Terrestrial plants have diverse and sophisticated mechanisms for defending against predation and pathogens, but the defenses of photosynthetic microbes in aquatic ecosystems are much less well understood. Synechococcus are aquatic/marine cyanobacteria that substantially contribute to global primary productivity. This project will employ two main approaches to study Synechococcus defenses against protist predators. Experiments will examine the mechanism(s) by which cell surface proteins provide a constitutive (continuously expressed) defense and assay the generality of this defense among Synechococcus strains and against diverse predator types. Secondly, co-culture techniques, in which Synechococcus are grown in the presence of predators, will be used to characterize protein-based induced defenses that may arise in response to predation. The formation of aggregates as a possible induced defense by Synechococcus will also be examined in field and laboratory settings. Aggregate formation is important for the biogeochemistry and ecological functioning of planktonic communities since aggregates can be the site of unique biogeochemical processes. The research is cross-cutting on a number of levels: it examines constitutive and induced defenses in both controlled laboratory and natural settings; it leverages the suite of available Synechococcus genomes to enable a mechanistic investigation of microbial defense processes; and it represents a synthesis of ecological and genomic approaches to the study of microbial interactions that are fundamental to biogeochemical cycling and the maintenance of biodiversity in aquatic ecosystems.
Broader impacts of this project include support of graduate and undergraduate students, including ethnic minority participants in Shannon Point Marine Center's Multicultural Initiatives in Marine Science Undergraduate Program (MIMSUP). Other impacts include development of instructional modules for elementary and high school students, and maintenance and distribution of culture collections.
Unicellular cyanobacteria are at the base of marine food chains. Their distributions and abundances are controlled in part by protist predators. The goal of this project was to determine whether unicellular cyanobacteria have developed defense mechanisms against predators. We focused on the cyanobacterial cell surface as this is the site of first encounter, and found that cell surface proteins are important in determining the susceptibility to predation. In addition, we discovered that certain of these cyanobacteria produce defensive, antibiotic-like compounds (MicrocinC-like) that inhibit the growth of closely related species. This had not been shown before, and in addition we showed that the mechanism of production was contact-dependent. This is a novel mechanism that has not been described in cyanobacteria before. In addition, at least one strain of cyanobacteria has an enzyme that allows it to produce bromoform, an environmentally relevant greenhouse gas, again a novel finding. Taken together, these results have provided important insights into ecological strategies employed by these primary producers in response to other microbes. These insights will inform our understanding on the growth, distributions, and ecological consequences of large numbers of unicellular cyanobacteria in the marine environment. As part of this work, three undergraduate students were trained in carrying out microbiological research, a graduate student completed a doctoral thesis, and two postdoctoral researchers carried out research and published their work.