Recent observational studies form oligotrophic waters provide ample evidence that mixotrophic flagellates often account for the bulk of bacterivory. However, we lack a general framework that allows a mechanistic understanding of success of mixotrophs in the competition with heterotrophic bacterivores. This is especially needed for integrating mixotrophy in models of the microbial loop. Based on general tradeoffs linked to the combined resource use in mixotrophs (generalist versus specialist), we propose a concept where mixotrophs are favored by conditions of high light – low losses, corresponding to the situation found in the surface waters of oligotrophic oceans. Under such conditions, they can achieve positive net growth at very low resource levels, allowing simultaneous competition with specialized protists. Conversely, heterotrophic bacterivores and photoautotrophs should be especially favored in more productive and low‐light conditions. We show experimentally that the combined effect of light and loss rates (dilution) predicts the success of mixotrophic bacterivorous flagellates. Moreover, our results suggest that total bacterivory, contrary as seen in the traditional microbial loop concept, has a more intricate coupling to light.

Mixotrophic protists are widespread and relevant primary producers and consumers in planktonic food webs. Given their dual mode of nutrition, mixotrophs face different constraints in allocating resources to cellular structures compared to strict photoautotrophs. However, little is known about their stoichiometric requirements and their flexibility in nutrient content and thus food quality, or how this affects consumer performance and nutrient recycling. In the present study, we tested for systematic differences in elemental composition between photoautotrophic and mixotrophic protists. We compiled intracellular nutrient ratios of mixotrophic and phototrophic species from culture experiments and from 2 lake surveys. Overall, both laboratory and field data indicated that mixotrophy has a stabilizing effect on the nutrient stoichiometry of primary producers under changing nutrient supply. In laboratory experiments, mixotrophs showed a lower variability in intracellular N:P ratios compared to strict phototrophs and were more stable in their elemental composition in response to a gradient of dissolved N:P availability. With increasing contributions of mixotrophic phytoplankton taxa to total lake phytoplankton, both the mean and variance in seston C:P ratios decreased, i.e. communities with higher proportion of mixotrophs overall exhibited more constrained seston stoichiometry. Our results show that mixotrophy may have direct implications for nutrient cycling and secondary production through regulation of seston stoichiometry, buffering stoichiometric constraints for herbivores and enabling a more stable secondary production compared to systems dominated by phototrophic specialists.

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Chemostats and their common alternative, semicontinuous cultures, play a pivotal role as model systems in aquatic ecology. Despite the theoretical and conceptual advantages chemostat systems offer, they can be challenging to set up and operate. One such challenge is to obtain a representative sample volume without changing the dilution rate, another is to ensure that the outflow constitutes an unbiased loss of the chemostat contents. Here, we present a specific system, the exponentially fed-batch culture (EFB), as an economic, simply operable albeit reliable alternative to conventional chemostats. The EFB is well-known in biotechnological research and in industrial bioengineering. Like a conventional chemostat, the EFB culture is a continuous culture method. In contrast to a conventional chemostat, there is no outflow. To ensure a constant dilution rate and therefore steady state, fresh medium is continuously supplied proportional to the current culture volume. Hence culture volume and medium supply rate both increase exponentially with time until sampling, when the volume is set back to the initial state. This allows taking large sample volumes without disturbing the steady state of the experimental unit. Here we compare the performance of the exponentially fed-batch culture with a conventional chemostat and a semicontinuous system by growing a green algae until steady state is reached. Most pronounced differences were found between semicontinuous system and the two continuous methods. Results from the EFB and chemostat were very similar. However, a bias in the sampling of the chemostat resulted in accumulation of biomass relative to the EFB.