Natural Phytoplankton Assemblages Research Articles

Preliminary Observations of the Effect of Temperature and Food Concentration on the Egg Production Rate and Hatching Success of Acartia amboinensis from the Central Red Sea.

The effects of temperature and food concentration on the egg production rate (EPR) of the tropical calanoid copepod Acartia amboinensis were studied from the coastal waters of the central Red Sea in March 2017. In the first experiment, adult females were incubated in glass bottles that were pre-filled with screened seawater containing a natural assemblage of phytoplankton. In the second experiment, the species were incubated in glass bottles that were enriched with different concentrations of Chaetoceros muelleri along with natural assemblages of phytoplankton. Both the experimental setups were then exposed to different temperatures (21, 24, 27, 30 and 33°C). The daily EPR varied significantly across different temperatures and the various food concentrations (p < 0.05). Within the natural food assemblage (Exp. 1), the EPR increased gradually to a peak mean of 13.7 eggs female-1 d-1 at 27°C, then declined as temperatures increased (at 30 and 33°C). In the second experiment when the water was enriched with algal culture, EPRs were significantly greater (maximum EPR: 63.9 eggs female-1 d-1 at 27°C) than those incubated in ambient water with natural food (maximum EPR: 17.4 eggs female-1 d-1 at 21°C). The hatching rate fluctuated between 42.4% and 88.6%. The present study revealed that the EPR of Acartia amboinensis responded rapidly to changes in food availability, suggesting an extreme food limitation in the central Red Sea.

Summer phytoplankton assemblages and carbon biomass in the northern south China sea

Phytoplankton assemblage composition, abundance, carbon biomass and distribution were studied in summer 2004 in the northern South China Sea (NSCS; 18°-22°N, 110°-117°E). This is the first report of natural phytoplankton assemblages which were analyzed by Utermöhl method and their biovolume conversion carbon biomass in the NSCS. A taxa of phytoplankton was identified, which belong to 4 classes and 76 genera, and mainly composed of Bacillariophyceae. Most of them belong to nanophytoplankton (Equivalent Sphere Diameter is 10–15 μm). Associated phytoplankton, with a share of 3% UPA (Utermöhl phytoplankton assemblage cell abundance), primarily consisted of Dinophyceae. Chrysophyceae and Cyanophyceae occurred in appreciable amounts only at upper layers waters in the NSCS. Combined the UPC (Utermöhl phytoplankton carbon) with PPC (picophytoplankton carbon) into the BPC (bulk phytoplankton carbon biomass), it ranged from 0.117 to 1467.172 μg C L−1, its average was 54.848 ± 197.578 μg C L−1. The ratios of PPC contribution to BPC ranged from 0.001% to 6.567%, its average was 0.667 ± 1.215%. This indicates that picophytoplankton contribution to phytoplankton carbon pool is much low than that of chlorophyll a partitioning. Two carbon biomass provinces, one was higher biomass province along continental surface and bottom layers, and the other one was lower biomass province in open sea waters, were clearly discerned along the isoline of 20 μg C L−1. The calculated budget amount BPC pool in the NSCS was 1.21 × 104 tC. The calculated Carbon: Chlorophyll a ratio (C: Chl a) in study area ranged from 0.21 to 217.648, its average was 37.31 ± 46.53. Several typical phytoplankton assemblages can be discerned based on Shannon-Wiener diversity index, Pielou's evenness and C: Chl a. Examination of phytoplankton abundances, biomass and environmental factors by multivariate techniques PCA (Principal Component Analysis) suggests BPC distribution in the NSCS was mainly controlled by salinity at Section along PRE (Pearl River estuary) to Dongsha Islands, and nitrate concentration at Section along the Yangjiang to Zhongsha Islands. The keystone phytoplankton species in the NSCS were mainly composed of large or chain form Bacillariophyceae, such as Pseudo-nitzschia delicatissima, Chaetoceros spp. and Asterionellopsis glacialis, and some Dinophyceae also take an important role in the NSCS, such as Gyrodinium spirale and Prorocentrum dentatum. In addition, the features of phytoplankton distribution were according to functional morphology.

Lake Phytoplankton Assemblage Altered by Irregularly Shaped PLA Body Wash Microplastics but Not by PS Calibration Beads

Microplastics are an emerging environmental pollutant, whose global ubiquity is becoming increasingly evident. Conventional wastewater treatment does not completely remove them, and there are growing concerns about microplastics in source water and post-treatment drinking water. Microplastics have been reported to alter the development, physiology, and behavior of various aquatic organisms; however, limited knowledge exists on their effect on natural phytoplankton communities. Many studies also use uniformly spherical plastic beads, while most scrub particles in consumer products and secondary microplastics in the environment have various shapes and sizes. We tested the effects of two types of microplastics, 50 µm polystyrene (PS) calibration beads and polylactic acid (PLA) plastic body wash scrub particles, and one type of plant-derived body wash scrub particle on a natural phytoplankton assemblage through a 7-day incubation experiment in a temperate, mesotrophic lake. The calibration beads and the plant-derived particles generally did not alter the taxonomic composition of the phytoplankton in the mesocosms, while the PLA body wash microplastics eliminated cryptophytes (p &lt; 0.001) and increased chrysophytes (p = 0.041). Our findings demonstrate differential effects of irregularly shaped PLA body wash microplastics vs. PS calibration beads on lake phytoplankters and empirically support potential bottom-up alteration of the aquatic food web by secondary microplastics.

Analyses of diet preference of larval orange-spotted grouper (Epinephelus coioides) grown under inorganic fertilization method using next-generation sequencing

Orange-spotted grouper (Epinephelus coioides) is one of the most important aquaculture species in tropical and subtropical Asia. Traditionally, rotifer are fed to the larvae after they hatched. However, larval survival rate has been very low, and prey of suitable size during the first week after hatching is a major constraint. In this study, we raised grouper larvae by using an inorganic fertilization method to produce natural assemblages of phytoplankton and zooplankton, and compared it to a traditional rotifer-fed group. The results showed higher larval survival rate in inorganic fertilization group, albeit not significantly different due to high variation. Using next-generation sequencing to examine the fish stomach content from inorganic fertilization group, we found that copepods and choanoflagellates were actively consumed, whereas rotifers were not. The results imply that the inorganic fertilization method can produce desirable forage species for the grouper larvae, and it can be an alternative way in grouper larviculture worldwide.

Effects of climate-induced changes in temperature and salinity on phytoplankton physiology and stress responses in coastal Antarctica

Coastal phytoplankton assemblages from Potter Cove in Antarctica were exposed to low salinity (S-) and high temperature (T+) conditions to simulate oceanic changes resulting from global warming. The treatments were: low salinity (30) and high temperature (S-T+); low salinity and ambient temperature (1–2 °C) (S-T0); ambient salinity (34) and increased temperature (4–5 °C) (S0T+) and ambient salinity with ambient temperature (control, S0T0). Experiments were conducted in 100-L microcosms and monitored for 6 days. Compared to the control treatment, micro-size diatoms (25–50 μm) dominated the phytoplankton assemblages while prasinophyceae were less abundant at the end of the S-T+ and S0T+ treatments. Nano-size diatoms (10–20 μm) also increased significantly at the end of the experiment but only when exposed to S0T+. In S- treatments, the production of reactive oxygen/ nitrogen species (ROS/RNS) increased while phytoplankton biomass decreased. Under T+ conditions, the production of ROS/RNS was significantly lower than in T0 treatments. Throughout the experiment, α-Tocopherol (α-T) consumption may have prevented lipid damage, allowing for increases in photosynthetic rate and growth when nutrients concentrations were sufficiently high. Our results indicate that an increase in temperature can compensate for the lipid damage produced by low salinity, and stimulate carbon uptake in both conditions. This study demonstrated that the final composition of phytoplankton assemblages in all experimental treatments was strongly influenced by the original composition. Future changes in natural phytoplankton assemblages in Antarctic coastal waters will therefore depend on the planktonic species present at the time of the perturbation, which can strongly impact energy flow along food webs and the magnitude of carbon and nutrient fluxes in Antarctic waters.

Effects of Temperature and Nutrient Supply on Resource Allocation, Photosynthetic Strategy, and Metabolic Rates of Synechococcus sp.

Temperature and nutrient supply are key factors that control phytoplankton ecophysiology, but theirrole is commonly investigated in isolation. Their combined effect on resource allocation, photosynthetic strategy, and metabolism remains poorly understood. To characterize the photosynthetic strategy and resource allocation under different conditions, we analyzed the responses of a marine cyanobacterium (Synechococcus PCC 7002) to multiple combinations of temperature and nutrient supply. We measured the abundance of proteins involved in the dark (RuBisCO, rbcL) and light (Photosystem II, psbA) photosynthetic reactions, the content of chlorophyll a, carbon and nitrogen, and the rates of photosynthesis, respiration, and growth. We found that rbcL and psbA abundance increased with nutrient supply, whereas a temperature-induced increase in psbA occurred only in nutrient-replete treatments. Low temperature and abundant nutrients caused increased RuBisCO abundance, a pattern we observed also in natural phytoplankton assemblages across a wide latitudinal range. Photosynthesis and respiration increased with temperature only under nutrient-sufficient conditions. These results suggest that nutrient supply exerts a stronger effect than temperature upon both photosynthetic protein abundance and metabolic rates in Synechococcus sp. and that the temperature effect on photosynthetic physiology and metabolism is nutrient dependent. The preferential resource allocation into the light instead of the dark reactions of photosynthesis as temperature rises is likely related to the different temperature dependence of dark-reaction enzymatic rates versus photochemistry. These findings contribute to our understanding of the strategies for photosynthetic energy allocation in phytoplankton inhabiting contrasting environments.

Effects of Salinity and Suspended Solids on Tropical Phytoplankton Mesocosm Communities

Phytoplankton play a fundamental role in marine food webs but are affected by both natural and anthropogenic fluctuations in environmental conditions. Here, to simulate a dynamic coastal environment, we used mesocosms to examine how different salinity levels and suspended solids concentrations (SSCs) impact a natural phytoplankton assemblage collected from a tropical estuary in Singapore. Significant differences in the phytoplankton composition between the baseline and treatments with medium and high SSC were found, but not among the three salinities tested. Differences can be attributed to nutrient limitation (particularly silicate) and the use of kaolinite for the suspended sediment. Silicate limitation is likely to have caused the observed switch in dominant genus from Skeletonema sp. to Chaetoceros sp. and the occurrence of weakly silicified genera such as Cylindrotheca. Kaolinite affected phytoplankton abundance through effects such as shading, flocculation, and nutrient adsorption. These results demonstrated how the combination of various physicochemical effects of suspended solids can influence tropical phytoplankton communities. Furthermore, as suspended solids such as kaolin can be found in the natural environment, this study showed that their potential effects should be evaluated beyond just their concentration.

Density-dependent oxylipin production in natural diatom communities: possible implications for plankton dynamics

Oxylipins are important signal transduction lipoxygenase-derived products of fatty acids that regulate a variety of physiological and pathological processes in plants and animals. In marine diatoms, these molecules can be highly bioactive, impacting zooplankton grazers, bacteria and other phytoplankton. However, the ultimate cause for oxylipin production in diatoms is still poorly understood, from an evolutionary perspective. Here we analysed production of particulate linear oxygenated fatty acids (LOFAs, previously named non-volatile oxylipins) from natural phytoplankton collected weekly for 1 year. We demonstrate for the first time that diatoms are the main LOFA producers in natural phytoplankton assemblages. Interestingly, LOFA-per-cell production decreased with increasing diatom density and was not due to major changes in diatom community composition. An inverse relation was confirmed at a global scale by analysing diatom lipoxygenase unigenes and metagenomes from Tara Oceans datasets. A network analysis suggested that different LOFAs could contribute to modulate co-variations of different diatom taxa. Overall, we offer new insights in diatom chemical ecology, possibly explaining the evolution of oxylipin synthesis in diatoms.

A single pulse of diffuse contaminants alters the size distribution of natural phytoplankton communities

The presence of a multitude of bioactive organic pollutants collectively classified as pharmaceuticals and personal care products (PPCPs) in freshwaters is of concern, considering that ecological assessments of their potential impacts on natural systems are still scarce. In this field experiment we tested whether a single pulse exposure to a mixture of 12 pharmaceuticals and personal care products, which are commonly found in European inland waters, can influence the size distributions of natural lake phytoplankton communities. Size is one of the most influential determinants of community structure and functioning, particularly in planktonic communities and food webs. Using an in-situ microcosm approach, phytoplankton communities in two lakes with different nutrient levels (mesotrophic and eutrophic) were exposed to a concentration gradient of the PPCPs mixture at five levels. We tested whether sub-lethal PPCPs doses affect the scaling of organisms' abundances with their size, and the slope of these size spectra, which describe changes in the abundances of small relative to large phytoplankton. Our results showed that a large proportion (approximately 80%) of the dataset followed a power-law distribution, thus suggesting evidence of scale invariance of abundances, as expected in steady state ecosystems. PPCPs were however found to induce significant changes in the size spectra and community structure of natural phytoplankton assemblages. The two highest treatment levels of PPCPs were associated with decreased abundance of the most dominant size class (nano-phytoplankton: 2–5 μm), leading to a flattening of the size spectra slope. These results suggest that a pulse exposure to PPCPs induce changes that potentially lead to unsteady ecosystem states and cascading effects in the aquatic food webs, by favoring larger non-edible algae at the expense of small edible species. We propose higher susceptibility due to higher surface to volume ratio in small species as the likely cause of these structural changes.

Impact of ocean acidification and high solar radiation on productivity and species composition of a late summer phytoplankton community of the coastal Western Antarctic Peninsula

AbstractThe Western Antarctic Peninsula (WAP), one of the most productive regions of the Southern Ocean, is currently undergoing rapid environmental changes such as ocean acidification (OA) and increased daily irradiances from enhanced surface‐water stratification. To assess the potential for future biological CO2 sequestration of this region, we incubated a natural phytoplankton assemblage from Ryder Bay, WAP, under a range of pCO2 levels (180 μatm, 450 μatm, and 1000 μatm) combined with either moderate or high natural solar radiation (MSR: 124 μmol photons m−2 s−1 and HSR: 435 μmol photons m−2 s−1, respectively). The initial and final phytoplankton communities were numerically dominated by the prymnesiophyte Phaeocystis antarctica, with the single cells initially being predominant and solitary and colonial cells reaching similar high abundances by the end. Only when communities were grown under ambient pCO2 in conjunction with HSR did the small diatom Fragilariopsis pseudonana outcompete P. antarctica at the end of the experiment. Such positive light‐dependent growth response of the diatom was, however, dampened by OA. These changes in community composition were caused by an enhanced photosensitivity of diatoms, especially F. pseudonana, under OA and HSR, reducing thereby their competitiveness toward P. antarctica. Moreover, community primary production (PP) of all treatments yielded similar high rates at the start and the end of the experiment, but with the main contributors shifting from initially large to small cells toward the end. Even though community PP of Ryder Bay phytoplankton was insensitive to the changes in light and CO2 availability, the observed size‐dependent shift in productivity could, however, weaken the biological CO2 sequestration potential of this region in the future.

Effects of typhoon Roke and Haitang on phytoplankton community structure in northeastern South China sea

ABSTRACT The influence of typhoon and Kuroshio intrusion process on the natural phytoplankton assemblages in the northeastern South China Sea (neSCS) was examined during a cruise in summer 2017. One hundred ninety-six seawater samples were collected and analyzed in the neSCS. In total, 299 species were identified belonging to 82 genera, mostly dominated by Bacillariophyta (159), followed by Dinophyta (132), Cyanophyta (4) and Chrysophyta (4). Among all, Thalassionema nitzschioides, Thalassionema frauenfeldii, Thalassiosira minima, in Bacillariophyta, Prorocentrum lenticulatum, Prorocentrum compressum in Dinophyta, and Trichodesmium thiebautii, in the Cyanophyta were the most dominated species, respectively. Correlation analysis shows compared with other groups Bacillariophyta was mostly influenced by silicate, whereas, Dinophyta and Cyanophyta showed significant correlation with temperature, salinity, phosphate, and nitrate. After the typhoon, the higher abundance of Dinophyta was observed, whereas it was lower in case of Bacillariophyta and Cyanophyta, shows typhoon “Roke“ and ”Haitang” had a significant influence on the phytoplankton assemblages, which could be manifested by the variations of total phytoplankton abundance in the surface water of neSCS. Overall, our present study provides the latest in-depth information about how the environmental factors influencing the phytoplankton density and diversity in the neSCS during summer.

Differential stimulation and suppression of phytoplankton growth by ammonium enrichment in eutrophic hardwater lakes over 16 years

AbstractPrevious research suggests that fertilization of surface waters with chemically reduced nitrogen (N), including ammonium (NH4+), may either enhance or suppress phytoplankton growth. To identify the factors influencing the net effect of NH4+, we fertilized natural phytoplankton assemblages from two eutrophic hardwater lakes with growth‐saturating concentrations of NH4Cl in 241 incubation experiments conducted biweekly May–August during 1996–2011. Phytoplankton biomass (as chlorophyll a) was significantly (p &lt; 0.05) altered in fertilized trials relative to controls after 72 h in 44.8% of experiments, with a marked rise in both spring suppression and summer stimulation of assemblages over 16 yr, as revealed by generalized additive models (GAMs). Binomial GAMs were used to compare contemporaneous changes in physico‐chemical (temperature, Secchi depth, pH, nutrients; 19.5% deviance explained) and biological parameters (phytoplankton community composition; 40.0% deviance explained) to results from fertilization experiments. Models revealed that that the likelihood of growth suppression by NH4+ increased with abundance of diatoms, cryptophytes, and unicellular cyanobacteria, particularly when water temperatures and soluble reactive phosphorus (SRP) concentrations were low. In contrast, phytoplankton was often stimulated by NH4+ when chlorophytes and non‐N2‐fixing cyanobacteria were abundant, and temperatures and SRP concentrations were high. Progressive intensification of NH4+ effects over 16 yr reflects changes in both spring (cooler water, increased diatoms and cryptophytes) and summer lake conditions (more chlorophytes, earlier cyanobacteria blooms), suggesting that the seasonal effects of NH4+ will vary with future climate change and modes of N enrichment.

Formation of planktonic chromophoric dissolved organic matter in the ocean

Chromophoric dissolved organic matter (CDOM) is an important fraction of the marine carbon cycle that controls most light absorption and many photochemical and biological processes in the ocean. Despite its importance, the chemical basis for the formation of oceanic CDOM remains unclear. Currently, CDOM's optical properties are best-described by an electronic interaction (EI) model of charge transfer (CT) complexes which form between electron-rich donors and electron-poor acceptors. While terrigenous compounds such as lignin best fit this model, planktonic sources of CDOM have not yet been tested. Here, we have tested CDOM formed during an incubation experiment using a natural phytoplankton assemblage and throughout active growth, stationary phase and algal biomass decomposition. Absorbance of the derived planktonic CDOM generally decreased with increasing wavelength, similar to the reference Pony Lake (PLFA) and Suwanee River (SRFA) fulvic acid solutions used as models of terrigenous CDOM. Further, after 60 d of microbial degradation in the dark, CDOM exhibited fluorescence emission maxima continuously red-shifted into the visible band, consistent with PLFA and SRFA. Reduction of carbonyl-containing groups, key to CT complex formation, with sodium borohydride (NaBH4) produced coherent results in planktonic CDOM and reference FAs. Absorption at 350 nm decreased by 50% for planktonic CDOM and by 30% for PLFA and SRFA, with corresponding increases in spectral slope (S) values, indicating preferential loss of absorption well into the visible. Fluorescence likewise responded with enhanced emission at shorter wavelengths. Apparent quantum yields (Φ) were similarly affected. Results from our work support prior observations that phytoplankton and bacteria are important sources of CDOM that color the ocean's “twilight zone”. We hypothesize that microbial processing of a variety of source substrates into more complex compounds represented as planktonic CDOM likely represents a semi-refractory pool of DOM in the ocean.

Impacts of elevated pCO2 on estuarine phytoplankton biomass and community structure in two biogeochemically distinct systems in Louisiana, USA

Ocean acidification has the potential to impact the ocean's biogeochemical cycles and food web dynamics, with phytoplankton in the distinctive position to profoundly influence both, as individual phytoplankton species play unique roles in energy flow and element cycling. Previous studies have focused on short-term exposure of monocultures to low pH, but do not reflect the competitive dynamics within natural phytoplankton communities. This study explores the use of experimental microcosms to expose phytoplankton assemblages to elevated pCO2 for an extended period of time. Phytoplankton communities were collected from two biogeochemically distinct Louisiana estuaries, Caillou Lake (CL) and Barataria Bay (BB), and cultured in lab for 16 weeks while bubbling CO2 enriched air corresponding to current (400 ppm) and future (1000 ppm) pCO2 levels. Results suggest that elevated pCO2 does not implicitly catalyze an increase in phytoplankton biomass (chlorophyll a). While pigment data showcased a parabolic trend and microscopic observations revealed a loss in species diversity within each major taxonomic class. By the end of the 16-week incubation, 10 out of the 12 cultures had a community structure analogous to that of the startup phytoplankton assemblage collected from the field. Natural phytoplankton assemblages exposed to elevated pCO2 experienced multiple transitional states over the course of a 16-week incubation, indicating that there is no deterministic successional pathway dictated by coastal acidification but community adaptation was observed.

Comparing static and dynamic incubations in primary production measurements under different euphotic and mixing depths

Since phytoplankton production is usually estimated from static incubations (fixed depths or light levels), a mesocosm study was performed to evaluate the significance of mixing depth, mixing intensity and load of humus of natural phytoplankton assemblages. Vertically rotated (dynamic) incubations usually gave higher results than static incubations in humus-rich water. Mixing intensity was of significant importance in one of 2 years tested, but strong interaction effects with humus complicated the explanation. Differences in primary production between dynamic incubations did not fully reflect the received PAR dose, and increased humus and increased mixing depth increased the photo-assimilation efficiency. Different single-depth incubations did not provide a shortcut method to measure water-column primary production with high accuracy. Results diverged from theoretical estimates based on recent combined photo-biological and physical environmental models. The large variability in responses to mixing is supposed to reflect species-specific adaptations and pre-history regarding quantity (photons) and quality (spectral distribution) of the optical environment in an assemblage of different species. The proportional abundance of each species with its specific characters will therefore strongly influence bulk primary production. Due to such variable responses, clear guidelines for a “best practice” in primary production measurements cannot be given, based on the present results.

Macroecological patterns of the phytoplankton production of polyunsaturated aldehydes

The polyunsaturated aldehydes (PUAs) are bioactive metabolites commonly released by phytoplankton species. Based primarily on laboratory experiments, PUAs have been implicated in deleterious effects on herbivores and competing phytoplankton species or in the regulation of the rates of bacterial organic matter remineralization; however, the role of the PUAs at an ecosystem level is still under discussion. Using data of PUA production in natural phytoplankton assemblages over a wide range of conditions, we analyzed macroecological patterns aiming for a comprehensive environmental contextualization that will further our understanding of the control and ecologic role played by these compounds. PUA composition changed from the predominance of decadienal in oligotrophy, octadienal in eutrophy, and heptadienal at intermediate conditions. The production of PUAs per unit biomass also showed a strong relationship with the trophic status, sharply increasing towards oligotrophic conditions and with small-sized cells reaching the highest production rates. High ratios of dissolved inorganic nitrogen to dissolved inorganic phosphorus also promoted PUA production, albeit to a considerably lesser extent. Although the allelopathic use of PUAs to outcompete other phytoplankton or reduce herbivory may be key in some environments and interactions, the macroecological patterns found here, showing higher production towards the poorest waters and among the small species typically populating these environments, support and link at the large scale the hypotheses of the nutrient-derived stress as driver for the production of PUAs together with the use of these compounds as boosters for the nutrient remineralization.

Environmental and nutrient controls of marine nitrogen fixation

Biological dinitrogen (N2) fixation by diazotrophic cyanobacteria has great biogeochemical implications in nitrogen (N) cycling in the ocean as this process represents the major source of new N input to the oceans, thereby controlling the marine primary productivity. Numerous factors can affect the extent of N2 fixation. A better understanding of the major environmental and nutrient factors governing this process is highly required. Iron (Fe) and/or phosphorus (P) are thought to be limiting factors in most oceanic regions. Special attention has been given in the present study to evaluate the effects of mineral dust deposition which is believed to stimulate N2 fixation as it increases the availability of both Fe and P. Through three laboratory bioassays (+Fe, +P, +Dust) via incubation experiments performed on Trichodesmium IMS101, we found that each addition of Fe, P or desert dust could stimulate the growth and N2 fixation of Trichodesmium IMS101. Several adaptive nutrient utilization strategies were observed, such as a Fe luxury uptake mechanism, a P-sparing effect and colony formation. In addition, during a field study using natural phytoplankton assemblages from the temperate Northeast Atlantic Ocean the critical role of dissolved Fe (DFe) was again highlighted by the remarkably enhanced N2 fixation rate observed after the addition of DFe under low temperature and P-depleted conditions. At the time of our study no Trichodesmium filaments were found in the studied region, the diazotrophic community was dominated by unicellular cyanobacteria symbiont (prymnesiophyte-UCYN-A1) and heterotrophic diazotrophs, therefore demonstrating that DFe could as well be the ultimate factor limiting N2 fixation of these smaller diazotrophs. Recently, the effects of ongoing climate change (ocean warming and acidification) on N2 fixation drew much attention, but various studies led to controversial conclusions. Semi-continuous dilution growth experiments were conducted on Trichodesmium IMS101 under present-day and future high pCO2 (400 and 800 µatm, respectively) and warming seawater (24 and 28 °C) conditions. The results indicate that higher pCO2 and therefore ocean acidification may be beneficial for Trichodesmium growth and N2 fixation. However, our observations suggest that Fe or P limitation in oligotrophic seawaters may offset the stimulation induced on Trichodesmium IMS101 resulting from ocean acidification. In contrast, ocean warming may not play an important role in Trichodesmium growth and N2 fixation with a 4 °C increase from 24 °C to 28 °C. Nevertheless, ocean warming is predicted to cause a shift in the geographical distribution of Trichodesmium species toward higher latitudes, extending its niche to subtropical ocean regions and potentially reducing its coverage in tropical ocean basins.

Shift towards larger diatoms in a natural phytoplankton assemblage under combined high-CO2 and warming conditions.

An indoor mesocosm experiment was carried out to investigate the combined effects of ocean acidification and warming on the species composition and biogeochemical element cycling during a winter/spring bloom with a natural phytoplankton assemblage from the Kiel fjord, Germany. The experimental setup consisted of a “Control” (ambient temperature of ~4.8 °C and ~535 ± 25 μatm pCO2), a “High-CO2” treatment (ambient temperature and initially 1020 ± 45 μatm pCO2) and a “Greenhouse” treatment (~8.5 °C and initially 990 ± 60 μatm pCO2). Nutrient replete conditions prevailed at the beginning of the experiment and light was provided at in situ levels upon reaching pCO2 target levels. A diatom-dominated bloom developed in all treatments with Skeletonema costatum as the dominant species but with an increased abundance and biomass contribution of larger diatom species in the Greenhouse treatment. Conditions in the Greenhouse treatment accelerated bloom development with faster utilization of inorganic nutrients and an earlier peak in phytoplankton biomass compared to the Control and High CO2 but no difference in maximum concentration of particulate organic matter (POM) between treatments. Loss of POM in the Greenhouse treatment, however, was twice as high as in the Control and High CO2 treatment at the end of the experiment, most likely due to an increased proportion of larger diatom species in that treatment. We hypothesize that the combination of warming and acidification can induce shifts in diatom species composition with potential feedbacks on biogeochemical element cycling.

Coastal eutrophication and freshening: Impacts on Pseudo-nitzschia abundance and domoic acid allelopathy

Species of the diatom genus Pseudo-nitzschia can produce domoic acid (DA), a neurotoxin responsible for amnesic shellfish poisoning. This release of DA by toxic Pseudo-nitzschia species in the water column may be a way to help the cells to outcompete other phytoplankton species (i.e. allelopathy). In this study, the influence of salinity and nutrient inputs on the effect of DA on natural phytoplankton community compositions was investigated using bioassays to measure phytoplankton responses to 400 ng DA ml−1. Dissolved DA (dDA) was added to natural phytoplankton assemblages from a low salinity site dominated by river runoff and from a relatively pristine, high salinity site in which nutrients were added simultaneously with dDA. At the low salinity site, the percent inhibition of cryptophytes and diatoms induced by the addition of dDA was negatively correlated with salinity. However, at the high salinity site with nutrients added, no correlation with salinity was observed. This study highlights how environmental factors such as salinity and nutrients can play important roles on toxin allelopathy by alleviating or exacerbating its effect.

Phytoplankton Do Not Produce Carbon‐Rich Organic Matter in High CO2 Oceans

AbstractThe ocean is a substantial sink for atmospheric carbon dioxide (CO2) released as a result of human activities. Over the coming decades the dissolved inorganic C concentration in the surface ocean is predicted to increase, which is expected to have a direct influence on the efficiency of C utilization (consumption and production) by phytoplankton during photosynthesis. Here we evaluated the generality of C‐rich organic matter production by examining the elemental C:N ratio of organic matter produced under conditions of varying pCO2. The data used in this analysis were obtained from a series of pelagic in situ pCO2 perturbation studies that were performed in the diverse ocean regions and involved natural phytoplankton assemblages. The C:N ratio of the resulting particulate and dissolved organic matter did not differ across the range of pCO2 conditions tested. In particular, the ratio for particulate organic C and N was found to be 6.58 ± 0.05, close to the theoretical value of 6.6.