Low rates of bacterivory enhances phototrophy and competitive advantage for mixoplankton growing in oligotrophic waters

With climate change, oceans are becoming increasingly nutrient limited, favouring growth of prokaryotic picoplankton at the expense of the larger protist plankton whose growth support higher trophic levels. Constitutive mixoplankton (CM), microalgal plankton with innate phototrophic capability coupled with phagotrophy, graze on these picoplankton, indirectly exploiting the excellent resource acquisition abilities of the prokaryotes. However, feeding rates can be very low (e.g., a few bacteria d−1). For the first time, the significance of such low consumption rates has been quantified. We find that while prokaryote-carbon (C) supply to CM grown at non-limiting light was so low that it may appear insignificant (<10%), contributions of nitrogen (N) and phosphorus (P) from ingestions of 1–12 prokaryotes d−1 were significant. Under limiting light, contributions of ingested C increased, also raising the contributions of N and P. The order of nutritional importance for CM growth from predation was P>N>C. Further, provision of N through internal recycling of ingested prey-N stimulates C-fixation through photosynthesis. Importantly, coupled photo-phago-mixoplanktonic activity improved CM resource afinities for both inorganic and prey-bound nutrients, enhancing the nutritional status and competitiveness of mixoplankton. With warming oceans, with increased prokaryote abundance, we expect CM to exhibit more phagotrophy