Award: OCE-1851012

Protists are integral to marine food webs and biogeochemical cycles; however, the specific ecological niches for some of the most abundant taxa detected in marker gene libraries, including dinoflagellate Syndiniales parasites, remain poorly understood. Syndiniales can comprise the majority of sequence reads recovered from some picoplankton samples from global surveys. We conducted a high-resolution (sampling ~each 3 days) time series (March-October 2018) in a productive coastal pond (Salt Pond, Falmouth, MA, USA) to examine seasonal shifts in protist populations, including parasitic Syndiniales. At nearly monthly intervals in July, August, and September, during periods of intensifying stratification we observed elevated concentrations of free-living parasite spores and hosts infected with Group II Syndiniales. Infections in some host populations were comparable to populations of previously reported large single-taxon dinoflagellate blooms. Syndinian parasites propagated through mixed protist assemblages over the timeframe of the study, highlighting patterns of host/parasite interactions likely typical of many other marine environments where single taxon blooms are uncommon. This study revealed a diversity of syndinian and host morphotypes, exhibiting both specific and generalist parasitic infections. In contrast to our results from Salt Pond, where Group II Syndiniales dominated parasite diversity and observed infections, at a nearshore coastal observatory site (Martha?s Vineyard Cabled Observatory, (MVCO)), Group I Syndiniales dominated the parasite marker gene sequences throughout that seasonal time series. Our field observations were combined with data from culture-based studies of Syndiniales parasites to model infection dynamics and their impact on release of nutrients for our high-resolution study of Salt Pond. Two different sized dinoflagellate hosts were examined in cultures to measure nutrients released during infection events. We found that equations described by Solomon and Stolte for host-parasite population dynamics can be applied to model Syndiniales infections in Salt Pond when populations of hosts and parasites exhibit a ?boom-bust? relationship. When host-parasite relationships do not exhibit this ?boom-bust? relationship, and parasites maintain low levels of infections in host populations, the Salomon and Stolte equations cannot be applied to model those Syndiniales infections. Rough estimates of carbon and nitrogen released during two boom-bust infection events in Salt Pond were 1130ug C/L and 920ug C/L of dissolved organic carbon, 1.6ug/L and 1.3ug/L of nitrate and nitrite, and 4.4ug/L and 3.6ug/L ammonium, respectively.Contributions to pools of dissolved and particulate organic matter likely differ for hosts of varying sizes, warranting further study and quantification to constrain the boundaries of these potential differences and to refine these models. One graduate student at WHOI was supported by the project and she successfully defended her thesis in 2021. Three undergraduate students received training in microscopy and field work, and several high school students participated in field work. Presentations on the importance of protist parasites in the marine food web and implications for pools of nutrients including dissolved and particulate organic carbon were made at national and international meetings, and to groups ranging from high school students to graduate students and postdocs. Last Modified: 04/01/2023 Submitted by: Virginia P Edgcomb