Prokaryotic aminopeptidase activity along a continuous salinity gradient in a hypersaline coastal lagoon (the Coorong, South Australia)

Dissolved proteins and peptides are important sources of energy and nitrogen in aquatic systems [1,2], but they must be hydrolysed to amino acids and oligopeptides to be useable by prokaryotes. Following the development of sensitive methods using fluorogenic substrates [3], proteolytic activity in natural aquatic systems has been assessed by measuring the activity of leucine-aminopeptidase as a model enzyme [4]. However, microbial cells living in aquatic systems are influenced by a variety of environmental factors which affect the molecular control of their enzyme synthesis. Among these variables, salinity has been identified as a major driving force in both the composition of bacterioplankton and their efficiency in degrading dissolved organic carbon (DOC) [5]. Previous studies focusing on the effect of salinity on the composition and metabolic activity of bacterial communities were mainly conducted in estuaries where salinity typically did not exceed 5% [6] and the effect of higher salinity conditions was mainly investigated in highly saline ponds from solar salterns [7]. To our knowledge, little is still known about the dynamic of prokaryotic aminopeptidase activity along natural continuous hypersaline gradients. The objective of this study was to investigate the changes in aminopeptidase activity of prokaryotic communities identified using flow cytometry from brackish to hypersaline waters.The Coorong is a South Australian shallow coastal lagoon characterized by a strong salinity gradient with salinity continuously ranging from brackish (1.8%) to hypersaline (15.5%). Constrained between the last interglacial dune and the modern dune that has been established from the mid-holocene, this lagoon receives inputs from the ocean through the Murray Mouth and from underground and freshwater inputs from Lake Alexandrina and Lake Albert, which are the terminal system of the River Murray (Fig. 1). If freshwater inputs lead to lower salinities in the northwest part of the C