Award: OCE-1436169

This project investigated the role of mixotrophy in marine algae. Mixotrophy involves the combination of eating (phagotrophy) and photosynthesis in the same organism, therefore combining animal and plant like characteristics. This phenomenon is commonly practiced in marine phytoplankton and is a key mechanism for acquiring limiting nutrients in aquatic environments. However, the phenomenology and role of feeding in mixotrophic protists varies widely, even among closely related species. This project investigated the role of mixotrophy in marine dinoflagellates and chrysophytes. In the coastal marine dinoflagellate, Prorocentrum minimum, mixotrophic grazing on cryptophyte algae was induced during nutrient limitation, and did not occur when inorganic nutrients were available or when cells were in stationary phase. Feeding in P. minimum was most strongly induced by limitation of phosphorus (P), followed by nitrogen (N), and only weakly so when starved of both N and P simultaneously. The dinoflagellate grew at its maximum rate when cultured phototrophically and given sufficient nutrients. However, mixotrophic grazing on bacteria occurred constitutively, and was most prominent during stationary phase of growth. Feeding on eukaryotic (cryptophyte) prey was induced only when cells were highly stressed, and had already accrued damage to their chloroplast photosystems. Feeding on cryptophyte prey helped to repair chloroplasts, their ability to photosynthesize, and return P. minimum cells to their maximum growth rate. Grazing rates by P. minimum on cryptophyte prey varied greatly among strains of the dinoflagellate, which were isolated from a variety of locations. In general, the longer a P. minimum isolate was in culture the less it fed. In contrast to P. minimum, feeding by the marine Chrysophyte Ochromonas, on either eukaryotic or prokaryotic prey, was a constitutive process that maximized growth rate. Mixotrophic feeding rates among three strains of Ochromonas were very different, but all increased with prey abundance and growth irradiance levels. These data were used to inform mathematical models aimed at understanding the interplay of light and prey availability on grazing and growth rates in Ochromonas. The information gained from this project helped to further understand the role of mixotrophy in marine phytoplankton, and the cellular and environmental factors that regulate it. This project provided research training for a postdoctoral scientist and undergraduate students. It also helped to fund educational activities for middle school and high school students to learn about oceanography, marine ecology, and microbial interactions in marine food webs. Student training involved boat field trips in coastal waters and classroom activities. Last Modified: 12/20/2018 Submitted by: Matthew D Johnson