Dominant Phytoplankton Group Research Articles

The Biological Pump and Seasonal Variability of pCO2 in the Southern Ocean: Exploring the Role of Diatom Adaptation to Low Iron

AbstractIron is known to limit primary production in the Southern Ocean (SO). To cope with the lack of this micronutrient, diatoms, a dominant phytoplankton group in this oceanic region, have been shown in cultures to have developed an original adaptation strategy to maintain efficient growth rates despite very low cellular iron quotas, even in low light conditions. Using a global ocean biogeochemical model, we explored the consequences of this physiological adaptation for the biological pump and the seasonal variability of both surface chlorophyll concentrations and surface partial pressure of carbon dioxide (pCO2) in this key region for global climate. In the model, we implemented a low intracellular Fe:C requirement in the SO for diatoms uniquely. This results in an increase of 10% in the relative contribution of diatoms to total SO primary production. The biological pump is also strengthened, which increases the biological contribution to the seasonal evolution of pCO2 relative to the thermodynamic component. Therefore, the seasonal evolution of both surface chlorophyll and surface pCO2 is significantly impacted, with a marked improvement, in our model, in the SO polar zone compared to the observations. Our model study underscores the potentially important consequences that this adaptive physiological behavior of diatoms could have on marine biogeochemistry in the SO. It is thus critical to improve our understanding of the physiology of this key phytoplankton group, in particular in the SO.

PAHs sensitivity of picophytoplankton populations in the Red Sea

In this study, we investigated the in situ responses of Red Sea picophytoplankton, the dominant phytoplankton group in the oligotrophic ocean, to two toxic polycyclic aromatic hydrocarbons (PAHs), phenanthrene and pyrene. The experiments were conducted across a latitudinal gradient of the Saudi Arabian Red Sea, an area sensitive to oil pollution. We observed significant adverse effects on the growth and abundance of the picocyanobacteria Synechococcus and picoeukaryotes, at all stations sampled. Prochlorococcus, which was abundant only at one of the stations, also appeared to be affected. Pyrene was found to be more toxic to phytoplankton at all stations. In general, picoeukaryotes exhibited higher sensitivity to PAHs than Synechococcus. Populations in the highly oligotrophic Northern region of the Red Sea were more tolerant to PAHs, presumably influenced by the natural selection of more resistant strains of phytoplankton due to the prolonged exposure to PAHs. Toxicity threshold values estimated here are higher than those reported for picophytoplankton from other oligotrophic marine waters and exceed by far the natural levels of PAHs in many oceans. Our findings reveal a possible adaptation of picophytoplankton populations to oil-related contaminants, which may clearly influence their spatial distribution patterns in the Red Sea.

210Po concentration in selected diatoms and dinoflagellates in the northern Arabian Gulf

Marine phytoplankton is a primary producer in the ocean that forms the base of the marine food web and supports the pelagic food chain. The two dominant groups of phytoplankton observed in northern Gulf waters are diatoms and dinoflagellates. The diatoms outnumber dinoflagellates in the Gulf waters. This study presents baseline information on the concentration of 210Po among selected phytoplankton in the northern Gulf. The concentration among diatoms varies between 6.99 and 11.4 Bq kg−1 wwt, whereas a higher concentration range of 8.51–15.41 Bq kg−1 wwt was observed among dinoflagellates. The diatoms analyzed includes Thalassiosira spp. – 10.2–11.4 Bq kg−1 wwt; Chaetoceros spp. – 6.99–7.14 Bq kg−1 wwt; Rhizosolenia spp. – 9.12–9.95 Bq kg−1 wwt. The analyzed dinoflagellate genera include Gymnodinium spp. – 8.51–8.78 Bq kg−1 wwt; Noctiluca spp. – 15.2–15.4 Bq kg−1 wwt; and Karenia spp. – 14.1–14.9 Bq kg−1 wwt. The 210Po concentration in seawater shows a seasonal variation, with a higher concentration range of 0.70 and 0.58 mBq L−1 during summer and autumn, whereas a lower range in concentration of 0.38 and 0.30 mBq L−1 occurring during winter and spring. The resultant computed concentration factors vary between 2 ∗ 104–5 ∗ 104 demonstrating a significant 210Po enrichment in the base of the pelagic food chain.

Mapping of trophic states based on nutrients concentration and phytoplankton abundance in Jatibarang Reservoir

Jatibarang Reservoir is one of the Indonesian Reservoirs, which used for human activities such as tourism and agriculture. These activities will provide input of organic matter and nutrients into the water. These materials will impact water quality and eutrophication process. Eutrophication is the water enrichment by nutrients, especially nitrogen and phosphorus which can promote the growth of phytoplankton. Some indicators of eutrophication are increasing nutrients, trophic states, and change of phytoplankton composition. The relationship between water quality and phytoplankton community can be used as an indicator of trophic states in Jatibarang Reservoir. The aim of this study was to analyze the effect of nutrients concentration and phytoplankton abundance to the trophic states and mapping trophic states based on nutrients concentration and phytoplankton in Jatibarang Reservoir. This study was conducted in June and July 2017 at 9 stations around Jatibarang Reservoir. The results showed that average concentration of nitrate, phosphate, and chlorophyll-a in Jatibarang Reservoir was 0.69 mg/L, 0.27 mg/L, and 1.66 mg/m3, respectively. The phytoplankton abundance ranged 16-62,200 cells/L, consists of 21 genera of four classes, i.e. Chlorophyceae, Cyanophyceae, Bacillariophyceae, and Dinophyceae. Cyanophyceae was a dominant phytoplankton group based on the composition of abundance (>80%). High nutrient concentrations and phytoplankton dominated by Anabaena (Cyanophyceae) which indicated that the waters in Jatibarang Reservoir were eutrophic.

Changes of the phytoplankton community as symptoms of deterioration of water quality in a shallow lake

Covering more than 60% of the lake surface, macrophytes determined the taxonomic composition of phytoplankton. We have found numerous indications of ecological deterioration and an increased trophic level year to year: an increased total number of taxa; a significantly increased number of species of Chlorophyta, Bacillariophyceae and Cyanoprokaryota; a decreased number of Chrysophyceae; increased Nygaard index, and high diversity and variability of phytoplankton functional groups. Within 2 years (2002 and 2003) algal biomass doubled: from 3.616 to 7.968 mg l−1. An increased contribution of Chlorococcales and Cyanoprokaryota indicates progressive eutrophication of the lake. The average size of planktonic algae increased, particularly Cyanoprokaryota, where small-celled decreased dramatically and were replaced by large colonies. Cyanoprokaryota remained the dominant group of phytoplankton after 10 years, and the ecosystem of the lake remained in the turbid state. This group of algae had the average biomass 9.734 mg l−1, which constituted almost 92% of the total biomass.

Estimation of the Potential Detection of Diatom Assemblages Based on Ocean Color Radiance Anomalies in the North Sea

Over the past years, a large number of new approaches in the domain of ocean-color have been developed, leading to a variety of innovative descriptors for phytoplankton communities. One of these methods, named PHYSAT, currently allows for the qualitative detection of five main phytoplankton groups from ocean-color measurements. Even though PHYSAT products are widely used in various applications and projects, the approach is limited by the fact it identifies only dominant phytoplankton groups. This current limitation is due to the use of biomarker pigment ratios for establishing empirical relationships between \textit{in-situ} information and specific ocean-color radiance anomalies in open ocean waters. However, theoretical explanations of PHYSAT suggests that it could be possible to detect more than dominance cases but move more towards phytoplanktonic assemblage detection. Thus, to evaluate the potential of PHYSAT for the detection of phytoplankton assemblages, we took advantage of the Continuous Plankton Recorder (CPR) survey, collected in both the English Channel and the North Sea. The available CPR dataset contains information on diatom abundance in two large areas of the North Sea for the period 1998-2010. {Using this unique dataset, recurrent diatom assemblages were retrieved based on classification of CPR samples}. Six diatom assemblages were identified \textit{in-situ}, \textcolor{blue}{each having indicators taxa or species}. Once this first step was completed, the \textit{in-situ} analysis was used to empirically associate the diatom assemblages with specific PHYSAT spectral anomalies. This step was facilitated by the use of previous classifications of regional radiance anomalies in terms of shape and amplitude, coupled with phenological tools. Through a matchup exercise, three CPR assemblages were associated \textcolor{blue}{with} specific radiance anomalies. The maps of detection of these specific radiances anomalies are in close agreement with current \textit{in-situ} ecological knowledge.

Phytoplankton Group Identification Using Simulated and In situ Hyperspectral Remote Sensing Reflectance

In the present study we investigate the bio-geo-optical boundaries for the possibility to identify dominant phytoplankton groups from hyperspectral ocean color data. A large dataset of simulated remote sensing reflectance spectra, Rrs(λ), was used. The simulation was based on measured inherent optical properties of natural water and measurements of five phytoplankton light absorption spectra representing five major phytoplankton spectral groups. These simulated data, named as C2X data, contain more than 105 different water cases, including cases typical for clearest natural waters as well as for extreme absorbing and extreme scattering waters. For the simulation the used concentrations of chlorophyll a (representing phytoplankton abundance), Chl, are ranging from 0 to 200 mg m-3, concentrations of non-algal particles, NAP, from 0 to 1500 g m-3, and absorption coefficients of chromophoric dissolved organic matter (CDOM) at 440 nm from 0 to 20 m-1. A second, independent, smaller dataset of simulated Rrs(λ) used light absorption spectra of 128 cultures from six phytoplankton taxonomic groups to represent natural variability. Spectra of this test dataset are compared with spectra from the C2X data in order to evaluate to which extent the five spectral groups can be correctly identified as dominant under different optical conditions. The results showed that the identification accuracy is highly subject to the water optical conditions, i.e., contribution of and covariance in Chl, NAP, and CDOM. The identification in the simulated data is generally effective, except for waters with very low contribution by phytoplankton and for waters dominated by NAP, whereas contribution by CDOM plays only a minor role. To verify the applicability of the presented approach for natural waters, a test using in situ Rrs(λ) dataset collected during a cyanobacterial bloom in Lake Taihu (China) is carried out and the approach predicts blue cyanobacteria to be dominant. This fits well with observation of the blue cyanobacteria Microcystis sp. in the lake. This study provides an efficient approach, which can be promisingly applied to hyperspectral sensors, for identifying dominant phytoplankton spectral groups purely based on Rrs(λ) spectra.

Controlling factors of the seasonal variability of productivity in the southern Humboldt Current System (30–40°S): A biophysical modeling approach

The spatial and seasonal variability of nutrients and chlorophyll in the southern Humboldt Current System were assessed using a high-resolution regional ocean circulation model (ROMS) coupled to a biogeochemical model (Pelagic-Interactions Scheme for carbon and Ecosystem Studies; PISCES). The simulated nutrients and chlorophyll fields were validated using satellite and in situ observations at a continental shelf time-series station. The annual cycles of modeled chlorophyll and nutrients were consistent with the highest values observed in spring and summer, which is in agreement with enhanced upwelling observations. Co-limitation of phytoplankton growth by nutrients and light was analyzed for diatoms, the dominant phytoplankton group in the simulations. The results showed that co-limitation, near the coast, was governed in autumn and winter by light, and by silicate in spring and summer, whereas other nutrients were limiting offshore between January and April. Nutrient transport in the surface layer was analyzed. Vertical advection reflected areas with higher coastal upwelling, and was partly offset by horizontal processes related to eddy-induced transport from the nearshore to the open ocean. Vertical mixing was shown to play a key role in replenishing the surface layer with nutrients.

Effect of eolian Fe-supply change on the phytoplankton productivity and community in central equatorial Pacific Ocean during the Pleistocene: A lipid biomarker approach

The effect of iron (Fe) fertilization on biological productivity and community structure in the equatorial Pacific, a region with high nutrient and low chlorophyll concentrations, has been debated for many years. Here we present lipid biomarker and geochemical proxy records in a sediment core from the central equatorial Pacific spanning from 1100 to 600ka to investigate the response of marine phytoplankton communities to eolian Fe supply and to assess links to environmental fluctuations during the Pleistocene. We found a strong link between change in eolian Fe input and phytoplankton community structure. Our data demonstrate that an increase in eolian Fe supply across the 800ka boundary resulted in a shift in the major primary producer from coccolithophores (alkenones) to diatoms (epi-brassicasterol) and dinoflagellates (dinosterol) at the study site. This strong link between changes in eolian Fe supply and the community structure of plankton during 800–600ka may have been associated with a change in atmospheric and ocean conditions due to southward migration of the Intertropical Convergence Zone (ITCZ). The lipid biomarker results show that such a high supply of eolian Fe might have led to a change in the dominant phytoplankton group. However, the opal record, which is often used asa primary productivity indicator, was not consistent with the biomarker results. Therefore, the results indicate that multi-proxy approaches should be used in paleoenvironmental studies because different proxies can reveal different aspects of environmental conditions, thus contributing to a better understanding of paleoceanographic conditions.

Reproduction of Spatio-Temporal Patterns of Major Mediterranean Phytoplankton Groups from Remote Sensing OC-CCI Data

During the last two decades, several satellite algorithms have been proposed to retrieve information about phytoplankton groups using ocean colour data. One of these algorithms, the so-called PHYSAT-Med, was developed specifically for the Mediterranean Sea due to the optical peculiarities of this basin. The method allows the detection from ocean colour images of the dominant Mediterranean phytoplankton groups, namely nanoeukaryotes, Prochlorococcus, Synechococcus, diatoms, coccolithophorids and Phaeocystis-like phytoplankton. Here, we present a new version of PHYSAT-Med applied to the Ocean Colour – Climate Change Initiative (OC-CCI) database. The OC-CCI database consists of a multi-sensor, global ocean-colour product that merges observations from four different sensors. This retuned version presents improvements with respect to the previous version, as it increases the temporal range (since 1998), decreases the cloud cover, improves the bias correction and a validation exercise was performed in the NW Mediterranean Sea. In particular, the PHYSAT-Med version has been used here to analyse the annual cycles of the major phytoplankton groups in the Mediterranean Sea. Wavelet analyses were used to explore the spatial variability in dominance both in the time and frequency domains in several Mediterranean sub-regions, such as the Alboran Sea, Ligurian Sea, Northern Adriatic Sea and Levantine basin. Results extended the interpretation of previously detected patterns, indicating the dominance of Synechococcus-like versus prochlorophytes throughout the year at the basin level, and the predominance of nanoeukaryotes during the winter months. The method successfully reproduced the diatom blooms normally detected in the basin during the spring season (March to April), especially in the Adriatic Sea. According to our results, the PHYSAT-Med OC-CCI algorithm represents a useful tool for the spatio-temporal monitoring of dominant phytoplankton groups in Mediterranean surface waters. The successful applications of other regional ocean colour algorithms to the OC-CCI database will give rise to extended time series of phytoplankton functional types, with promising applications to the study of long-term oceanographic trends in a global change context.

Spring phytoplankton communities of the Labrador Sea (2005–2014): pigment signatures, photophysiology and elemental ratios

Abstract. The Labrador Sea is an ideal region to study the biogeographical, physiological, and biogeochemical implications of phytoplankton community composition due to sharp transitions between distinct water masses across its shelves and central basin. We have investigated the multi-year (2005–2014) distributions of late spring and early summer (May to June) phytoplankton communities in the various hydrographic settings of the Labrador Sea. Our analysis is based on pigment markers (using CHEMTAX analysis), and photophysiological and biogeochemical characteristics associated with each phytoplankton community. Diatoms were the most abundant group, blooming first in shallow mixed layers of haline-stratified Arctic shelf waters. Along with diatoms, chlorophytes co-dominated at the western end of the section (particularly in the polar waters of the Labrador Current (LC)), whilst Phaeocystis co-dominated in the east (modified polar waters of the West Greenland Current (WGC)). Pre-bloom conditions occurred in deeper mixed layers of the central Labrador Sea in May, where a mixed assemblage of flagellates (dinoflagellates, prasinophytes, prymnesiophytes, particularly coccolithophores, and chrysophytes/pelagophytes) occurred in low-chlorophyll areas, succeeding to blooms of diatoms and dinoflagellates in thermally stratified Atlantic waters in June. Light-saturated photosynthetic rates and saturation irradiance levels were highest at stations where diatoms were the dominant phytoplankton group ( > 70 % of total chlorophyll a), as opposed to stations where flagellates were more abundant (from 40 up to 70 % of total chlorophyll a). Phytoplankton communities from the WGC (Phaeocystis and diatoms) had lower light-limited photosynthetic rates, with little evidence of photoinhibition, indicating greater tolerance to a high light environment. By contrast, communities from the central Labrador Sea (dinoflagellates and diatoms), which bloomed later in the season (June), appeared to be more sensitive to high light levels. Ratios of accessory pigments (AP) to total chlorophyll a (TChl a) varied according to phytoplankton community composition, with polar phytoplankton (cold-water related) having lower AP : TChl a. Polar waters (LC and WGC) also had higher and more variable particulate organic carbon (POC) to particulate organic nitrogen (PON) ratios, suggesting the influence of detritus from freshwater input, derived from riverine, glacial, and/or sea ice meltwater. Long-term observational shifts in phytoplankton communities were not assessed in this study due to the short temporal frame (May to June) of the data. Nevertheless, these results add to our current understanding of phytoplankton group distribution, as well as an evaluation of the biogeochemical role (in terms of C : N ratios) of spring phytoplankton communities in the Labrador Sea, which will assist our understanding of potential long-term responses of phytoplankton communities in high-latitude oceans to a changing climate.

Fluorescence-Based Approach to Estimate the Chlorophyll-A Concentration of a Phytoplankton Bloom in Ardley Cove (Antarctica)

A phytoplankton bloom occurred in Ardley Cove, King George Island in January 2016, during which maximum chlorophyll-a reached 9.87 mg/m3. Records show that blooms have previously not occurred in this area prior to 2010 and the average chlorophyll-a concentration between 1991 and 2009 was less than 2 mg/m3. Given the lack of in situ measurements and the poor performance of satellite algorithms in the Southern Ocean and Antarctic waters, we validate and assess several chlorophyll-a algorithms and apply an improved baseline fluorescence approach to examine this bloom event. In situ water properties including in vivo fluorescence, water leaving radiance, and solar irradiance were collected to evaluate satellite algorithms and characterize chlorophyll-a concentration, as well as dominant phytoplankton groups. The results validated the nFLH fluorescence baseline approach, resulting in a good agreement at this high latitude, high chlorophyll-a region with correlation at 59.46%. The dominant phytoplankton group within the bloom was micro-phytoplankton, occupying 79.58% of the total phytoplankton community. Increasing sea ice coverage and sea ice concentration are likely responsible for increasing phytoplankton blooms in the recent decade. Given the profound influence of climate change on sea-ice and phytoplankton dynamics in the region, it is imperative to develop accurate methods of estimating the spatial distribution and concentrations of the increasing occurrence of bloom events.

Constraining the Distribution of Photosynthetic Parameters in the Global Ocean

Global and regional ocean primary production estimates are highly dependent on assumptions concerning the photosynthetic potential of the resident phytoplankton communities. Little is known, however, about global patterns in the distribution of photosynthetic potential and their causes. Here, we review existing literature reporting photosynthetic characteristics of natural populations. From this, we formulate hypotheses regarding abiotic and biotic factors of potential importance in determining photosynthetic performance. These hypotheses are then tested using data we have compiled from nearly all major ocean basins on the maximum rate of photosynthesis, PBmax, and the slope of the photosynthesis vs. light curve, αB (both parameters normalised to chlorophyll) as well as standard environmental variables, size fractioned chlorophyll, taxonomic data (to group), size and biovolume data for pico-, nano-, and micro-phytoplankton. In terms of abiotic variables, depth of sampling, temperature and nutrient availability all can be related to photosynthetic parameters. The most important biotic variable influencing photosynthetic performance was found to be community size distribution and the small component (i.e. the proportion of the phytoplankton community passing through a 10 µm filter) is shown to have both higher PBmax and αB than the larger phytoplankton component. A simple model was used to derive best fit values for PBmax (1.53/2.50 µgC l-1 h-1) and αB (0.025/0.040) for the large/small groups in the subset of the data where taxonomic data were available (both surface and sub-surface samples) using fractioned chlorophyll data and bulk community photosynthetic parameters. Non-metric multidimensional scaling (NMDS) was used to relate the distribution of photosynthetic parameters and dominant (by biovolume) phytoplankton groups. High PBmax was recorded in communities dominated by dinoflagellates, small flagellates and, in warmer waters, picoeukaryotes and Synecococcus. Diatom dominated communities exhibited lower PBmax and were associated with high inorganic nutrients and colder temperatures. That photosynthetic parameters appear closely related to community size distributions and taxonomic group provides some hope for improving the parameterization of photosynthetic performance in global ocean primary production estimates as both of these parameters can be made from remotely sensed optical characteristics of surface waters.

The first continuous plankton sampling by VOR (Towed Oceanographic Vehicle) in southeastern and southern Brazil waters

Plankton sampling with the Continuous Plankton Sampling survey has been conducted since 1938 in the Northern Atlantic Ocean and North Sea, where in both cases historical records have documented climate change. Plankton sampling with the VOR (Towed Oceanographic Vehicle) on industrial fishing boats was performed in May-June of 2013 off the southeastern and southern coasts of Brazil (between 23o00`S44o21`W and 31o15`S-50o06`W). The speed of VOR towing onboard the fishing vessels was 8-10 knots to 10- 12 m of depth. The size of mesh silk used was 250-270 μm. The total of taxa is 73 phytoplankton, 26 zooplankton, 10 protozooplankton (Ciliophora) and ichityoplankton (fish eggs) were recorded. The dominant phytoplankton groups detected were diatoms and dinoflagellates, although cyanobacteria were also abundant in the southeast region. The zooplankton with a size class of less than 2 mm was the most frequent. A positive correlation was observed between herbivorous zooplankton and phytoplankton in the coastal waters.

Struktur Komunitas Fitoplankton di Danau Toba, Sumatera Utara

Lake Toba is the largest lake in Indonesia, located in the Province of North Sumatra. Lake Toba is used to aquaculture, agricultural, tourism, and residential areas. These activities will produce organic and anorganic matter that can affect water quality and can cause eutrophication. Some indicators of eutrophication are an increase in phytoplankton biomass and changes in phytoplankton community structure. The aim of this study was to describe the phytoplankton community structure in Lake Toba. This study was conducted on 20-24 October 2014 at 23 stations around the Samosir Island, Lake Toba. The results showed that the phytoplankton abundance ranged 216-68.319.716 cells/m3. The phytoplankton composition consists of 35 genera from four classes: Cyanophyceae, Chlorophyceae, Bacillariophyceae, and Dinophyceae. Chyanophyceae is a dominant phytoplankton groups based on the composition of abundance (>40%). Structure of phytoplankton communities in Lake Toba dominated by Anabaena.

Phytoplankton community structure in local water types at a coastal site in north-western Bay of Bengal.

A comprehensive analysis on seasonal distribution of phytoplankton community structure and their interaction with environmental variables was carried out in two local water types (type 1<30m isobath and Type 2>30m isobath) at a coastal site in north-western Bay of Bengal. Phytoplankton community was represented by 211 taxa (146 marine, 37 fresh, 2 brackish, 20 marine-fresh, and 6 marine-brackish-fresh) belonging to seven major groups including 45 potential bloom forming and 22 potential toxin producing species. The seasonal variability depicted enrichment of phytoplankton during pre-monsoon in both water types. Total phytoplankton abundance pattern observed with inter-annual shift during monsoon and post-monsoon period at both water types. In both water types, diatom predominance was observed in terms of species richness and abundance comprising of centric (82 sp.) and pennate (58 sp.) forms. Pennate diatoms, Thalassiothrix longissima and Skeletonema costatum preponderated in both the water types. The diatom abundance was higher in type 1 in comparison to type 2. In general, SiO4 found to fuel growth of the dominant phytoplankton group, diatom in both the water types despite comparative lower concentration of other macronutrients in type 2.

Dynamics of phytoplankton communities in eutrophying tropical shrimp ponds affected by vibriosis

Tropical shrimp aquaculture systems in New Caledonia regularly face major crises resulting from outbreaks of Vibrio infections. Ponds are highly dynamic and challenging environments and display a wide range of trophic conditions. In farms affected by vibriosis, phytoplankton biomass and composition are highly variable. These conditions may promote the development of harmful algae increasing shrimp susceptibility to bacterial infections. Phytoplankton compartment before and during mortality outbreaks was monitored at a shrimp farm that has been regularly and highly impacted by these diseases. Combining information from flow cytometry, microscopy, pigment and phylogenetic analysis, the presence of Picocyanobacteria, Prasinophyceae and Diatomophyceae were detected as dominant phytoplankton groups and Cryptophyceae, Prymnesiophyceae and Dinophyceae as minor components. At the onset of the first shrimp mortalities, Bacillariophyceae increased while Cyanobacteria, Prymnesiophyceae and Dinophyceae decreased in the water column, followed by proliferation of Prasinophyceae. Several taxa were identified as potential harmful algae (Cyanobacteria, dinoflagellates and Phaeocystis).

Genetic Manipulation of Competition for Nitrate between Heterotrophic Bacteria and Diatoms.

Diatoms are a dominant group of eukaryotic phytoplankton that contribute substantially to global primary production and the cycling of important elements such as carbon and nitrogen. Heterotrophic bacteria, including members of the gammaproteobacteria, are commonly associated with diatom populations and may rely on them for organic carbon while potentially competing with them for other essential nutrients. Considering that bacterioplankton drive oceanic release of CO2 (i.e., bacterial respiration) while diatoms drive ocean carbon sequestration vial the biological pump, the outcome of such competition could influence the direction and magnitude of carbon flux in the upper ocean. Nitrate availability is commonly a determining factor for the growth of diatom populations, particularly in coastal and upwelling regions. Diatoms as well as many bacterial species can utilize nitrate, however the ability of bacteria to compete for nitrate may be hindered by carbon limitation. Here we have developed a genetically tractable model system using the pennate diatom Phaeodactylum tricornutum and the widespread heterotrophic bacteria Alteromonas macleodii to examine carbon-nitrogen dynamics. While subsisting solely on P. tricornutum derived carbon, A. macleodii does not appear to be an effective competitor for nitrate, and may in fact benefit the diatom; particularly in stationary phase. However, allochthonous dissolved organic carbon addition in the form of pyruvate triggers A. macleodii proliferation and nitrate uptake, leading to reduced P. tricornutum growth. Nitrate reductase deficient mutants of A. macleodii (ΔnasA) do not exhibit such explosive growth and associated competitive ability in response to allochthonous carbon when nitrate is the sole nitrogen source, but could survive by utilizing solely P. tricornutum-derived nitrogen. Furthermore, allocthonous carbon addition enables wild-type A. macleodii to rescue nitrate reductase deficient P. tricornutum populations from nitrogen starvation, and RNA-seq transcriptomic evidence supports nitrogen-based interactions between diatoms and bacteria at the molecular level. This study provides key insights into the roles of carbon and nitrogen in phytoplankton-bacteria dynamics and lays the foundation for developing a mechanistic understanding of these interactions using co-culturing and genetic manipulation.

Risk assessment of excessive CO2 emission on diatom heavy metal consumption

Diatoms are the dominant group of phytoplankton in the modern ocean, accounting for approximately 40% of oceanic primary productivity and critical foundation of coastal food web. Rising dissolution of anthropogenic CO2 in seawater may directly/indirectly cause ocean acidification and desalination. However, little is known about dietary diatom-associated changes, especially for diatom heavy metal consumption sensitivity to these processes, which is important for seafood safety and nutrition assessment. Here we show some links between ocean acidification/desalination and heavy metal consumption by Thalassiosira weissflogii. Excitingly, under desalination stress, the relationships between Cu, Zn, and Cd were all positively correlated, especially between Cu and Zn (r=0.989, total intracellular concentration) and between Zn and Cd (r=0.962, single-cell intracellular concentration). Heavy metal consumption activity in decreasing order was acidification<acidification+desalination<desalination for Zn, acidification<desalination<acidification+desalination for Cu and Cd, i.e., heavy metal uptake (or release) were controlled by environmental stress. Our findings showed that heavy metal uptake (or release) was already responded to ongoing excessive CO2 emission-driven acidification and desalination, which was important for risk assessment of climate change on diatom heavy metal consumption, food web and then seafood safety in future oceans.

Contrasting Phytoplankton Structure and Morphologically Based Functional Groups of Reservoirs That Differ in the Adjacent Surrounding

Increasing human activity has resulted in the transformation of waters and their landscape leading to fragmentation and degradation of natural ecosystems. We hypothesized that adjacent surrounding structures of water reservoir is weighty factor explaining changes in phytoplankton. The aim of the study was to find what level of identification is adequate to routine analyses and assess the best criteria that reflect structure of adjacent surrounding of reservoirs. Lithuanian and Polish reservoirs located in similar climatic conditions and of similar morphology were studied. The adjacent land use was described based on Corine Land Cover 2006. The physico‐chemical parameters of waters, structure and biomass of phytoplankton, Shannon index and obligatory autotrophic to mixotrophic species ratio were analyzed. The main difference between reservoirs was natural (Lithuanian reservoir: 67.8%) and agricultural landscape structure (Polish reservoir: 78.4%). Both ecosystems showed significant statistical differences in a few chemical features and in phytoplankton taxonomic and morphologically based functional groups (MBFG), Shannon index and autotrophic to mixotrophic species ratio. The dominant phytoplankton groups and MBFG better reflected stresses resulted differences of adjacent surrounding structure, compared with single species. In conclusion, the best criteria indicated the differences in the structure of adjacent surrounding of water ecosystems are MBFG and autotrophic to mixotrophic species ratio were presented.