Seasonality of SAR Clades in a Temperate, Estuarine Environment

SAR clades, including SAR11, are the most abundant microorganisms in the modern ocean. Despite their ubiquity, studies of SAR groups often center around oligotrophic ocean environments, leaving the roles of these organisms in eutrophic waters understudied. We examined the dynamics of the abundant SAR11, SAR86, SAR324, and SAR202 clades in an eutrophic estuary in Georgia, United States. Over one year, weekly sampling of water followed by 16S rDNA sequencing revealed these clades to be among some of the most abundant orders, as well as among the most commonly occurring ASVs across seasons. The temporal nature of the study allowed for the tracking of seasonal dynamics of these clades and their specific ASVs. Additionally, co-occurrence networks were constructed to find which organisms SAR groups correlate with, both positively and negatively. We found that SAR11 reads were abundant across all sampling dates, whereas SAR86, SAR324, and SAR202 clades were more present in warmer temperatures ( 20-31०C ) . Diversity of SAR ASVs showed a similar trend, within each order, ASVs increased in diversity as temperature increased. SAR11 ASVs showed a high level of co-occurrences both positive and negative, ( SparCC value > 0.6 or < -0.6) but with a higher number of connections in the spring and fall relative to the summer and winter months. This study demonstrates the vast seasonal diversity of SAR groups in estuarine environments, and suggested such diversity is controlled by seasonal factors. Further temporal studies of these groups in eutrophic environments will help predict how these abundant marine organisms may respond to changing oceans.