VIII European Congress of Protistology (ECOP) - Italian Society of Protistology (ISOP) Joint Meeting, 28 July - 2 August 2019, Rome.-- 1 page, 2 figures, Mixotrophs are now widely recognised as important members of the aquatic food webs, but their presence/absence is not yet fully understood. Processes like energy transfer efficiency may be enhanced by the presence of mixotrophic protists and, consequently, affect the secondary production of an ecosystem. Therefore, in order to improve the performance of ecosystem predictive models, it is necessary to properly address the mixotrophic paradigm. An immediate step in this direction would be to develop a technique/combination of techniques to measure mixotrophic predation on phytoplankton.
One way to solve the issue could be to adapt the dilution technique developed by Landry & Hassett in 1982 for micro-zooplankton. The technique is in itself very elegant in the way that it provides the grazing rate of the micro-zooplankton community without an overwhelming workload. Despite having been refined by diverse authors over the years, mixotrophs have always caused distress, as they mask the results obtained by applying this technique.
In this study, we did some laboratory experiments with mixotrophic and heterotrophic cultures (one dinoflagellate and one ciliate for each group) and the addition of the toxic compound Rotenone. Rotenone inhibits the electron transport chain in the mitochondria by blocking the transport between the flavoprotein and the ubiquinone. Therefore, it is expected to inhibit all non-photosynthetic organisms, as their energy factories become impaired. Indeed, several studies exist where the effect of rotenone on metazoans is well-documented, with notes on the low toxicity to phytoplankton species.
After the short-term incubations with rotenone (ca. 24h), the effect on protists was not as one would predict considering previous metazoan studies. Although chloroplast-bearing organisms displayed a better survivability at low concentrations of rotenone, their predation was more affected than that of the heterotrophs. Regarding heterotrophic groups, the dinoflagellate displayed the highest tolerance to the compound, when balancing the growth and ingestion rates, whereas the ciliate was the most sensitive. The difference between taxonomic groups could be critical when assessing the predatory impact of the mixotrophic community and further studies are required, This project has received funding from the European Union’s Horizon 2020 research and innovation programm eunder the Marie Skłodowska-Curie Grant agreement No 766327