Phytoplankton phagotrophy across nutrients and light gradients using different measurement techniques

Abstract

Mixotrophy is important to ecosystems functioning. Assuming that limiting resources induce phagotrophy in mixotrophs, we used a factorial experimental design to evaluate how nutrient and light affects phagotrophy in two mixotrophic phytoflagellates belonging to different lineages. We estimated cell-specific grazing rates (CSGR) by analyzing prey ingestion using microscopy and flow cytometry (FC). Furthermore, we tested if the acidotropic probe LysoTracker green (LyTG) can be used to differentiate autotrophs from mixotrophs. Cryptomonas marssonii (cryptophyte) had higher CSGR in high-nutrient treatments. Although it seems counterintuitive, phytoflagellates likely uses phagotrophy to obtain organic growth factors instead of inorganic nutrients when photosynthesis is more favorable. In contrast, CSGR in Ochromonas tuberculata (chrysophyte) increased when light decreased, suggesting that it uses phagotrophy to supplement carbon when autotrophic growth conditions are suboptimal. Measurements of CSGR obtained by FC and microscopy were significantly correlated and displayed the same trend among treatments, although FC rates tended to be higher. Fluorescence with LyTG did not differ from the control in the non-phagotrophic chlorophyte. Contrarily, addition of LyTG significantly increased the fluorescence in chrysophytes and cryptophytes, although no differences were observed among treatments. This approach allowed for differentiation between phagotrophic and non-phagotrophic flagellates but failed to quantify mixotrophy.