth were incubated at in situ depths in light and dark bottles with 3 H-labelled amino acid mixture and glucose in nanomolar concentrations. Moreover, we measured silica deposition and chlorophyll content in diatom cells with and without unlabelled glucose addition (100 μM). Phytoplankton uptake of both glucose and amino acids was low regardless of the incubation depth. The major portion of incorporated organic carbon was found in the fraction <2 μm indicating that bacteria are better competitors for organic substrate than phytoplankton. Additionally, direct visualization of organic carbon uptake using microautoradiography showed that bacteria attached to the phytoplankton accounted for the activity in the large size fractions. Thus, mixotrophic nutrition of the phytoplankton was of low importance under natural conditions. However, glucose addition in micromolar concentrations resulted in a significant increase in diatom silification rates in the light bottles. Interestingly, in both light and dark bottles, chlorophyll fluorescence in the diatom cells markedly decreased after the glucose addition indicating a potential restructuring of cell metabolism to benefit from a remarkably high concentration of available organic carbon in the environment.

Seasonal Dynamics of the Sulphate Reduction Rate on the North-western Black Sea Shelf

The microbial sulphate reduction rate (SRR) in the surface layer (0–30 cm) of shallow water sediments of the north-western Black Sea shelf has been measured using the 35S radioisotopic technique. It was shown that SRR is accompanied by changes of salt composition of porewaters and isotopic (δ 34S) composition of sulphate. The hydrogen sulphide-derived minerals are formed in the solid phase of sediments. An expected exponential dependence of the isotopic composition of reduced sulphur compounds on the SRR was observed. In near-shore sediments, terrestrial organic carbon (δ 13C≈−26‰) as well as marine phytoplankton organic carbon (δ 13C≈−22‰) are associated with the SR processes. In August 1995 and May 1997–1998, anoxic conditions in near-bottom waters of coastal areas were not detected. The average value of H 2S production in nearshore sediments varied from 3·62 mmol S m −2 day −1 in spring to 8·89 mmol S m −2 day −1 in summer 1995; these fluxes were twice as high as those measured in spring 1997 and 1998.

Microbial communities in the chemocline of a hypersaline deep-sea basin (Urania basin, Mediterranean Sea).

The Urania basin is a hypersaline sulfidic brine lake at the bottom of the eastern Mediterranean Sea. Since this basin is located at a depth of approximately 3,500 m below the sea surface, it receives only a small amount of phytoplankton organic carbon. In the present study, the bacterial assemblages at the interface between the hypersaline brine and the overlaying seawater were investigated. The sulfide concentration increased from 0 to 10 mM within a vertical interval of 5 m across the interface. Within this chemocline, the total bacterial cell counts and the exoenzyme activities were elevated. Employing 11 cultivation methods, we isolated a total of 70 bacterial strains. The 16S ribosomal DNA sequences of 32 of the strains were identical to environmental sequences detected in the chemocline by culture-independent molecular methods. These strains were identified as flavobacteria, Alteromonas macleodii, and Halomonas aquamarina. All 70 strains could grow chemoorganoheterotrophically under oxic conditions. Sixty-six strains grew on peptone, casein hydrolysate, and yeast extract, whereas only 15 strains did not utilize polymeric carbohydrates. Twenty-one of the isolates could grow both chemoorganotrophically and chemolithotrophically. While the most probable numbers in most cases ranged between 0.006 and 4.3% of the total cell counts, an unusually high value of 54% was determined above the chemocline with media containing amino acids as the carbon and energy source. Our results indicate that culturable bacteria thriving at the oxic-anoxic interface of the Urania basin differ considerably from the chemolithoautotrophic bacteria typical of other chemocline habitats.

The effect of fish farming organic waste on food availability for bivalve molluscs (Gaeta Gulf, Central Tyrrhenian, MED): stable carbon isotopic analysis

Stable carbon isotope ( δ 13C) analysis was used in a fish-farming impacted Mediterranean area (the Gulf of Gaeta, Central Tyrrhenian) to determine the predominant carbon sources available to bivalve molluscs cultivated around fish cages. Whether the organic matter generated by fish farming was taken up by the bivalve molluscs was also investigated. Stable carbon isotope values were measured in the particulate organic carbon (POC) of samples from potential organic matter sources such as fish-pelleted feed, mollusc faecal waste and bivalve flesh. The sources of organic matter affecting the study area water column and benthic communities appeared to be terrigenous-continental, autochthonous (phytoplankton) and anthropogenic inputs due mainly to fish-farming and bivalve mollusc activities. The POC was dominated by organic waste isotopic signatures, while the bivalve mixed diet was composed of organic matter with different isotopic signatures (phytoplankton, waste material from the bivalves themselves and surplus uneaten pelletted feed). Organic waste appears to be the dominant trophic resource in the deeper-cultivated clam diet, while phytoplankton organic carbon plays a more important role in the diet of the mussel. We propose that bivalve organic matter uptake may play an effective role in reducing the environmental impact of fish organic waste. The organic matter produced by bivalves (faecal material) under these hydrodynamic conditions (low current velocities) can be recycled through the filtration activities of the bivalves themselves, together with most of the organic matter produced by fish-farming activities (uneaten feed and faecal material). Bivalve cultivation around cages may reduce the environmental impact of organic waste from fish-farming activities and increase the profitability of fish culture activities.

A Comparison of Phytoplankton Biomass Estimators and Their Environmental Correlates in the Mali Ston Bay (Southern Adriatic)

Abstract. Phytoplankton biomass estimators – total cell number (CN), total cell volume (CV), chlorophyll a (Chl a). phytoplankton organic carbon (PC), and their response to environmental variables, with a special emphasis on the vertical variability in the structure of the water column – were examined in the Mali Ston Bay (Southern Adriatic). Correlations among biomass estimators as well as with physical‐chemical parameters varied according to hydrodynamic characteristics of the water column. The amount of Chl a per cell ranged from 0.28.06–10‐6μg during stratification to 0.7–2.0.10‐6μg during the mixing period. Most frequent values for PC/Chl a ratios and for Chl a/CV in mixed natural phytoplankton populations were 20–25 and 1–2, respectively.

Acclimatization of Antarctic natural phytoplankton assemblages when exposed to solar ultraviolet radiation

The effects of solar ultraviolet radiation (UVR) on photosynthetic rates of natural assemblages of Antarctic phytoplankton were determined in both 1 day and 2 week incubations during austral spring at Palmer Station, Antarctica. During the first day of the long-term incubations, photosynthetic rates were enhanced by ∼40% when UV-B radiation was excluded from the culture and by an additional 80% when UV-A radiation was also excluded. In spite of this UVR-induced photoinhi-bition of photosynthetic rates during the first day of each long-term experiment, cultures with and without exposure to UVR both showed exponential growth after the first few days, so that at the end of the 2 week growth period the chlorophyll-a concentrations in the samples grown with exposure to UVR were quite similar to those in the cultures from which UVR had been excluded. Phytoplankton organic carbon concentrations increased at rates comparable to those of chlorophyll concentrations during the long-term experiments. The effects of UVR were evident, however, in changes in the Holistic and chemical composition of the phytoplankton during the long-term incubations. The proportion of the phytoplankton biomass accounted for by diatoms in the cultures exposed to UVR increased, while that of flagellates deceased. Cultures exposed to UVR also showed higher concentrations of UV-absorbing compounds as compared to cultures grown without exposure to UVR. This apparent acclimatization of the cultures to UVR was reflected in decreased sensitivity to UVR when the responses of subsamples were determined in 1 day incubation tests.

Phytoplankton around Elephant Island, Antarctica

Phytoplankton studies were conducted in a 4·104 km2 grid around Elephant Island, Antarctica, during January–March 1991. In addition to profiling studies at 50 stations during each of two cruises, pumped surface water was used to continuously record in vivo chlorophyll-a (chl-a) fluorescence and beam attenuation coefficients (ct). Measurements on discrete samples included chl-a, phytoplankton organic carbon (Ac), and total particulate organic carbon (POC). Equations were developed which permitted contour-mapping of chl-a and POC throughout the entire grid based on continuous measurements of in vivo chl-a fluorescence and ct values. Phytoplankton abundance increased from January–February (cruise I) to February–March (cruise II) as evidenced by mean surface chl-a values (0.8 and 1.34 μg l−1, respectively) and mean Ac values for the upper 20 m of the water column (21.3 and 39.8 μg Cl−1, respectively). Microplankton accounted for 29% (cruise I) and 40% (cruise II) of the total phytoplankton, based on direct measurements of chl-a. Patterns for integrated values of chl-a, Ac and POC were similar to patterns of concentrations in surface waters. The richest areas of phytoplankton were north of Elephant and King George Islands, which coincided with high krill density areas.

Dynamics of carbon assimilation by St Lawrence estuary phytoplankton at the end of summer

This study presents data of in situ measurements of inorganic carbon assimilation by phytoplankton communities of the St Lawrence estuary during the end of summer 1982. We used carboxylase activity measurements (ribulose-1,5-bisphosphate carboxylase, β carboxylases) and the 13C/12C ratio of phytoplankton organic carbon, expressed as ∂13C, to study patterns of assimilation. Upper estuary phytoplankton communities showed a smaller turn-over rate in carbon assimilation than lower estuary phytoplankton communities. Carbon assimilation was limited by light intensity in the upper estuary and by CO2 availability in the lower estuary. In the St Lawrence estuary, stable carbon isotope ratios of phytoplankton organic carbon seemed to be controlled by inorganic carbon availability rather than by phytoplankton metabolism.

Micro-zooplankters in the plankton communities of the upper waters of the eastern tropical Pacific

The numerical abundance, biomass, and taxonomic composition of the micro-zooplankton and chlorophyll, phaeophytin, and total seston levels were determined for six depth intervals within the upper 200 m at 12 locations from approximately 10°N to 12°S latitude on longitude 105°W. Data on the hydrography, phytoplankton inorganic nutrients, larger zooplankton standing stocks, and phytoplankton productivity at the various sites was available through the EASTROPAC program. Average micro-zooplankton volume over the euphotic zone showed a threefold range, from 15 mm 3/m 3 at the southerly extreme of our sampling to 47 mm 3/m 3 near the Equator. The relative taxonomic composition of the micro-zooplankton populations within the three size classes of material separated was similar to that described from earlier studies of California Current populations. Highly significant positive correlations were found between micro-zooplankton abundance and chlorophyll levels in the various depth intervals when ‘groups’ of stations, delineated on the basis of similar hydrographic features, or all stations in the study were considered. Standing stock biomass of micro-zooplankton averaged 34% (range, 23–66%) of that of the phytoplankton, both estimated in terms of dry weight over the euphotic zone at the different sites. This was considerably greater than was earlier indicated for stations in the California Current. Calculations, based on assumptions regarding micro-zooplankton feeding rates, suggested that these small animal plankters might be consuming an average of 70% (range, 39–104%)_of the daily phytoplankton organic carbon production at stations along the transect. The same calculation using data from weekly observations taken between April and September at a site five miles off the La Jolla coast indicated an average of only 23% (range, 7–52%) for this relationship. Micro-zooplankton biomass within the euphotic zone at the various locations relative to the larger zooplankton was suggested to be about an average 24%, but this calculation was complicated by the different depths of sampling for the two populations and the methods of determining their volumes.