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The growth of autotroph communities is frequently (co-)limited by essential nutrients such as nitrogen (N) and phosphorus (P). Several mechanisms have been proposed to explain the observed co-limitation patterns at different levels of biological organization, especially at the biochemical level for individual species. When considering communities, the presence of different species and functional groups with contrasting physiologies and nutrient requirements leads to a more difficult understanding of the mechanisms involved in nutrient (co-) limitation. To investigate what drives co-limitation patterns and possible underlying mechanisms based on biomass responses in autotroph communities, we grew phytoplankton communities differing in species composition in experimental microcosms on three N:P ratios to impose different limiting conditions. Afterwards, N, P, both, or none were factorially supplied to the communities to test which nutrients were limiting growth. We measured the biovolume of single species in the communities to assess how they responded to nutrient additions and compared it to the response of the overall community biovolume. The types of nutrient (co-)limitation identified, i.e. the factorial limitation scenarios for community biomass were single N limitation or simultaneous co-limitation by N and P, and were strongly driven by the dominant species. The phytoplankton species in the communities responded differently to the nutrient addition treatments, i.e. they showed contrasting limitation outcomes and therefore likely different nutrient requirements. Our experiment indicates that phylogenetically distantly-related phytoplankton species grown in a community can have different resource use efficiencies and thus can be limited by different nutrients. We suggest that the dominance of species or groups with similar traits in nutrient requirements and acquisition is one of the leading mechanisms that determines the biomass pattern of nutrient (co-)limitation observed at the community level. This work also highlights the potential of predicting community growth limitation outcomes based on knowledge of nutrient use efficiencies of one or few dominant species, which can be a suitable tool for lake restoration and oligotrophication efforts. [ABSTRACT FROM AUTHOR]

We analyzed a model of phytoplankton competition for light in a well-mixed water column. The model, proposed by Gerla et al. (Oikos 120:519-527, ), assumed inhibition of photosynthesis at high irradiance (photoinhibition). We described the global behavior through mathematical analyses, providing a general solution to the multi-species competition for light with photoinhibition. We classified outcomes of 2- and 3-species competitions as examples, and evaluated feasibility of the theoretical predictions using empirical relationships between photosynthetic production and irradiance. Numerical simulations with published p- I curves indicate that photoinhibition may often lead to strong Allee effects and competitive facilitation among species. Hence, our results suggest that photoinhibition may play a major role in organizing phytoplankton communities. [ABSTRACT FROM AUTHOR]

Abstract: What determines the vertical distribution of phytoplankton in different aquatic environments remains an open question. To address this question, we develop a model to explore how phytoplankton respond through growth and movement to opposing resource gradients and different mixing conditions. We assume stratification creates a well-mixed surface layer on top of a poorly mixed deep layer and nutrients are supplied from multiple depth-dependent sources. Intraspecific competition leads to a unique strategic equilibrium for phytoplankton, which allows us to classify the distinct vertical distributions that can exist. Biomass can occur as a benthic layer (BL), a deep chlorophyll maximum (DCM), or in the mixed layer (ML), or as a combination of BL+ML or DCM+ML. The ML biomass can be limited by nutrients, light, or both. We predict how the vertical distribution, relative resource limitation, and biomass of phytoplankton will change across environmental gradients. We parameterized our model to represent potentially light and phosphorus limited freshwater lakes, but the model is applicable to a broad range of vertically stratified systems. Increasing nutrient input from the sediments or to the mixed layer increases light limitation, shifts phytoplankton towards the surface, and increases total biomass. Increasing background light attenuation increases light limitation, shifts the phytoplankton towards the surface, and generally decreases total biomass. Increasing mixed layer depth increases, decreases, or has no effect on light limitation and total biomass. Our model is able to replicate the diverse vertical distributions observed in nature and explain what underlying mechanisms drive these distributions. [ABSTRACT FROM AUTHOR]

Planktonic microorganisms are affected by various size-dependent processes both from the bottom up and from the top down. We developed a simple resource-consumer model to explore how size-dependent resource uptake and resource loss influence the growth of, and competition between, planktonic microorganisms. We considered three steps of resource uptake: diffusive transport of resource molecules, uptake by membrane transporters, and cellular enzymatic catalysis, and we investigated optimal cell size when one, two, or three of those steps limit resource uptake. Optimal cell size depends negatively on the size of resource molecules when resource uptake is limited by diffusive transport and membrane uptake. When competing for two resources of different molecular sizes, two different-sized consumers can coexist if the inputs of resources and sizes of consumers are correctly chosen. The model suggests that mixtures of various-sized resources can promote coexistence and size diversity of microorganisms even if the availability of one element, such as carbon, nitrogen, or phosphorus, limits the whole community. Model predictions include that bacteria grown on maltose or polysaccharides should be smaller compared with those grown on glucose under carbon limitation. Our results suggest that size of resource molecules can be an important factor in microbial resource competition in aquatic environments. [ABSTRACT FROM AUTHOR]