Effect of light and nutrients on phytoplankton phagotrophy: comparison of different techniques to estimate mixotrophy

COSTA, M. R. A.; SARMENTO, H.; BECKER, V.; LACATIVA BAGATINI, I.; UNREIN, F.

Congreso; International Society for Microbial Ecology - Latin America (ISME-LA); 2019

Mixotrophy is a nutritional strategy that combines autotrophy and heterotrophy in the same organism and represents an adaptive strategy in limiting environmental conditions. Although mixotrophy is recognized to be widespread, it is hard to quantify its effects and contribution to carbon flux in aquatic food webs. Thus, it is necessary to develop new methodologies that allow obtaining rapid and accurate results. The goal of this study was to determine grazing rates (GRs) by phytoflagellates in distinct light and nutrient conditions using different protocols, including epifluorescence microscopy (EM) and flow cytometry (FC). We performed short-term grazing experiments by two phytoflagellates (Ochromonas tuberculata and Cryptomonas marssonii) using fluorescent latex beads as preys surrogates, manipulating different conditions of light and nutrients. We also performed experiments using food vacuole markers (Lyso-Tracker - LyT), to investigate mixotrophic metabolism. The comparison of the GRs between FC and microscopy demonstrated a significant and strong relationship (R2 = 0.50, p < 0.001), being FC estimations higher than EM. The highest GRs by Cryptomonas were observed at high nutrient concentrations, which seems to be the most important factor for GR by this species, whereas for Ochromonas GR increased at low light availability. Moreover, the use of LyT showed that both phytoflagellates were actively feeding on preys. We conclude that, at least in culture, the use of flow cytometry is a useful tool to estimate mixotrophy.