Award: OCE-1559356

In a simplified view of the planktonic food web, there are two main roles: 1. primary producers that employ photosynthesis to grow (phytoplankton) and 2. consumers that rely on the phytoplankton as a source of food (bacterial decomposers, protists, animals). Often the primary producers and the consumers are thought of as distinct organisms. However, many phytoplankton in the open ocean do not just photosynthesize like plants, they are also predators that eat other cells in a battle for the energy and nutrients they need to survive (like single-cell versions of a venus fly trap). There has been growing appreciation that these predatory phytoplankton, known as mixotrophs (because they are a "mix" of photosynthetic and predatory modes), are very common and exceptionally diverse. Furthermore, like any planktonic organism, a mixotroph can itself be eaten or can be infected by viruses, adding to the complexity of the planktonic food web. Despite the importance of these types of microscopic plankton, very few of them have been cultivated and studied under controlled conditions in the lab. There is also very limited information about the viruses that infect them. Intellectual merit: Support from this grant enabled significant advances in our understanding of the ecology of plankton in the open ocean. We took two complementary approaches: The first was to synthesize and model existing field and lab data on the properties of mixotrophs and different types of viruses to predict when and where they should thrive. The second was to cultivate, and perfom experiments on, diverse mixotrophs in order to fill in major gaps in our knowledge about how they live, grow, and die. In the course of cultivating a wide variety of different mixotrophic phytoplankton, we also isolated and investigated the viruses that infect them. The first approach led to advances in our theoretical understanding of plankton ecology and resulted in publications with specific predictions that can be tested experimentally. For example, we found that mixotrophs should be favored when nutrients are limiting but light is plentiful, and that the largest viruses should infect organisms in the open ocean that eat bacteria. The second approach led to the most comprehensive, quantitative dataset on grazing for a wide variety of different mixotrophs. Many of these mixotrophs are from groups that are widespread in the ocean but had never before been studied in the lab, or had never before been shown to be mixotrophs. Our cultivation efforts also resulted in the isolation and description of a broad spectrum of new marine viruses, from one of the smallest to some of the largest phytoplankton viruses on record. The latter are remarkable "giant" viruses that produce viral particles with dimensions and genome sizes similar to those of a small bacterium, and which code for an unusually large number of metabolic genes. Broader Impacts: The efforts supported by this grant to cultivate and characterize new phytoplankton and their viruses led to new lines of research and discoveries beyond the initial focus. In one instance, our giant virus that infects a green alga was provided upon request to a group interested in genetic engineering for improved algal biofuel production. In a second instance, the genomic sequence of one of our giant virus isolates led to new exprimental work on the biophysics of potassium ion channels. In a third instance, our cultivation efforts led to the isolation of two new species of open-ocean diatoms capable of fixing nitrogen. This project also supported two post-doctoral researchers, one of whom has moved on to a tenure-track faculty position, the other has been granted Research Affiliate staus in the department allowing promotion to co-PI status on grants. The project has also supported the research of several graduate and undergraduate students, including a member of a group underrepresented in the sciences. Last Modified: 05/03/2021 Submitted by: Grieg F Steward