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Exploring microbial interactions, functions and diversity under euxinic conditions in Rogoznica Lake (Croatia) - an unique, extreme environment on the Adriatic coast

The closed, isolated small systems, as the representative of a “unique environmental feature”, are valuable natural laboratories for studying different biogeochemical processes. Saline Rogoznica Lake (“Dragon Eye”), situated on the Eastern Adriatic coast is such a system. Rogoznica Lake is a seawater lake located on the Gradina Peninsula (43°32′N, 15°58′E) approximately 100 m from the open Adriatic Sea, close to village Rogoznica on the Dalmatian coast. The lake has a circular shape with an area of 10, 276m2, maximum length of 143 m, and a maximum depth of 15 m. It is sheltered from the wind by 4–23 m high cliffs that prevent wind-shear mixing. Rogoznica Lake can be characterized as a typical extreme, euxinic environment (Ciglenečki et al., 2006). The water column is seasonally stratified. Depth of mixolimnion changes seasonally and it is greatly influenced by meteorological conditions, i.e. temperature and rainfall. Vertical mixing usually occurs during winter when cold, oxygen-rich water from the surface sinks downwards (Ciglenečki et al 2005). From winter to late summer, the less saline top layer (0–10 m) is well oxygenated due to very high biological activity (O2 saturation up to 300%). The more saline bottom layer are enriched with reduced sulfur species i.e. sulfide which appears there in very high concentration (up to 5 mM) (Bura-Nakić et al., 2009). At the boundary oxia-anoxia usually develops a pinky colored chemocline layer, cca 50 cm thick. In this layer the hydrogen sulfide concentration is below 1 μmol L-1 and oxygen concentration is below 5 μmol L-1. The chemocline hosts a dense population of purple phototrophic sulfur bacteria Chromatium (up to 4.3×108 cells mL-1 in July 1997) that store sulfur intracellularly and comprises up to 51% of total bacteria (Ciglenečki et al., 1998). Latest work on microbiological activity in the Lake showed high activity of the green sulfur phototrophic bacteria (Chlorobium sp)( Bura-Nakić et al, 2012). In the present study we collected water and sediment samples in July 2013 and August 2013 at two stations (oxic and anoxic) of the lake: A) at 14 m depth, where permanent anoxic conditions exist in the overlying water column and B) at 8 m depth, which is usually always exposed to the oxic water layer (Figure 1). Water column samples were taken by Niskin bottles in surface, chemocline and bottom layers and the sediment was collected manually and by gravity corers. Samples were analyzed to determine viral and prokaryotic abundance (epifluorescence microscopy), viral and prokaryotic heterotrophic production (by dilution technique and 3H leucine incorporation respectively), viral and prokaryotic diversity ( metagenomic analysis), and extracellular enzymatic activities (by fluorometric analysis). These results will provide new insights into the dynamics, composition and activity of viral and prokaryotic assemblages living under the extreme conditions of the lake of Rogoznica.

Rogoznica Lake ; microbial interaction ; sulfur bacteria

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