Caveats on the use of rotenone to estimate mixotrophic grazing in the oceans

Guilherme D Ferreira,Albert Calbet

doi:10.1038/s41598-020-60764-2

Abstract

Phagotrophic mixotrophs (mixoplankton) are now widely recognised as important members of food webs, but their role in the functioning of food webs is not yet fully understood. This is due to the lack of a well-established technique to estimate mixotrophic grazing. An immediate step in this direction would be the development of a method that separates mixotrophic from heterotrophic grazing that can be routinely incorporated into the common techniques used to measure microplankton herbivory (e.g., the dilution technique). This idea was explored by the addition of rotenone, an inhibitor of the respiratory electron chain that has been widely used to selectively eliminate metazoans, both in the field and in the laboratory. Accordingly, rotenone was added to auto-, mixo-, and heterotrophic protist cultures in increasing concentrations (ca. 24 h). The results showed that mixotrophs survived better than heterotrophs at low concentrations of rotenone. Nevertheless, their predation was more affected, rendering rotenone unusable as a heterotrophic grazing deterrent. Additionally, it was found that rotenone had a differential effect depending on the growth phase of an autotrophic culture. Altogether, these results suggest that previous uses of rotenone in the field may have disrupted the planktonic food web.

Highlights

Phagotrophic mixotrophs are widely recognised as important members of food webs, but their role in the functioning of food webs is not yet fully understood
The photosynthetic activity of marine phytoplankton is responsible for nearly half of the carbon (C) sequestration by autotrophs on Earth
Most of this C will be processed in the food web by microzooplankton[1,2]; it is currently accepted that a substantial part of this grazing activity might be mediated by phagotrophic “phytoplankton”[3]

Summary

Introduction

Phagotrophic mixotrophs (mixoplankton) are widely recognised as important members of food webs, but their role in the functioning of food webs is not yet fully understood. An immediate step in this direction would be the development of a method that separates mixotrophic from heterotrophic grazing that can be routinely incorporated into the common techniques used to measure microplankton herbivory (e.g., the dilution technique). This idea was explored by the addition of rotenone, an inhibitor of the respiratory electron chain that has been widely used to selectively eliminate metazoans, both in the field and in the laboratory. It was found that rotenone had a differential effect depending on the growth phase of an autotrophic culture These results suggest that previous uses of rotenone in the field may have disrupted the planktonic food web. The major issue regarding mixoplankton is the change in bulk nutrient circulation whereas at the individual level is to elucidate how mixoplankton affect the structure and biodiversity of a community[16]

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