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Phagotrophy and competitive ability of the mixotrophic Ochromonas minima were investigated in a three-factorial experiment where light intensity (low: 1.0μmolm−2 s−1 and high: 60μmolm−2 s−1 PPFD), nutrient concentration (ambient: 7.0μmolNl−1, 0.11μmolPl−1 and enriched: 88μmolNl−1, 6.3μmolPl−1) and DOC supply (without and with enrichment, 250μmolCl−1) were manipulated. Ochromonas minima and bacterial abundance were monitored for 12 days. We found significant and interacting effects of light and nutrients on Ochromonas minima growth rate and abundance. At high light intensity, nutrient enrichment resulted in increased growth rates and population sizes. In contrast, reduced growth rates and population sizes were observed for nutrient enrichment when light intensity was low. Although, Ochromonas minima was able to ingest bacteria under both high and low light conditions, it grew only when light intensity was high. At high light intensity, Ochromonas minima grew exponentially under nutrient conditions that would have been limiting for photoautotrophic microalgae. In non-enriched low light treatments, Ochromonas minima populations survived, probably by using background DOC as an energy source, indicating that this ability can be of relevance for natural systems even when DOC concentrations are relatively low. When competing with photoautotrophic microalgae, the ability to grow under severe nutrient limitation and to survive under light limitation should be advantageous for Ochromonas minima. [Copyright &y& Elsevier]

Microbial parasites have only recently been included in planktonic food web studies, but their functional role in conveying dietary energy still remains to be elucidated. Parasitic fungi (chytrids) infecting phytoplankton may constitute an alternative trophic link and promote organic matter transfer through the production of dissemination zoospores. Particularly, during proliferation of inedible or toxic algal species, such as large Cyanobacteria fostered by global warming, parasites can constitute an alternative trophic link providing essential dietary nutrients that support somatic growth and reproduction of consumers. Using phytoplankton-parasites associations grown under laboratory controlled conditions we assessed the fatty acids and biochemical composition of species with different nutritional quality and followed the metabolic pathway from the algal host and their parasites zoospores using compound-specific stable isotope analysis. This study demonstrated that chytrids are trophic upgraders able to retain essential nutrients that can be transferred to upper trophic levels both in terms of organic matter quantity and nutritional quality. Through the production of zoospores, nutritionally important long-chain polyunsaturated fatty acids that can be consequently assimilated by consumers. We conclude that parasitism at the base of aquatic food webs may represent a crucial trophic link for dietary nutrients and essential biomolecules alternative to herbivory or bacterivory, which can be particularly crucial during the proliferation of inedible or nutritionally inadequate algal species fostered by climate change. [ABSTRACT FROM AUTHOR]

The author comments on an analysis made by I. V. Telesh and colleagues in an article which claimed that plankton diversity peaks at intermediate salinities in the Baltic Sea. In this paper, the concept of Artenminimum or minimum diversity at intermediate salinities that was described for macrozoobenthos in the Baltic Sea by A. Remane was challenged by Telesh. A correlation between the plankton diversity along the Baltic salinity gradient and the size of the area having a given salinity was also suggested by Telesh.