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We present a model of the growth rate and elemental stoichiometry of phytoplankton as a function of resource allocation between and within broad macromolecular pools under a variety of resource supply conditions. The model is based on four, empirically-supported, cornerstone assumptions: that there is a saturating relationship between light and photosynthesis, a linear relationship between RNA/protein and growth rate, a linear relationship between biosynthetic proteins and growth rate, and a constant macromolecular composition of the light-harvesting machinery. We combine these assumptions with statements of conservation of carbon, nitrogen, phosphorus, and energy. The model can be solved algebraically for steady state conditions and constrained with data on elemental stoichiometry from published laboratory chemostat studies. It interprets the relationships between macromolecular and elemental stoichiometry and also provides quantitative predictions of the maximum growth rate at given light intensity and nutrient supply rates. Themodel is compatible with data sets from several laboratory studies characterizing both prokaryotic and eukaryotic phytoplankton from marine and freshwater environments. It is conceptually simple, yet mechanistic and quantitative. Here, the model is constrained only by elemental stoichiometry, but makes predictions about allocation to measurable macromolecular pools, which could be tested in the laboratory. [ABSTRACT FROM AUTHOR]

The ratio of elements such as carbon:nitrogen:phosphorus (C:N:P) in phytoplankton is known to vary substantially within single isolates and across environmental gradients. In addition, C:N:P is known to vary throughout the day due to diel patterns in nutrient acquisition and storage. It has been hypothesized that small phytoplankton such as marine cyanobacteria have relatively invariable elemental ratios during a 24 h period, whereas larger phytoplankton have a greater capacity to store elements and thus a wider diel range of C:N:P. To test this hypothesis, we examined diel variability in cellular C:N:P, using a chemostat culturing system, for one of the most abundant marine cyanobacteria, Synechococcus (WH8102) during two 24 h periods. The cellular C quota nearly doubled during the 14 h light period and was subsequently reduced during the dark period. The cellular N quota also varied considerably, whereas the P quota remained relatively stable. These daily changes in elemental quotas led to highly variable C:Ncell and C:Pcell. Furthermore, the magnitude of variability in cellular elemental stoichiometry of Synechococcus was positively related to the growth rate. We constructed a model to test the extent to which variation in C:Ncell and C:Pcell is related to reserve carbon accumulation and depletion over each light-dark cycle. Results imply that, in addition to growth-related respiratory losses, Synechococcus also purges excess C during the dark period in order to maintain a nutritive balance within cells. Our data suggest that diel variation in C:Ncell and C:Pcell of Synechococcus is of the same order of magnitude as stoichiometric variation within plankton communities between major ocean environments.

Recent metanalyses suggest that microzooplankton biomass density scales linearly with phytoplankton biomass density, suggesting a simple, general rule may underpin trophic structure in the global ocean. Here, we use a set of highly simplified food-web models, solved within a global general circulation model, to examine the core drivers of linear predator-prey scaling. We examine a parallel food-chain model which assumes microzooplankton grazers feed on distinct size-classes of phytoplankton, and contrast this with a diamond food-web model allowing shared microzooplankton predation on a range of phytoplankton size classes. Within these two contrasting model structures, we also evaluate the impact of fixed vs. density-dependent microzooplankton mortality. We find that the observed relationship between microzooplankton predators and prey can be reproduced with density-dependent mortality on the top predator, regardless of choices made about plankton food-web structure. Our findings point to the importance of parameterizing mortality of the top predator for models to recapitulate trophic structure in the global ocean. [ABSTRACT FROM AUTHOR]

It has been proposed that microbial predator and prey densities are related through sublinear power laws. We revisited previously published biomass and abundance data and fitted Power‐law Biomass Scaling Relationships (PBSRs) between marine microzooplankton predators (Z) and phytoplankton prey (P), and marine viral predators (V) and bacterial prey (B). We analysed them assuming an error structure given by Type II regression models which, in contrast to the conventional Type I regression model, accounts for errors in both the independent and the dependent variables. We found that the data support linear relationships, in contrast to the sublinear relationships reported by previous authors. The scaling exponent yields an expected value of 1 with some spread in different datasets that was well‐described with a Gaussian distribution. Our results suggest that the ratios Z/P, and V/B are on average invariant, in contrast to the hypothesis that they systematically decrease with increasing P and B, respectively, as previously thought. [ABSTRACT FROM AUTHOR]

The influence of viruses on nutrient cycles and energy transfer in aquatic systems is important, yet still being determined. We developed a dynamic model to capture the population dynamics of algal hosts and their viruses. The model was fitted to literature data of population dynamics during laboratory one‐step infection experiments. Model parameters that underlie population dynamics were quantified in diverse algal groups, covering 7 different algal classes, 14 different host genera, and 32 different virus genotypes. The hypothesis that trade‐offs exist between traits that underlie population dynamics was evaluated. We report a possible virus size‐dependent trade‐off between the rate at which viruses can initiate infection, and the efficiency of carbon transfer from hosts to viruses upon lysis. We hypothesize that slower rates of infection in large viruses, due to slower rates of molecular diffusion, are compensated by enhanced utilization of host resources. Our approach provides a quantitative trait‐framework for understanding and quantifying virus activity in diverse natural communities. [ABSTRACT FROM AUTHOR]

Light drives phytoplankton productivity, so phytoplankton must exploit variable intensities and durations of light exposure, depending upon season, latitude, and depth. We analyzed the growth, photophysiology and composition of small, Thalassiosira pseudonana, and large, Thalassiosira punctigera, centric diatoms from temperate, coastal marine habitats, responding to a matrix of photoperiods and growth light intensities. T. pseudonana showed fastest growth rates under long photoperiods and low to moderate light intensities, while the larger T. punctigera showed fastest growth rates under short photoperiods and higher light intensities. Photosystem II function and content responded primarily to instantaneous growth light intensities during the photoperiod, while diel carbon fixation and RUBISCO content responded more to photoperiod duration than to instantaneous light intensity. Changing photoperiods caused species-specific changes in the responses of photochemical yield (e−/photon) to growth light intensity. These photophysiological variables showed complex responses to photoperiod and to growth light intensity. Growth rate also showed complex responses to photoperiod and growth light intensity. But these complex responses resolved into a close relation between growth rate and the cumulative daily generation of reductant, across the matrix of photoperiods and light intensities. [ABSTRACT FROM AUTHOR]

Free viruses are the most abundant type of biological particles in the biosphere, but the lack of quantitative knowledge about their consumption by heterotrophic protists and bacterial degradation has hindered the inclusion of virovory in biogeochemical models. Using isotope-labeled viruses added to three independent microcosm experiments with natural microbial communities followed by isotope measurements with single-cell resolution and flow cytometry, we quantified the flux of viral C and N into virovorous protists and bacteria and compared the loss of viruses due to abiotic vs biotic factors. We found that some protists can obtain most of their C and N requirements from viral particles and that viral C and N get incorporated into bacterial biomass. We found that bacteria and protists were responsible for increasing the daily removal rate of viruses by 33% to 85%, respectively, compared to abiotic processes alone. Our laboratory incubation experiments showed that abiotic processes removed roughly 50% of the viruses within a week, and adding biotic processes led to a removal of 83% to 91%. Our data provide direct evidence for the transfer of viral C and N back into the microbial loop through protist grazing and bacterial breakdown, representing a globally significant flux that needs to be investigated further to better understand and predictably model the C and N cycles of the hydrosphere., (© The Author(s) [2024]. Published by Oxford University Press on behalf of the International Society for Microbial Ecology.)