SALINE SYSTEMS Pollet et al. Saline Systems 2010, 6:5 http://www.salinesystems.org/content/6/1/5 Open Access SHORT REPORT © 2010 Pollet et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Short Report Prokaryotic aminopeptidase activity along a continuous salinity gradient in a hypersaline coastal lagoon (the Coorong, South Australia) Thomas Pollet*1,2, Mathilde Schapira 1,3 , Marie-Jeanne Buscot 1 , Sophie C Leterme 1,4 , James G Mitchell 1 and Laurent Seuront 1,4,5,6 Abstract The distribution and aminopeptidase activity of prokaryotes were investigated along a natural continuous salinity gradient in a hypersaline coastal lagoon, the Coorong, South Australia. The abundance of prokaryotes significantly increased from brackish to hypersaline waters and different sub-populations, defined by flow cytometry, were observed along the salinity gradient. While four sub-populations were found at each station, three additional ones were observed for 8.3% and 13.4%, suggesting a potential modification in the composition of the prokaryotic communities and/or a variation of their activity level along the salinity gradient. The aminopeptidase activity highly increased along the gradient and salinity appeared as the main factor favouring this enzymatic activity. However, while the aminopeptidase activity was dominated by free enzymes for salinities ranging from 2.6% to 13.4%, cell-attached aminopeptidase activity was predominant in more saline waters (i.e. 15.4%). Changes in substrate structure and availability, strongly related to salinity, might (i) modify patterns of both aminopeptidase activities (free and cell- associated enzymes) and (ii) obligate the prokaryotic communities to modulate rapidly their aminopeptidase activity according to the nutritive conditions available along the gradient. Findings 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], prote- olytic activity in natural aquatic systems has been assessed by measuring the activity of leucine-aminopep- tidase 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 composi- tion 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 condi- tions was mainly investigated in highly saline ponds from solar salterns [7]. To our knowledge, little is still known about the dynamic of prokaryotic aminopeptidase activ- ity 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 hyper- saline 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 Mur- ray (Fig. 1). If freshwater inputs lead to lower salinities in the northwest part of the Coorong, the excess in evapora- * Correspondence: thomas.pollet@thonon.inra.fr 1 School of Biological Sciences, Flinders University, GPO Box 2100, Adelaide SA 5001, Australia Full list of author information is available at the end of the article