Many marine microbial eukaryotes combine photosynthetic with phagotrophic nutrition, but the functional diversity and underlying physiological and metabolic mechanisms within these mixotrophs remain poorly understood. Chrysophytes in particular represent an ideal model group to study strategies of marine mixotrophy in closely related organisms as they contain species with diverse nutritional modes. Here, we developed an experimental system to study responses of mixotrophs to availability of living prey and light and use it to compare the mixotrophic growth strategies of a coastal and an open ocean isolate of the genus Ochromonas. We quantify the relative importance of photosynthesis and ingestion of bacterial prey to the nutrition of both strains and the phenotypic plasticity in this nutritional balance. We further test for the potential interaction or independence of nutritional pathways, by assessing the role of mitochondrial respiration in photosynthesis and the ability to regulate the photosynthetic capacity in response to light intensity and prey availability.
Results/Conclusions
We find contrasting eco-physiological strategies based on large differences in phenotypic plasticity of the two Ochromonas isolates that share 97% 18S rRNA gene identity. The open ocean isolate CCMP1393 is shown to be an obligate mixotroph that requires both light and prey as complementary resources for growth. Its sensitivity to photoinhibition further reduces the niche width of this specialized strategy. The interdependence of photosynthesis and heterotrophy in this strain comprises a significant role of mitochondrial respiration in photosynthetic electron transport. In contrast the coastal isolate CCMP2951 is a facultative mixotroph with high phenotypic plasticity that is able to grow purely heterotrophically in darkness. In this strain photosynthesis and phagotrophy act as substitutable routes of resource acquisition. Effective non-photochemical quenching alleviates photoinhibition and mitochondrial respiration further acts as overflow for photosynthetically produced reducing equivalents at high light intensities. Facultative mixotrophs similar to CCMP2951 will be well adapted to fluctuating environments, while obligate mixotrophs similar to CCMP1393 appear capable of very resource efficient growth in oligotrophic waters.