Phytoplankton Mortality Research Articles

Growth and Grazing Mortality of Microbial Plankton in a Shallow Temperate Coastal Lagoon (Ria Formosa, SW Iberia)

Microzooplankton grazing is widely recognized as an important process of heterotrophic prokaryote and phytoplankton biomass removal. However, few studies have specifically addressed microbial mortality in the Ria Formosa coastal lagoon. This study aimed to assess the growth and mortality of heterotrophic prokaryotes and phytoplankton in this ecosystem using the dilution technique. The results revealed significant seasonal variations in the growth and grazing rates of both heterotrophic prokaryotes and phytoplankton, with mean grazing rates slightly exceeding the mean potential instantaneous growth rates. This indicates that microzooplankton consume a substantial proportion of both microbial groups in the lagoon. For specific phytoplankton taxa, the wide range of observed grazing rates suggests grazer selectivity, highlighting the need for future research to examine the dynamics of each phytoplankton group more closely.

Mortality partitioning between viral lysis and microzooplankton grazing in successive phytoplankton blooms using dilution and molecular methods

Phytoplankton play crucial roles in aquatic ecosystems, serving as the foundation of marine food webs and being responsible for ~50% of the world’s oxygen production. Predation by microzooplankton and viral lysis are the major sources of phytoplankton mortality, with the balance between these 2 processes affecting microbial food webs and biogeochemical cycles. However, determining the dominant mortality process in time and space remains an open question. This study investigated microzooplankton grazing and viral lysis rates during a mesocosm experiment in western Norway. High-resolution measurements were determined on phytoplankton groups using flow cytometry to observe changes in mortality rates and carbon flow during phytoplankton blooms. Digital droplet PCR was employed to detect Emiliania huxleyi and Micromonas spp. and associated viruses within environmental samples, to explore its use for determining mortality processes and understanding the impact on the phytoplankton community. The results showed that grazing and viral lysis dominated mortality at different times, with only one significant instance of both processes being observed. Microzooplankton grazing primarily affected picoplankton, while nanoeukaryotes and E. huxleyi were more susceptible to viral lysis. Molecular detection did not always match with abundances or rates determined by flow cytometry; however, it did provide insights into their dynamics throughout the mesocosm. These findings provide insights into the complex interactions between microzooplankton, viruses and phytoplankton communities. Understanding the balance between microzooplankton grazing and viral lysis can contribute to a more comprehensive understanding of carbon flow in aquatic ecosystems, which has significant implications for food webs and biogeochemical cycles.

BIFURCATION ANALYSIS AND CHAOS CONTROL OF THE DISSOLVED OXYGEN-PHYTOPLANKTON DYNAMICAL MODEL

The production of oxygen through phytoplankton photosynthesis is a crucial phenomenon in the dynamics of marine ecosystems. A generic oxygen-phytoplankton interaction model is considered to comprehend its underlying mechanism. This paper investigates the discrete-time dynamics of oxygen and phytoplankton in aquatic ecosystems, incorporating factors that cause phytoplankton mortality due to external influences. We explore the conditions for the local stability of steady states concerning the oxygen content in dissolved water and phytoplankton density. The analysis reveals that the model undergoes a co-dimension one bifurcation, encompassing flip and Neimark–Sacker bifurcations, utilizing the center manifold theorem and bifurcation theory. To manage the chaos resulting from the Neimark–Sacker bifurcation, we apply the OGY feedback control method and a hybrid control methodology. Finally, we present numerical simulations to validate the theoretical discussion.

Role of Syndiniales parasites in depth-specific networks and carbon flux in the oligotrophic ocean.

Microbial associations that result in phytoplankton mortality are important for carbon transport in the ocean. This includes parasitism, which in microbial food webs is dominated by the marine alveolate group, Syndiniales. Parasites are expected to contribute to carbon recycling via host lysis; however, knowledge on host dynamics and correlation to carbon export remain unclear and limit the inclusion of parasitism in biogeochemical models. We analyzed a 4-year 18S rRNA gene metabarcoding dataset (2016-19), performing network analysis for 12 discrete depths (1-1000m) to determine Syndiniales-host associations in the seasonally oligotrophic Sargasso Sea. Analogous water column and sediment trap data were included to define environmental drivers of Syndiniales and their correlation with particulate carbon flux (150m). Syndiniales accounted for 48-74% of network edges, most often associated with Dinophyceae and Arthropoda (mainly copepods) at the surface and Rhizaria (Polycystinea, Acantharea, and RAD-B) in the aphotic zone. Syndiniales were the only eukaryote group to be significantly (and negatively) correlated with particulate carbon flux, indicating their contribution to flux attenuation via remineralization. Examination of Syndiniales amplicons revealed a range of depth patterns, including specific ecological niches and vertical connection among a subset (19%) of the community, the latter implying sinking of parasites (infected hosts or spores) on particles. Our findings elevate the critical role of Syndiniales in marine microbial systems and reveal their potential use as biomarkers for carbon export.

Chaotic behavior and controlling chaos in a fast-slow plankton-fish model

<abstract><p>The interaction of different time scales in predator-prey models has become a common research topic. In the present article, we concentrated on the dynamics of interactions at two time scales in a plankton-fish system. To investigate the effects of the two time scales on plankton-fish dynamics, we constructed a new parameter with a corrected type that differs from the traditional slow parameter. In addition, zooplankton's refuge from the predator and phytoplankton mortality due to competition are incorporated into the model. Positivity and boundedness of solutions were proved. We then discussed feasibility and stability conditions of the equilibrium. We used a variety of means to support the existence of chaos in the system. Hopf bifurcation conditions were also obtained. Chaos control in the plankton-fish model is one of the main motivations for this study. In the slow-variable parameter case, we explored the control mechanism of gestation delay on chaotic systems, which are calmed by different periodic solutions. Moreover, under seasonal mechanisms, external driving forces can stabilize the system from chaos to periodic oscillations. Meanwhile, the sliding mode control (SMC) approach quickly calms chaotic oscillations and stabilizes it to an internal equilibrium state. The necessary numerical simulation experiments support the theoretical results.</p></abstract>

Seasonality and Strain Specificity Drive Rapid Co-evolution in an Ostreococcus-Virus System from the Western Baltic Sea

Marine viruses are a major driver of phytoplankton mortality and thereby influence biogeochemical cycling of carbon and other nutrients. Phytoplankton-targeting viruses are important components of ecosystem dynamics, but broad-scale experimental investigations of host-virus interactions remain scarce. Here, we investigated in detail a picophytoplankton (size 1 µm) host’s responses to infections by species-specific viruses from distinct geographical regions and different sampling seasons. Specifically, we used Ostreococcus tauri and O. mediterraneus and their viruses (size ca. 100 nm). Ostreococcus sp. is globally distributed and, like other picoplankton species, play an important role in coastal ecosystems at certain times of the year. Further, Ostreococcus sp. is a model organism, and the Ostreococcus-virus system is well-known in marine biology. However, only few studies have researched its evolutionary biology and the implications thereof for ecosystem dynamics. The Ostreococcus strains used here stem from different regions of the Southwestern Baltic Sea that vary in salinity and temperature and were obtained during several cruises spanning different sampling seasons. Using an experimental cross-infection set-up, we explicitly confirm species and strain specificity in Ostreococcus sp. from the Baltic Sea. Moreover, we found that the timing of virus-host co-existence was a driver of infection patterns as well. In combination, these findings prove that host-virus co-evolution can be rapid in natural systems.

Investigating biophysical control of marine phytoplankton dynamics via Bayesian mechanistic modeling

Marine phytoplankton possess a key position in multiple ecological, biological, and environmental processes; therefore, constructing a systematic and comprehensive mechanistic understanding of the leading mechanisms and key biophysical drivers of phytoplankton dynamics is crucial for evaluating biophysical forcings and ecological responses. Here, we develop a process-based model to improve the characterization of controlling mechanisms and assess the relative importance of nutrients, light, temperature, and oceanic mixing for explaining chlorophyll-a variability in the western North Pacific Ocean. The model is calibrated using 20-year observational data of chlorophyll-a concentrations from 1993 to 2012 in a Bayesian framework to provide data-driven inference and a rigorous uncertainty quantification of important biophysical rates. The model explains approximately 61% of the variability in chlorophyll-a. Our results, supported by cross-validation, show that the chlorophyll-a concentration is much more responsive to temperature variability than nutrients, light, or mixing. By synthesizing the effects of multiple nutrients (i.e., nitrogen, phosphorus, silica, and iron) on phytoplankton dynamics, we find that iron and nitrogen largely determine the chlorophyll-a variability. Sensitivity analysis shows that future warming conditions with +3°C change in temperature will impede phytoplankton production by 24% due to an intensified phytoplankton mortality impact. Our findings highlight the potential of “deductive-and-inductive” modeling approaches combining embedded biophysical mechanisms and probabilistic uncertainty quantification to effectively integrate and leverage sporadic ocean monitoring efforts and ultimately improve the marine water quality predictions.

Ecological Importance of Viral Lysis as a Loss Factor of Phytoplankton in the Amundsen Sea

Whether phytoplankton mortality is caused by grazing or viral lysis has important implications for phytoplankton dynamics and biogeochemical cycling. The ecological relevance of viral lysis for Antarctic phytoplankton is still under-studied. The Amundsen Sea is highly productive in spring and summer, especially in the Amundsen Sea Polynya (ASP), and very sensitive to global warming-induced ice-melt. This study reports on the importance of the viral lysis, compared to grazing, of pico- and nanophytoplankton, using the modified dilution method (based on apparent growth rates) in combination with flow cytometry and size fractionation. Considerable viral lysis was shown for all phytoplankton populations, independent of sampling location and cell size. In contrast, the average grazing rate was 116% higher for the larger nanophytoplankton, and grazing was also higher in the ASP (0.45 d−1 vs. 0.30 d−1 outside). Despite average specific viral lysis rates being lower than grazing rates (0.17 d−1 vs. 0.29 d−1), the average amount of phytoplankton carbon lost was similar (0.6 µg C L−1 d−1 each). The viral lysis of the larger-sized phytoplankton populations (including diatoms) and the high lysis rates of the abundant P. antarctica contributed substantially to the carbon lost. Our results demonstrate that viral lysis is a principal loss factor to consider for Southern Ocean phytoplankton communities and ecosystem production.

Understanding plankton ecosystem dynamics under realistic micro-scale variability requires modeling at least three trophic levels

Plankton ecosystems are complex, multi-trophic networks of biotic and abiotic interactions among physical and chemical components. Still, nutrient-phytoplankton (NP) interactions are in many cases assumed to be representative of higher trophic-level interactions in plankton ecosystems. Here, we investigate the degree to which NP interactions capture the overall dynamics of multi-trophic plankton ecosystems when accounting for realistic levels of micro-scale variability. Plankton models are typically developed based on the mean-field approach, which considers only first central moments (i.e., spatio-temporal means). Such conventional plankton models may be appropriate for meso‑ or larger-scales, but inappropriate for the highly intermittent spatial fluctuations of phytoplankton that are ubiquitous at the micro (mm) scale. Using Reynold's decomposition, the closure approach accounts for spatial variability and temporal fluctuations (higher central moments) of the phytoplankton distributions. We apply closure models of various combinations of Nutrient (N), Phytoplankton (P), and Zooplankton (Z), each with linear, hyperbolic, sigmoidal and quadratic phytoplankton mortality forms to test i) the previously advanced hypothesis ‘fluctuations enhance higher trophic level biomass in plankton models’ and ii) the suitability of nutrient-phytoplankton interactions as a proxy for higher trophic ecological processes. We find that in NP models with linear, hyperbolic and sigmoidal phytoplankton mortality (P-mortality), phytoplankton biomass (P-biomass) increases with variability, but in the NP model with quadratic P-mortality, P-biomass decreases with variability. The results are robust in the sense that similar qualitative behavior is obtained for a wide range of biologically feasible parameter sets. We investigate the characteristics of the measurable quantity coefficient of variation of phytoplanktonCVp for models with different numbers of compartments. The NP models with density-independent form (linear form) and density-dependent forms (hyperbolic, sigmoidal and quadratic) for P-mortality term behave quite opposite to each other: NP models with linear P-mortality produce coefficient of variation CVp > 1 for all stable solutions while NP models with hyperbolic, sigmoidal and quadratic P-mortality forms produce CVp < 1 for all stable solutions. Depending on the mortality rate assumed, the NP model produces qualitatively different results at both micro- and macro-scales, and therefore is unable to consistently capture the true underlying dynamics of plankton ecosystems. By contrast, NPZ models behaved consistently and supported the previously advanced hypotheses, regardless of the functional form assumed for P-mortality.

Elevated pCO2 changes community structure and function by affecting phytoplankton group-specific mortality

The rise of atmospheric pCO2 has created a number of problems for marine ecosystem. In this study, we initially quantified the effects of elevated pCO2 on the group-specific mortality of phytoplankton in a natural community based on the results of mesocosm experiments. Diatoms dominated the phytoplankton community, and the concentration of chlorophyll a was significantly higher in the high-pCO2 treatment than the low-pCO2 treatment. Phytoplankton mortality (percentage of dead cells) decreased during the exponential growth phase. Although the mortality of dinoflagellates did not differ significantly between the two pCO2 treatments, that of diatoms was lower in the high-pCO2 treatment. Small diatoms dominated the diatom community. Although the mortality of large diatoms did not differ significantly between the two treatments, that of small diatoms was lower in the high-pCO2 treatment. These results suggested that elevated pCO2 might enhance dominance by small diatoms and thereby change the community structure of coastal ecosystems.

Phytoplankton growth regulation by dissolved P and mortality regulation by endogenous cell death over 35years of P control in a Mountain Lake.

Dynamics of phytoplankton and phosphorus were quantified in Lake Dillon, Colorado, over 35 years of P control. The lake provides an example of early intervention for P enrichment rather than remediation of advanced eutrophication. Phosphorus control began with tertiary treatment of effluent, which caused a phytoplankton decline (8.1–4.5 μg L−1 chla); a second decline (4.6–2.5 μg L−1 chla) occurred later following replacement of failing septic systems. Results showed that bioavailable phosphorus (BAP) loading was the only significant correlate of phytoplankton biomass; total P loading was not significantly related to biomass measured as chlorophyll. Phytoplankton composition changed greatly over the study interval, even though there was no long-term trend in potential causes of phytoplankton abundance other than reduction in BAP. Gradual decline of BAP loading also appears to have been the cause of large, gradual changes in phytoplankton community composition. Factors typically assumed to control phytoplankton mortality accounted for only ~50% of phytoplankton biomass turnover; the balance of mortality appears to be accounted for by endogenous cell mortality.

Spring Accumulation Rates in North Atlantic Phytoplankton Communities Linked to Alterations in the Balance Between Division and Loss.

For nearly a century, phytoplankton spring blooms have largely been explained in the context of abiotic factors regulating cellular division rates (e.g., mixed-layer light levels). However, the accumulation of new phytoplankton biomass represents a mismatch between phytoplankton division and mortality rates. The balance between division and loss, therefore, has important implications for marine food webs and biogeochemical cycles. A large fraction of phytoplankton mortality is due to the combination of microzooplankton grazing and viral lysis, however, broad scale simultaneous measurements of these mortality processes are scarce. We applied the modified dilution assay along a West-to-East diagonal transect in the North Atlantic during spring. Our results demonstrate positive accumulation rates with losses dominated by microzooplankton grazing. Considering the dynamic light environment phytoplankton experience in the mixed surface layer, particularly in the spring, we tested the potential for incubation light conditions to affect observed rates. Incubations acted as short-term ‘light’ perturbations experiments, in which deeply mixed communities are exposed to elevated light levels. These “light perturbations” increased phytoplankton division rates and resulted in proportional changes in phytoplankton biomass while having no significant effect on mortality rates. These results provide experimental evidence for the Disturbance-Recovery Hypothesis, supporting the tenet that biomass accumulation rates co-vary with the specific rate of change in division.

Phytoplankton mortality in a changing thermal seascape.

Predicting spatiotemporal distributions of phytoplankton biomass and community composition heavily relies on experimental studies that document how environmental conditions influence population growth rates. In unicellular phytoplankton, the net population growth rate is the difference between the cell division rate and the death rate. Along with predation and disease, phytoplankton mortality arises from abiotic stress. Although the effect of temperature on the net population growth rate is well understood, studies examining thermally induced death rates in phytoplankton are scarce. We investigated how cell division and death rates of the diatom Phaeodactylum tricornutum varied within its thermal tolerance limits (thermal niche), and at temperatures just above its upper thermal tolerance limit. We show that death rates were largely independent of temperature when P. tricornutum was grown within its thermal niche, but increased significantly at temperatures that approached or exceeded its upper thermal tolerance limit. Furthermore, the sensitivity of mortality increased with the duration of exposure to heat stress and was affected by the pre-acclimation temperature. Heat waves can be expected to significantly affect phytoplankton mortality episodically. The increasing frequency of heat waves accompanying global warming can be expected to drive changes in phytoplankton community structure due to interspecific variability of thermal niches with potential implications for food web dynamics and biogeochemical cycles.

Seasonal variations in viral distribution, dynamics, and viral-mediated host mortality in the Arabian Sea

Viruses are key players in the marine ecosystem. It is critical to study specific viral processes and their inter-relationship with various biotic and abiotic variables to quantify their impact on the marine environment. This study investigates the influence of seasonality on viral distribution and their mediated processes in the coastal region of the southeastern Arabian Sea (India) for two consecutive years (2014 and 2015). Water samples were collected from four sampling stations, S1, S2, S3 and S4, in the southeastern Arabian Sea on a monthly basis. Samples were analyzed for the variations in viral abundance, viral production, and viral-induced host mortality covering three seasons namely, pre-monsoon, monsoon and post-monsoon. Viruses and bacteria were enumerated by epifluorescence microscopy following staining with SYBR Green I and viral production was estimated using viral dilution method. Seasonal variations in viral-mediated mortality and microzooplankton grazing mortality of phytoplankton hosts were estimated by modified dilution method. The results revealed a pronounced seasonal pattern of viral abundance (0.04–4.03 × 107 viruses mL−1) and viral production (0.71 × 1010–4.94 × 1010 L−1d−1). The observed high viral-induced mortality of prokaryotes (25.85%) and phytoplankton (7.98–28.9%) during the pre-monsoon season was eventually linked to high viral abundance and production rates. Viral-mediated processes were essentially linked to host density and water temperature. Transmission electron microscopy analysis revealed that myoviruses and non-tailed viruses were dominant in the study region with rod-shaped bacteria being more susceptible to viral infection. The results indicated the prevalence of seasonally induced active viral processes in this tropical marine ecosystem which may significantly contribute to rapid recycling of nutrients in this region.

Significance of Viral Activity for Regulating Heterotrophic Prokaryote Community Dynamics along a Meridional Gradient of Stratification in the Northeast Atlantic Ocean.

How microbial populations interact influences the availability and flux of organic carbon in the ocean. Understanding how these interactions vary over broad spatial scales is therefore a fundamental aim of microbial oceanography. In this study, we assessed variations in the abundances, production, virus and grazing induced mortality of heterotrophic prokaryotes during summer along a meridional gradient in stratification in the North Atlantic Ocean. Heterotrophic prokaryote abundance and activity varied with phytoplankton biomass, while the relative distribution of prokaryotic subpopulations (ratio of high nucleic acid fluorescent (HNA) and low nucleic acid fluorescent (LNA) cells) was significantly correlated to phytoplankton mortality mode (i.e., viral lysis to grazing rate ratio). Virus-mediate morality was the primary loss process regulating the heterotrophic prokaryotic communities (average 55% of the total mortality), which may be attributed to the strong top-down regulation of the bacterivorous protozoans. Host availability, encounter rate, and HNA:LNA were important factors regulating viral dynamics. Conversely, the abundance and activity of bacterivorous protozoans were largely regulated by temperature and turbulence. The ratio of total microbial mediated mortality to total available prokaryote carbon reveals that over the latitudinal gradient the heterotrophic prokaryote community gradually moved from a near steady state system regulated by high turnover in subtropical region to net heterotrophic production in the temperate region.

Virus-induced spore formation as a defense mechanism in marine diatoms.

Summary Algal viruses are important contributors to carbon cycling, recycling nutrients and organic material through host lysis. Although viral infection has been described as a primary mechanism of phytoplankton mortality, little is known about host defense responses.We show that viral infection of the bloom‐forming, planktonic diatom Chaetoceros socialis induces the mass formation of resting spores, a heavily silicified life cycle stage associated with carbon export due to rapid sinking.Although viral RNA was detected within spores, mature virions were not observed. ‘Infected’ spores were capable of germinating, but did not propagate or transmit infectious viruses.These results demonstrate that diatom spore formation is an effective defense strategy against viral‐mediated mortality. They provide a possible mechanistic link between viral infection, bloom termination, and mass carbon export events and highlight an unappreciated role of viruses in regulating diatom life cycle transitions and ecological success.

Diagnosing Mechanisms of Ocean Carbon Export in a Satellite-Based Food Web Model

The net primary productivity (NPP) of marine phytoplankton is ~ 50 Pg C yr-1, and roughly 10-20% of this NPP is exported out of the surface ocean as sinking particulate organic carbon (POC). Numerous mechanisms are hypothesized to control POC export out of the surface ocean but the relative importance of the various mechanisms remains poorly quantified on global scales. Here, we use a previously published satellite-based mechanistic model of POC export to examine the effects on global POC export of size-specific physical aggregation, size-specific and temperature-dependent zooplankton fecal pellet production, and size-specific and temperature-dependent non-grazing phytoplankton mortality. We test these mechanisms in different model configurations to determine if these processes improve the ability of the model to match POC export observations, and to assess the role of each process in controlling global POC export. We find that all model configurations predict that over 60% of the global POC export is from small zooplankton fecal pellets. All model configurations predict similar total POC export, and we find only small differences in the magnitude, timing, and geographical variations of total POC export. However, the fraction of total POC export due to sinking phytoplankton aggregates, and that due to the fecal pellets of large zooplankton, vary by more than a factor of two across the different model configurations. The POC export in all models is most sensitive to parameters controlling zooplankton fecal fluxes and non-grazing phytoplankton mortality. We compared zooplankton grazing rates predicted by the models to results of experimental data, and found that some models match the experimental grazing rates better than others, although data uncertainties remain large. More field measurements of bulk ecosystem rates (i.e., phytoplankton aggregation and zooplankton grazing), as well as explicit determinations of of the proportion of fecal matter to phytoplankton aggregation, will help to better constrain mechanistic models of global POC export.

Dynamics of a nutrient-phytoplankton model with random phytoplankton mortality

This study formulates a stochastic nutrient-phytoplankton model which incorporates the effect of white noise on phytoplankton growth. The global existence and uniqueness of a positive solution, stochastic boundedness, and stochastically asymptotic stability are well explored. A stochastic ecological reproductive index R0s is formulated to characterize the global dynamics. The theoretical analysis demonstrates that, if R0s<1, then phytoplankton dies out with probability one; if R0s>1 and some other conditions hold, then there exists an invariant and asymptotically stable density of the system and the approach involves integral Markov semigroups theory. Numerical simulations are presented to illustrate the analytical findings and to investigate the long-time effect of water temperature, light, nutrients, and environmental noise on the dynamic evolution of phytoplankton.

Transcriptomic responses to grazing reveal the metabolic pathway leading to the biosynthesis of domoic acid and highlight different defense strategies in diatoms

BackgroundA major cause of phytoplankton mortality is predation by zooplankton. Strategies to avoid grazers have probably played a major role in the evolution of phytoplankton and impacted bloom dynamics and trophic energy transport. Certain species of the genus Pseudo-nitzschia produce the neurotoxin, domoic acid (DA), as a response to the presence of copepod grazers, suggesting that DA is a defense compound. The biosynthesis of DA comprises fusion of two precursors, a C10 isoprenoid geranyl pyrophosphate and l-glutamate. Geranyl pyrophosphate (GPP) may derive from the mevalonate isoprenoid (MEV) pathway in the cytosol or from the methyl-erythritol phosphate (MEP) pathway in the plastid. l-glutamate is suggested to derive from the citric acid cycle. Fragilariopsis, a phylogenetically related but nontoxic genus of diatoms, does not appear to possess a similar defense mechanism. We acquired information on genes involved in biosynthesis, precursor pathways and regulatory functions for DA production in the toxigenic Pseudo-nitzschia seriata, as well as genes involved in responses to grazers to resolve common responses for defense strategies in diatoms.ResultsSeveral genes are expressed in cells of Pseudo-nitzschia when these are exposed to predator cues. No genes are expressed in Fragilariopsis when treated similarly, indicating that the two taxa have evolved different strategies to avoid predation. Genes involved in signal transduction indicate that Pseudo-nitzschia cells receive signals from copepods that transduce cascading molecular precursors leading to the formation of DA. Five out of seven genes in the MEP pathway for synthesis of GPP are upregulated, but none in the conventional MEV pathway. Five genes with known or suggested functions in later steps of DA formation are upregulated. We conclude that no gene regulation supports that l-glutamate derives from the citric acid cycle, and we suggest the proline metabolism to be a downstream precursor.ConclusionsPseudo-nitzschia cells, but not Fragilariopsis, receive and respond to copepod cues. The cellular route for the C10 isoprenoid product for biosynthesis of DA arises from the MEP metabolic pathway and we suggest proline metabolism to be a downstream precursor for l-glutamate. We suggest 13 genes with unknown function to be involved in diatom responses to grazers.

Physics or biology? Persistent chlorophyll accumulation in a shallow coastal sea explained by pathogens and carnivorous grazing.

One of the most striking patterns at the land–ocean interface is the massive increase of chlorophyll-a (CHL) from continental shelves towards the coast, a phenomenon that is classically linked to physical features. Here I propose that the coastal–offshore CHL gradient in a shallow sea has biological origins related to phytoplankton mortality that are neglected in state-of-the-art biogeochemical models. I integrate a trait-based ecosystem model into a modular coupling framework that is applied to the southern North Sea (SNS). The coupled model very well reproduces daily, seasonal and inter-annual (2000-2014) dynamics and meso-scale patterns in macronutrients, zooplankton biomass, and CHL as observed in situ and by remote sensors. Numerical experiments reveal that coast–offshore CHL gradients may predominantly arise from a trophic effect as resolved by an increase in carnivorous grazing towards shallow waters. This carnivory gradient reflects higher near-coast abundance of juvenile fish and benthic filter feeders. Furthermore, the temporal evolution of CHL can be much affected by viral infection as a fast-responding loss process at intermediate to high phytoplankton concentrations. Viral control in the model also prevents excessive and unrealistic blooms during late spring. Herbivores as often only ecological factor considered for explaining the spatio-temporal phytoplankton distribution are in this study supplemented by pathogens as well as pelagic and benthic carnivores as powerful agents, which are barely represented in current modeling but can mediate physical drivers of coastal ecosystems.