Natural Phytoplankton Communities Research Articles

Multi-Element Composition of Diatom Chaetoceros spp. from Natural Phytoplankton Assemblages of the Russian Arctic Seas.

Simple SummaryDespite the long history of studying the elemental composition of phytoplankton and its individual ecological and systematic groups or specific algae species, the global dataset is far from completed. Our original research aims to study the elemental composition of a certain taxonomic group of marine diatoms, whose representatives make a significant contribution to primary production in the Arctic Ocean. The data on the chemical composition of diatom microalgae are discussed concerning their role in the global biogeochemical circulation of elements in the ocean. In particular, the obtained data make a prominent input to the study of the multi-element composition of marine diatom species, namely Chaetoceros spp., inhabiting the shelf seas of the Arctic Ocean. These data may be used as a basis for the cultivation of marine diatom strains for obtaining commercially promising producers of biogenic silica or valuable biological products that can be used as raw materials in the production of feed and nutrition for agriculture and aquaculture.Data on the elemental composition of the diatom Chaetoceros spp. from natural phytoplankton communities of Arctic marine ecosystems are presented for the first time. Samples were collected during the 69th cruise (22 August–26 September 2017) of the R/V Akademik Mstislav Keldysh in the Kara, Laptev, and East Siberian Seas. The multi-element composition of the diatom microalgae was studied by ICP-AES and ICP-MS methods. The contents of major (Na, Mg, Al, Si, P, S, K and Ca), trace (Li, Be, B, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, As, Se, Rb, Sr, Mo, Ag, Cd, Sn, Sb, Cs, Ba, Hg, Tl, Pb, Bi, Th and U) and rare earth (Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) elements varied greatly, which was probably associated with the peculiarities of the functional state and mineral nutrition of phytoplankton in the autumn period. Biogenic silicon was the dominant component of the chemical composition of Chaetoceros spp., averaging 19.10 ± 0.58% of dry weight (DW). Other significant macronutrients were alkaline (Na and K) and alkaline earth (Ca and Mg) metals as well as biogenic (S and P) and essential (Al and Fe) elements. Their total contents varied from 1.26 to 2.72% DW, averaging 2.07 ± 0.43% DW. The Al:Si ratio for natural assemblages of Chaetoceros spp. of the shelf seas of the Arctic Ocean was 5.8 × 10−3. The total concentrations of trace and rare earth elements on average were 654.42 ± 120.07 and 4.14 ± 1.37 μg g−1 DW, respectively. We summarize the scarce data on the average chemical composition of marine and oceanic phytoplankton and discuss the limitations and approaches of such studies. We conclude on the lack of data and the need for further targeted studies on this issue.

Underwater light climate and wavelength dependence of microalgae photosynthetic parameters in a temperate sea.

Studying how natural phytoplankton adjust their photosynthetic properties to the quantity and quality of underwater light (i.e. light climate) is essential to understand primary production. A wavelength-dependent photoacclimation strategy was assessed using a multi-color pulse-amplitude-modulation chlorophyll fluorometer for phytoplankton samples collected in the spring at 19 locations across the English Channel. The functional absorption cross section of photosystem II, photosynthetic electron transport (PETλ) parameters and non-photochemical quenching were analyzed using an original approach with a sequence of three statistical analyses. Linear mixed-effects models using wavelength as a longitudinal variable were first applied to distinguish the fixed effect of the population from the random effect of individuals. Population and individual trends of wavelength-dependent PETλ parameters were consistent with photosynthesis and photoacclimation theories. The natural phytoplankton communities studied were in a photoprotective state for blue wavelengths (440 and 480 nm), but not for other wavelengths (green (540 nm), amber (590 nm) and light red (625 nm)). Population-detrended PETλ values were then used in multivariate analyses (partial triadic analysis and redundancy analysis) to study ecological implications of PETλ dynamics among water masses. Two wavelength ratios based on the microalgae saturation parameter Ek (in relative and absolute units), related to the hydrodynamic regime and underwater light climate, clearly confirmed the physiological state of microalgae. They also illustrate more accurately that natural phytoplankton communities can implement photoacclimation processes that are influenced by in situ light quality during the daylight cycle in temporarily and weakly stratified water. Ecological implications and consequences of PETλ are discussed in the context of turbulent coastal ecosystems.

Chlorination in power station cooling water systems: Effect on biomass, abundance and physiology of natural phytoplankton communities.

Chlorination is a widely used method to prevent biofouling in power station cooling water systems in coastal and estuarine environments. This study evaluated the impact of chlorination together with temperature increase to simulate primary entrainment of a phytoplankton community. Biomass, diversity, and photosynthetic activity were monitored over 72 hours to establish impacts on the phytoplankton community. Biomass was significantly reduced after treatment. The mean cell size of the population significantly increased immediately after treatment highlighting an impact on the smaller cell size species of the community (picophytoplankton). Changes in accessory pigments composition suggest an effect on groups such as Prasinophyceae, Cyanobacteria and Chlorophycea. Species composition, dominated by diatoms, was also affected with Skeletonema marinoi and Asterionellopsis glacialis amongst the most sensitive species. Photosynthetic activity was affected in the short term but recovered after 48 hours. This study shows that by using a combination of measurements (e.g biomass, diversity, and physiology) the effects of entrainment in power station cooling water systems, that may be of longer-term significance for specific functional groups of phytoplankton communities, can be discerned. These changes would not necessarily be seen using individual techniques alone such as cell number counts or biomass assessment which may indicate apparent community recovery.

Biogeochemical thallium cycling during a mesocosm phytoplankton spring bloom: Biotic versus abiotic drivers

Thallium (Tl) is classified as a non-(bio)-essential and highly toxic element in the marine environment. Despite its active and passive involvement in bio-cycling processes, it is considered a conservative type element in open ocean settings. Previous studies on the Tl-behavior in the coastal waters of the southern North Sea, however, documented non-conservative Tl-behavior in seasonal and tidal patterns. As drivers for the non-conservative depletion, Tl-fixation in redox stratified adjacent sediments as well as its complexation with algae-bloom derived organic matter were suggested. Due to superimposition by resuspended lithogenic particles, it was not possible to distinguish whether the Tl concentration pattern was induced by biotic or abiotic processes. The main motivation of the present study was to investigate the non-conservative Tl-behavior during a phytoplankton bloom in coastal ocean water masses and to identify potential key drivers. We conducted an indoor mesocosm experiment where artificial seawater was inoculated with a natural phytoplankton and bacteria community from the southern North Sea and incubated under natural light and temperature conditions, mimicking a neritic North Sea water column. The incubation of six weeks covered the different stages of two distinct phytoplankton bloom events as well as a subsequent bacteria bloom. Our results reveal a non-conservative Tl-depletion, which seemed to be primarily caused by the coupling to algae bloom derived OM-cycling. The extent of Tl-depletion was dependent on the amount and the composition of organic matter. While the first phytoplankton bloom, dominated by Diatom-species, did not induce significant deviations of Tl from theoretical conservative behavior, especially the colonial stage (hydrogel formation) of the secondary occurring Phaeocystis sp. bloom induced significant depletions of dissolved Tl with rates up to ~27% d-1. Global extrapolations of potential algae-induced deficits in dissolved Tl and its potential export Tl from the open water column have shown that the processes identified for Tl removal in this study could be responsible for a flux in the range of 4–20% of the total removal previously assumed in Tl mass balances. Our study emphasizes that although Tl is classified as a conservative-type element, biological processes have an impact on the global Tl budget and thus must be considered in the respective oceanographic models.

Effects of temperature and food availability on the filtration and excretion rates of Diplodon parallelopipedon (Hyriidae)

AbstractBivalves can consume detritus, bacteria, phytoplankton, and zooplankton by filtering the water column. Ecological attributes like filtration rate (FR) and excretion rate (ER) are particularly important to better understand the role of bivalves in ecosystem. Here, we aimed to elucidate the FR/ER of Diplodon parallelopipedon in a five‐times replicated laboratory experiment under five levels of temperature (10°C, 15°C, 20°C, 25°C, and 30°C) and three levels of phytoplankton biomass (low, middle, and high). Temperature was the main factor regulating FR and ER in our experimental conditions, as we hypothesized the experimental results showed a nonlinear relationship between FR and water temperature. D. parallelopipedon exerted top‐down control over phytoplankton biomass throughout the temperature gradient tested. Contrarily to our expectation, FR and ER did not vary within the phytoplankton biomass range offered. The experimental evidence suggests D. parallelopipedon might control phytoplankton biomass in different temperature scenarios. Moreover, the excretion of nutrients by D. parallelopipedon, together with a capacity to avoid grazing shown by several phytoplankton species (i.e., buoyance regulation) reveal relevant challenges to our understanding of bivalve‐phytoplankton dynamics, and consequently to the whole ecosystem response, particularly in the presence of more diverse natural phytoplankton communities.

Changes in spectral quality of underwater light alter phytoplankton community composition

AbstractLight is a fundamental resource for phytoplankton. To utilize the available light, most phytoplankton species possess pigments in taxon‐specific combinations and quantities, which in turn result in a specific use of certain wavelengths. This optimizes the light use efficiency, allows for a complementary use of light, and may be an additional driver for community structure. While the effects of light intensity on phytoplankton biomass production and community composition have been intensively studied, here we focused on the effects of specific light spectrum quality (thus light color) on a natural phytoplankton community. In a controlled mesocosm experiment we reduced the supplied wavelength range to its blue, green, or red part of the light spectrum and compared the responses of each treatment to a full spectrum control over 28 d. Highest community growth rates were observed under blue, lowest under red light. Light absorption by the communities showed adaptation toward the supplied wavelength range. Community composition was significantly affected by light quality treatments, driven by Bacillariophyta and Chlorophyta, whereas pigment composition was not. Furthermore, lower species richness but higher evenness occurred when communities were exposed to red light compared to the full spectrum. We expected the response of phytoplankton communities to changes in the light spectrum to be driven by a combination of species sorting and pigment acclimation; however, the effect of species sorting turned out to be stronger. Our study showed that, even if species might acclimate, changes in the available light spectrum affect primary production and phytoplankton community composition.

Viral lysis modifies seasonal phytoplankton dynamics and carbon flow in the Southern Ocean

Phytoplankton form the base of marine food webs and are a primary means for carbon export in the Southern Ocean, a key area for global pCO2 drawdown. Viral lysis and grazing have very different effects on microbial community dynamics and carbon export, yet, very little is known about the relative magnitude and ecological impact of viral lysis on natural phytoplankton communities, especially in Antarctic waters. Here, we report on the temporal dynamics and relative importance of viral lysis rates, in comparison to grazing, for Antarctic nano- and pico-sized phytoplankton of varied taxonomy and size over a full productive season. Our results show that viral lysis was a major loss factor throughout the season, responsible for roughly half (58%) of seasonal phytoplankton carbon losses. Viral lysis appeared critically important for explaining temporal dynamics and for obtaining a complete seasonal mass balance of Antarctic phytoplankton. Group-specific responses indicated a negative correlation between grazing and viral losses in Phaeocystis and picoeukaryotes, while for other phytoplankton groups losses were more evenly spread throughout the season. Cryptophyte mortality was dominated by viral lysis, whereas small diatoms were mostly grazed. Larger diatoms dominated algal carbon flow and a single ‘lysis event’ directed >100% of daily carbon production away from higher trophic levels. This study highlights the need to consider viral lysis of key Antarctic phytoplankton for a better understanding of microbial community interactions and more accurate predictions of organic matter flux in this climate-sensitive region.

Impact of light quality on freshwater phytoplankton community in outdoor mesocosms.

In shallow lakes, wind wave turbulence alters underwater spectral composition, but the influence of this phenomenon on phytoplankton community structure is poorly understood. We used 100L mesocosms to investigate the influence of light quality on a natural phytoplankton community collected from Taihu Lake in China. The communities in mesocosms were exposed to sunlight filtered for white, blue, green, and red light, while wave-making pumps simulated wind wave turbulence similar to Taihu Lake. Over the course of experiment, each filtered light reduced the total phytoplankton abundance compared to white light. The mean abundance of phytoplankton in controls was 1.72, 1.78, and 7.89 times of that in the red, blue, and green light treatments. Red, blue, and green light significantly promoted the growth of cyanobacteria, green algae, and diatoms, respectively, and induced successional change of the phytoplankton species under the tested conditions. The proportion of Microcystis to total phytoplankton abundance in controls and red light shifted from 87.09% at the beginning to 37.95% and 56.30% at the end of the experiment, respectively, and maintained its dominance, whereas Microcystis lost its dominance and was replaced by Scenedesmus (53.78%) and Synedra (53.18%) in the blue and green light, respectively. Given the process of how these phytoplankton compete in designated spectrum, exploring these influences could help provide new insights into the dominance formation of toxic cyanobacteria.

Intraguild predation dampens trophic cascades in shallow aquatic mesocosms in the subtropics: Implications for lake restoration by biomanipulation

Abstract Intraguild predation (IGP), defined as killing and eating among potential competitors, is commonly observed in shallow lakes and is predicted to dampen trophic cascades and affect ecosystem properties (e.g. phytoplankton biomass or primary production). We tested this hypothesis by manipulating the density of two common lake predators, the small‐sized fish Pelteobagrus fulvidraco (the intraguild predator) and the shrimp Exopalaemon modestus (the intraguild prey), in outdoor mesocosms containing natural phytoplankton and zooplankton communities. In single predator treatments, both predators induced a strong trophic cascade, as evidenced by extinction of the key herbivore, Daphnia pulex, and increasing phytoplankton biomass and chlorophyll a concentration. When the two predators were added together, however, the strength of collective predator effects on zooplankton and the cascading effects on phytoplankton growth were weaker than the sum of the individual predator effects. We attributed this antagonism among predators to intraguild predation as demonstrated by the lower shrimp catch per unit effort in the presence of fish. Our study suggests that common predators in subtropical shallow lakes may act antagonistically and thereby dampen trophic cascades in food webs. Thus, when implementing biomanipulation efforts to suppress algal growth via enhancing zooplankton herbivory, both the quantity and diversity of lake predator assemblies should be considered.

Comparison of Rapid Methods Used to Determine the Concentration, Size Structure and Species Composition of Algae

Traditionally, the abundance, cell size distribution and species identification of algae are determined by microscopic counts. In recent years, various rapid methods have been developed for routine algal studies. However, each of these methods has its drawbacks. It is important for aquatic ecologists to understand the advantages, disadvantages, and limitations of these methods. We compared the sensitivity of three rapid methods (multichannel fluorimeter FluoroProbe, imaging flow cytometer FlowCam, and CASY particle counter) to changes in cell abundance of three algae species (Chlorella vulgaris Beyerinck, Arthrospira platensis Gomont, and Nostoc sp.). We also assessed the ability of rapid methods to estimate the cell abundance of different species in the mixed samples. All instruments showed high sensitivity to changes in the cell abundance of different algae species and a mixture of these species. Any one of these methods, once calibrated, can be reliably used to estimate the abundance of a single-species/laboratory culture of microalgae. At the same time, FlowCam, without preliminary calibration, recorded the cell abundance closest to microscopic counts. When analysing a mixture of three microalgae differing in their cell sizes and spectral characteristics, FluoroProbe showed the highest accuracy in assessing the proportions of species in the mixture and FlowCam – in assessing their abundance. To study mixtures of algae and/or natural phytoplankton communities, it is advisable to use jointly a flow cytometer and a multichannel fluorimeter. The images of algae saved by the flow cytometer, if necessary, can be used to identify them, with a certain accuracy, to the species. Information on cells size and spectral characteristics obtained by two methods will be detailed enough to perform such common tasks as studying trophic interactions between phyto- and zooplankton or creating warning systems to inform of unwanted blooms of phytoplankton and their individual groups (for example, cyanobacteria)

Arctic sea ice algae differ markedly from phytoplankton in their ecophysiological characteristics

Photophysiological and biochemical characteristics were investigated in natural communities of Arctic sea ice algae and phytoplankton to understand their respective responses towards variable irradiance and nutrient regimes. This study revealed large differences in photosynthetic efficiency and capacity between the 2 types of algal assemblages. Sea ice algal assemblages clearly displayed increased photoprotective energy dissipation under the highest daily average irradiance levels (>8 µmol photons m-2 s-1). In contrast, phytoplankton assemblages were generally light-limited within the same irradiance ranges. Furthermore, phytoplankton assemblages exhibited more efficient carbon assimilation rates in the low irradiance range compared to sea ice algae, possibly explaining the ability of phytoplankton to generate substantial under-ice blooms. They were also able to readily adjust and increase their carbon production to higher irradiances. The Arctic is warming more rapidly than any other oceanic region on the planet, and as a consequence, irradiance levels experienced by microalgae are expected to increase due to declining ice thickness and snow cover, as well as enhanced stratification. The results of this study suggest that sea ice algae may have less capacity to adapt to the expected environmental changes compared to phytoplankton. We therefore anticipate a change in sea ice-based vs. pelagic primary production with respect to timing and quantity in a future Arctic. The clearly distinct responses of sea ice algae vs. phytoplankton need to be incorporated into model scenarios of current and future Arctic algal blooms and considered when predicting implications for the entire ecosystem and associated biogeochemical fluxes.

Impact of Increased Nutrients and Lowered pH on Photosynthesis and Growth of Three Marine Phytoplankton Communities From the Coastal South West Atlantic (Patagonia, Argentina)

Effect of global change variables on the structure and photosynthesis of phytoplankton communities was evaluated in three different sites of the Patagonian coast of Argentina: enclosed bay (Puerto Madryn, PM), estuarine (Playa Unión, PU), and open waters (Isla Escondida, IE). We exposed samples to two contrasting scenarios: Present (nutrients at in situ levels) vs. Future (with lowered pH and higher nutrients inputs), and determined growth and photosynthetic responses after 2 days of acclimation. Under the Future condition phytoplankton growth was higher in the estuarine site compared to those in PM and IE. This effect was the most pronounced on large diatoms. While the increase of photosynthetic activity was not always observed in the Future scenario, the lower photosynthetic electron requirement for carbon fixation (Φe,C = ETR/PmB) in this scenario compared to the Present, suggests a more effective energy utilization. Long-term experiments were also conducted to assess the responses along a 4 days acclimation period in PU. Diatoms benefited from the Future conditions and had significantly higher growth rates than in the Present. In addition, Φe,C was lower after the acclimation period in the Future scenario, compared to the Present. Our results suggest that the availability, frequency and amount of nutrients play a key role when evaluating the effects of global change on natural phytoplankton communities. The observed changes in diatom growth under the Future scenario in PU and IE and photosynthesis may have implications in the local trophodynamics by bottom up control.

Metabolomics-derived marker metabolites to characterize Phaeocystis pouchetii physiology in natural plankton communities

Phaeocystis pouchetii (Hariot) Lagerheim, 1893 regularly dominates phytoplankton blooms in higher latitudes spanning from the English Channel to the Arctic. Through zooplankton grazing and microbial activity, it is considered to be a key resource for the entire marine food web, but the actual relevance of biomass transfer to higher trophic levels is still under discussion. Cell physiology and algal nutritional state are suggested to be major factors controlling the observed variability in zooplankton grazing. However, no data have so far yielded insights into the metabolic state of Phaeocystis populations that would allow testing this hypothesis. Therefore, endometabolic markers of different growth phases were determined in laboratory batch cultures using comparative metabolomics and quantified in different phytoplankton blooms in the field. Metabolites, produced during exponential, early and late stationary growth of P. pouchetii, were profiled using gas chromatography-mass spectrometry. Then, metabolites were characterized that correlate with the growth phases using multivariate statistical analysis. Free amino acids characterized the exponential growth, whereas the early stationary phase was correlated with sugar alcohols, mono- and disaccharides. In the late stationary phase, free fatty acids, sterols and terpenes increased. These marker metabolites were then traced in Phaeocystis blooms during a cruise in the Barents Sea and North Norwegian fjords. About 50 endometabolites of P. pouchetii were detected in natural phytoplankton communities. Mannitol, scyllo-inositol, 24-methylcholesta-5,22-dien-3β-ol, and several free fatty acids were characteristic for Phaeocystis-dominated blooms but showed variability between them. Distinct metabolic profiles were detected in the nutrient-depleted community in the inner Porsangerfjord (< 0.5 µM NO3−, < 0.1 µM PO43−), with high relative amounts of free mono- and disaccharides indicative for a limited culture. This study thereby shows how the variable physiology of phytoplankton can alter the metabolic landscape of entire plankton communities.

No Clear Response in the Stormwater Phytoplankton Community to Biocide Contamination

This study investigated the responses of natural phytoplankton communities of an urban stormwater pond to biocide contamination. The biocides carbendazim, terbutryn, diuron, and irgarol 1051, and their mixture, were used in two laboratory microcosm and one outdoor mesocosm studies at concentrations of 10, 100, and 1000 ng L−1. The water samples were collected in a pond receiving significant biocide contamination. The mesocosm study was carried out in the same pond. The phytoplankton community response was evaluated after 10–15 days of exposure, with respect to its taxonomic composition, abundance and biovolume. No significant changes were observed in any of the experiments. Only at the highest applied terbutryn concentration were lower numbers of taxa identified in both microcosm and mesocosm experiments. Additionally, increases in Chlorophyta abundance and biovolume, along with an increase in irgarol concentration, were observed throughout the three experiments. Nevertheless, the statistical analysis did not confirm any significant differences among the biocide treatments. These results suggest that the biocide concentrations probably were below the harmful or toxic threshold to the stormwater pond phytoplankton. On the other hand, as the investigated pond phytoplankton taxa face biocide inputs throughout the year, they could have already adapted to the tested biocide contamination.

Nonrandom species loss in phytoplankton communities and its effect on ecosystem functioning

AbstractThe effects of species diversity on ecosystem functioning have been broadly studied, mostly considering random artificial assemblages. However, natural communities are shaped by ecological interactions and environmental conditions often leading to nonrandom species extinctions. Here, we manipulated a natural phytoplankton community by generating a taxonomic diversity gradient based on rare species exclusions and conducted a mesocosm experiment to investigate the diversity effects on ecosystem functioning (resource use efficiency and biomass) under two nutrient levels. We quantified two functional traits (size and photosynthetic pigments) to evaluate the relation of functional diversity and ecosystem functioning. In a second experimental phase we simulated temperature fluctuations to assess the role of diversity on temporal stability of ecosystem functioning. We did not find a significant effect of diversity on ecosystem functioning and the temporal stability of ecosystem functioning, regardless of nutrient level. These results indicated that loss of biomass caused by rare species extinctions was compensated by the species retained in the diversity gradient. Phytoplankton size diversity was positively related to diversity, but this was not transferred into a positive diversity effect on ecosystem functioning. Additionally, the loss of species did not result in a loss of pigment diversity. The lack of a biodiversity–ecosystem functioning (BEF) relationship in our study may be due to the weak coupling of functional and species diversity and a low manifestation of functional diversity under the evaluated conditions. We emphasize that more realistic biodiversity loss scenarios in experiments can yield different results from those in classical BEF research paradigms.

Proteomic and Transcriptomic Patterns during Lipid Remodeling in Nannochloropsis gaditana.

Nutrient limited conditions are common in natural phytoplankton communities and are often used to increase the yield of lipids from industrial microalgae cultivations. Here we studied the effects of bioavailable nitrogen (N) and phosphorus (P) deprivation on the proteome and transcriptome of the oleaginous marine microalga Nannochloropsis gaditana. Turbidostat cultures were used to selectively apply either N or P deprivation, controlling for variables including the light intensity. Global (cell-wide) changes in the proteome were measured using Tandem Mass Tag (TMT) and LC-MS/MS, whilst gene transcript expression of the same samples was quantified by Illumina RNA-sequencing. We detected 3423 proteins, where 1543 and 113 proteins showed significant changes in abundance in N and P treatments, respectively. The analysis includes the global correlation between proteomic and transcriptomic data, the regulation of subcellular proteomes in different compartments, gene/protein functional groups, and metabolic pathways. The results show that triacylglycerol (TAG) accumulation under nitrogen deprivation was associated with substantial downregulation of protein synthesis and photosynthetic activity. Oil accumulation was also accompanied by a diverse set of responses including the upregulation of diacylglycerol acyltransferase (DGAT), lipase, and lipid body associated proteins. Deprivation of phosphorus had comparatively fewer, weaker effects, some of which were linked to the remodeling of respiratory metabolism.

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.

Using chlorophyll fluorescence kinetics to determine photosynthesis in aquatic ecosystems

AbstractVariable fluorescence techniques are increasingly used to assess phytoplankton photosynthesis. All fluorescence techniques and models for photosynthetic electron transport rates (ETRs) are amplitude‐based and are subject to errors, especially when phytoplankton growth is nutrient‐limited. Here we develop a new, kinetic‐based approach to measure, directly and in absolute units, ETRs and to estimate growth rates in phytoplankton. We applied this approach to investigate the effects of nitrogen limitation on phytoplankton photophysiology and growth rates. Nutrient stress leads to a decrease in the quantum yield of photochemistry in Photosystem II (Fv/Fm); however, the relationship between Fv/Fm and growth rates is highly nonlinear, which makes it impossible to quantify the reduction in phytoplankton growth rates from Fv/Fm alone. In contrast, the decline in growth rates under nitrogen stress was proportional to the decrease in kinetic‐based photosynthetic rates. Our analysis suggests the kinetic fluorescence measurements markedly improve the accuracy of ETR measurements, as compared to classical amplitude‐based measurements. Fluorescence‐based methods for primary production rely on measurements of ETRs and then conversion to carbon fixation rates by using the electron yields of carbon fixation. The electron yields exhibit 10‐fold variability in natural phytoplankton communities and are strongly affected by nutrient limitation. Our results reveal that a decrease in the growth rates and the electron yields of carbon fixation is driven by, and can be quantified from, a decrease in photosynthetic turnover rates. We propose an algorithm to deduce the electron yields of carbon fixation, which greatly improve fluorescence‐based measurements of primary production and growth rates.

High CO2 concentration and iron availability determine the metabolic inventory in an Emiliania huxleyi‐dominated phytoplankton community

Ocean acidification (OA), a consequence of anthropogenic carbon dioxide (CO2 ) emissions, strongly impacts marine ecosystems. OA also influences iron (Fe) solubility, affecting biogeochemical and ecological processes. We investigated the interactive effects of CO2 and Fe availability on the metabolome response of a natural phytoplankton community. Using mesocosms we exposed phytoplankton to ambient (390 μatm) or future CO2 levels predicted for the year 2100 (900 μatm), combined with ambient (4.5 nM) or high (12 nM) dissolved iron (dFe). By integrating over the whole phytoplankton community, we assigned functional changes based on altered metabolite concentrations. Our study revealed the complexity of phytoplankton metabolism. Metabolic profiles showed three stages in response to treatments and phytoplankton dynamics. Metabolome changes were related to the plankton group contributing respective metabolites, explaining bloom decline and community succession. CO2 and Fe affected metabolic profiles. Most saccharides, fatty acids, amino acids and many sterols significantly correlated with the high dFe treatment at ambient pCO2 . High CO2 lowered the abundance of many metabolites irrespective of Fe. However, sugar alcohols accumulated, indicating potential stress. We demonstrate that not only altered species composition but also changes in the metabolic landscape affecting the plankton community may change as a consequence of future high-CO2 oceans.

Testing algal-based pCO2 proxies at a modern CO2 seep (Vulcano, Italy)

Understanding long-term trends in atmospheric concentrations of carbon dioxide (pCO2) has become increasingly relevant as modern concentrations surpass recent historic trends. One method for estimating past pCO2, the stable carbon isotopic fractionation associated with photosynthesis (Ɛp) has shown promise over the past several decades, in particular using species-specific biomarker lipids such as alkenones. Recently, the Ɛp of more general biomarker lipids, organic compounds derived from a multitude of species, have been applied to generate longer-spanning, more ubiquitous records than those of alkenones but the sensitivity of this proxy to changes in pCO2 has not been constrained in modern settings. Here, we test Ɛp using a variety of general biomarkers along a transect taken from a naturally occurring marine CO2 seep in Levante Bay of the Aeolian island of Vulcano in Italy. The studied general biomarkers, loliolide, cholesterol, and phytol, all show increasing depletion in 13C over the transect from the control site towards the seep, suggesting that CO2 exerts a strong control on isotopic fractionation in natural phytoplankton communities. The strongest shift in fractionation was seen in phytol, and pCO2 estimates derived from phytol confirm the utility of this biomarker as a proxy for pCO2 reconstruction.