Lessons learned on toxic cyanobacterial blooms from studying the microbial ecology of Lake Okeechobee

Lake Okeechobee, the largest freshwater lake in the southeastern United States, has both economic and ecological importance, yet recurring toxic cyanobacterial blooms continue to have negative impacts that expand to coastal oceans. Our studies over the past several years have aimed to better understand the drivers of the toxic blooms that occur that include, but expand beyond, nutrient inputs. We investigated this through studying the microbial biodiversity of the lake, which encompasses both the diversity and function of the microbial communities, using a suite of molecular techniques, including 16S rRNA sequencing, metagenomics, and metatranscriptomics, on samples collected from Lake Okeechobee and on a major outflow, the Caloosahatchee River, over the course of three consecutive bloom seasons. Analysis of the 16S rRNA sequencing showed that microbial communities reflected the ecological zones of Lake O, which are distinct in their physical and chemical properties. These observations also revealed bloom hotspots at major inflows in the northwest portion of the lake. Metagenomes and 16S rRNA sequences from Lake O over three bloom seasons revealed that the cyanobacterial communities in Lake O were diverse, yet a genetically identical toxic Microcystis, responsible for the most expansive blooms, persisted over three years. We identified key functional traits of Microcystis and other bloom formers that contributed to their success over competing taxa, which included phage defense and storage of nitrogen (N). Further evidence that N dynamics and storage of nutrients were important was shown through functional analysis of microbial communities associated with blooms where genes involved in organic N and phosphorus (P) utilization and carbon and P storage were enriched. Experimental mesocosms conducted on the Caloosahatchee River to investigate the influence of nutrients demonstrated that bloom forming cyanobacteria had stronger responses to N additions over P, especially in early and mid-bloom season. Together, these results can be used to help inform bloom management practice on Lake O, and they have expanded our physiological knowledge of toxic cyanobacterial blooms in freshwater ecosystems.