Award: DEB-1233788

Synechococcus are ecologically-important cyanobacteria that are some of the most abundant phytoplankton in the world?s oceans. These small phototrophic cells utilize sunlight, carbon dioxide and nutrients to fuel growth and production of biomass, which in turn serves as food for single-celled protozoan grazers at the very base of the marine food webs. Grazing on Synechococcus represents one of the crucial ?first steps? in the transfer of nutrients and energy from minute primary producers to organisms that occupy higher trophic levels of the marine food web. Although Synechococcus cells are present year-round in temperate coastal waters, their abundance increases by several orders of magnitude during summertime blooms when environmental conditions become favorable. These blooms are typically short-lived events that are followed by sharp reductions in cell abundance, suggesting control of Synechococcus by a combination of biological factors including predation and viral lysis. Prior to this study, very little was known about the interactions between Synechococcus and the specific grazers that consume Synechococcus in marine food webs. It was not known whether the rise and fall of Synechococcus blooms was due primarily to changes in seasonal environmental conditions (e.g. temperature and sunlight) or due to mortality-based losses inflicted by microbial grazers and viruses. We have documented the seasonal blooms of the two dominant, and genetically-distinct clades of Synechococcus at our coastal time-series station in the Gulf of Maine and estimated their rates of growth and mortality during several stages of the seasonal bloom. Our data suggest that both clades of Synechococcus are impacted proportionally by mortality-based events, with changes in the abundance of the two Synechococcus groups occurring over the same time-scales. These growth and mortality rate estimates contribute to our broader understanding of the ecological role of Synechococcus in the microbial food web and suggest that Synechococcus growth is often balanced by nearly simultaneous losses. Although we focused primarily on estimating Synechococcus mortality due to grazing pressure, losses due to viral lysis were typically lower than grazer-based mortality estimates. DNA sequences obtained from natural samples and experimental seawater incubations suggest that a wide variety of heterotrophic protistan grazers respond to high abundance of Synechococcus. These grazers include a taxonomically diverse array of single-celled microbial eukaryotes including; cercozoans, choanoflagellates, ciliates, cryptophytes, dinoflagellates, picozoans, stramenopiles, and telonemids. It is probable that blooms of Synechococcus help to support a relatively high diversity of protozoan grazers by serving as a broadly-accessible food source. Trends in the overall diversity and composition of the bacterial and protistan community across the spring to fall bloom period suggest a highly-dynamic microbial community at our study site, but one that reassembles with respect to the composition of the assemblage from one year to the next. The genetic diversity of both bacterial and single-celled eukaryote communities attained maximal levels during the fall relative to estimates of diversity for spring and summer microbial assemblages. This study provides novel microbial data for one of the highest resolution time-series studies in the coastal Gulf of Maine, characterizing the genetic diversity at the base of the marine food-web in great detail. In a highly productive ecosystem like the Gulf of Maine, it is crucial to understand all of the linkages and interactions in the marine food-web, from the organisms in the highest trophic levels all the way to the organisms occupying the very base of the food-web, where our research was focused. Our study captured a large amount of the genetic diversity at the base of the food web in the coastal Gulf of Maine and suggests some of the specific microbial interactions that may be occurring around the annual blooms of Synechococcus. This project supported a variety of outreach activities that enhanced the broader impacts of our research. Activities included our participation in Bigelow Laboratory?s Café Scientifique summer lecture series to present "Moving DNA-technology out of the lab into the field and beyond". The goal of this lecture was to familiarize members of the public with DNA technology, its many applications, and its increasing availability to citizen scientists. Additional outreach included participation in ?5th Grade Science Night? at a local elementary school for each of the past three years with a ?plankton research? display table. We trained several dozen undergraduate students and visiting professionals in the specific molecular biology and flow-cytometry techniques that we utilized in this project during short courses at Bigelow Laboratory. Four undergraduate interns received extensive training through their assistance with this project. Two of these students are now pursuing PhD degrees in aquatic microbiology and one is a full-time marine research technician. The associated DNA sequence data for this project will be released under NCBI BioProject number PRJNA420516. Additionally, raw sequence reads from DNA amplicon sequencing, cell counts, and rate measurements will be available via the BCO-DMO data repository under project number 720200. Last Modified: 03/12/2018 Submitted by: Peter Countway