Shifts of Bacterioplankton Metabolic Profiles along the Salinity Gradient in a Subtropical Estuary

Understanding the biodegradation potential of river bacterioplankton communities is crucial for watershed management. We investigated the shifts in bacterioplankton metabolic profiles along the salinity gradient of the Caloosahatchee River Estuary, Florida. The carbon source utilization patterns of river bacterioplankton communities were determined by using Biolog EcoPlates. The number of utilized substrates was generally high in the upstream freshwater dominated zone and low in the downstream zone, suggesting a shift in metabolic profiles among bacterioplankton assemblages along the estuarine gradient. The prokaryotic cell numbers also decreased along the estuarine salinity gradient. Seasonal and site-specific differences were found in the numbers of utilized substrates, which were similar in summer and fall (wet season) and winter and spring (dry season). Bacterioplankton assemblages in summer and fall showed more versatile substrate utilization patterns than those of winter and spring communities. Therefore, our data suggest that microbial metabolic patterns in the subtropical estuary are likely influenced by the water discharge patterns created by dry and wet seasons along the salinity gradient. 1. Introduction An estuary is a partially enclosed body of water along the coast where freshwater from rivers mixes with saltwater from the ocean. Estuarine environments are among the most productive regions on Earth and create habitats hosting unique floral and faunal communities especially adapted for life at the margin of the sea [1]. Various habitat types are found in and around estuaries and serve as places for organisms to live, feed, and reproduce. Estuaries are among the most heavily populated areas; consequently, life-forms routinely suffer substantial habitat loss and alteration as well as exposure to various forms of anthropogenic pollution, including chemical contaminants and eutrophication, one of the most significant issues facing coastal zone management [2]. Bacterioplankton communities in riverine and estuarine environments are responsible for water purification processes and are influenced by watershed usage [3–8]. Shifts in microbial communities along the salinity gradient formed by the mixing of freshwater and seawater are commonly observed in estuaries [9–13]. Although microbial communities are an important metabolic component in estuaries, their metabolic versatilities are poorly understood. Therefore, physiological profiling of microbial populations is an important subject for a better understanding of the biodegradation potential of