Impact of lower salinity waters on bacterial heterotrophic production and community structure in the offshore NW Mediterranean Sea

Abstract

We investigated the impact of water masses originating from freshwater input on bacterial heterotrophic metabolism and community structure at an offshore site in the oligotrophic NW Mediterranean Sea in 2007 and 2008. By combining 16S rRNA gene clone libraries and MICRO-CARD-FISH we determined the dominant operational taxonomic units (OTU) and their contribution to bulk abundance and activity in the presence of buoyant water masses characterized by lower salinity (LSW, < 37.9) and compared these with the winter and spring phytoplankton blooms. We demonstrate that organic matter associated with LSW markedly stimulated bacterial heterotrophic production as determined by [(3) H]-leucine incorporation. The OTUs SAR11-IA, SAR11-IIB, SAR86-I and SAR86-III were dominant in all clone libraries, while the Roseobacter clade and the Bacteroidetes OTU NorSea72 were more specific to the spring phytoplankton bloom. The relative contribution of these OTUs to leucine incorporation varied between 23% and 69% for SAR11, 2% and 17% for Roseobacter and was up to 4% for NorSea72. Together, they accounted for roughly 50% of bulk abundance and leucine incorporation during the four situations investigated. Our results suggest that a few cosmopolitan OTUs respond to different DOM sources in the NW Mediterranean Sea.

A method for the use of the radiotracer55Fe for microautoradiography and CARD-FISH of natural bacterial communities

“…Our results strongly suggest that the 55 Fe quota is a critical parameter for the production of useful microautoradiograms of heterotrophic bacteria, as was already pointed out by Fuhrman & Azam (). For 3 H, a frequently used radioisotope that emits electrons with similar energy as 55 Fe, this issue was never problematic because the activity per cell is estimated in the range 7–14 × 10 −3 dpm per cell, based on data from the same study area (Laghdass et al ., ), and therefore much higher than the per cell activity observed in the present study.…”

Structure and variability of a microbial community at a transect crossing a double gyre structure in the northeastern Adriatic Sea

“…Since these waters differed in salinity and not in phytoplankton biomass, it seems, in accordance with Laghdass et al (2010), that the lower salinity waters, containing riverine discharges of organic matter, stimulated prokaryotic activities. This influence was apparently propagated into deeper layers of the waters below P. Thus, the surface gyres CG and AG, which typically appear in the northern Adriatic and are persistent for several months or even more, appear to be 'hotspots' of prokaryotic activities.…”

“…The modification in the concentrations and nature of DOC are due to biological and mixing processes (Christaki et al 2009). These changes in DOC are more pronounced if transit time of freshened waters from source areas into open-sea lower-salinity water structures is longer (Laghdass et al 2010). However, since the transit times of freshened waters from the Po delta to areas in which the AG or CG are located are approximately similar, in this case the heterogeneity in organic matter between gyres could result Leu…”

Effects of Saharan dust on the microbial community during a large in situ mesocosm experiment in the NW Mediterranean Sea

“…The more specific probes used in this study were SAR11-152R, SAR11-441R, SAR11-542R, SAR11-732R (SAR11 cluster) (Morris et al 2002), Ros537 (Roseobacter-Sulfitobacter-Silicibacter group) (Eilers et al 2001), SAR86 (SAR86 cluster) (Eilers et al 2000) and ALT1413 (Alteromonas group) (Eilers et al 2000). The recently designed probe NorSea72_88 (5'-GCC ACT CGT CAT CAG ATT-3') and 2 helper probes NorSea72_69H (5'-CCA AGC AAT CCA TGT TAC-3' and NorSea72_ 107H (5'-GCG TTA CGC ACC CGT GC-3') (NorSea72 group belonging to Bacteroidetes) (Laghdass et al 2010) were also applied in the present study. Two probes were specifically designed to target the group Thiothrix belonging to Gammaproteobacteria (Thiothrix_577a and Thiothrix_577b) and a part of the NS5 cluster (NS5-1) within Bacteroidetes (NS5_175) (see Table 2).…”