Dissolved organic matter released by two marine heterotrophic bacterial strains and its bioavailability for natural prokaryotic communities.

Marine heterotrophic prokaryotes (HP) play a key role in organic matter processing in the ocean; however, the view of HP as dissolved organic matter (DOM) sources remains underexplored. In this study, we quantified and optically characterized the DOM produced by two single marine bacterial strains. We then tested the availability of these DOM sources to in situ Mediterranean Sea HP communities. Two bacterial strains were used: Photobacterium angustum (a copiotrophic gammaproteobacterium) and Sphingopyxis alaskensis (an oligotrophic alphaproteobacterium). When cultivated on glucose as the sole carbon source, the two strains released from 7% to 23% of initial glucose as bacterial derived DOM (B-DOM), the quality of which (as enrichment in humic or protein-like substances) differed between strains. B-DOM induced significant growth and carbon consumption of natural HP communities, suggesting that it was partly labile. However, B-DOM consistently promoted lower prokaryotic growth efficiencies than in situ DOM. In addition, B-DOM changed HP exoenzymatic activities, enhancing aminopeptidase activity when degrading P. angustum DOM, and alkaline phosphatase activity when using S. alaskensis DOM, and promoted differences in HP diversity and composition. DOM produced by HP affects in situ prokaryotic metabolism and diversity, thus changing the pathways for DOM cycling (e.g. respiration over biomass production) in the ocean.

Impact of an intense water column mixing (0-1500 m) on prokaryotic diversity and activities during an open-ocean convection event in the NW Mediterranean Sea.

Open-ocean convection is a fundamental process for thermohaline circulation and biogeochemical cycles that causes spectacular mixing of the water column. Here, we tested how much the depth-stratified prokaryotic communities were influenced by such an event, and also by the following re-stratification. The deep convection event (0-1500 m) that occurred in winter 2010-2011 in the NW Mediterranean Sea resulted in a homogenization of the prokaryotic communities over the entire convective cell, resulting in the predominance of typical surface Bacteria, such as Oceanospirillale and Flavobacteriales. Statistical analysis together with numerical simulation of vertical homogenization evidenced that physical turbulence only was not enough to explain the new distribution of the communities, but acted in synergy with other parameters such as exported particulate and dissolved organic matters. The convection also stimulated prokaryotic abundance (+21%) and heterotrophic production (+43%) over the 0-1500 m convective cell, and resulted in a decline of cell-specific extracellular enzymatic activities (-67%), thus suggesting an intensification of the labile organic matter turnover during the event. The rapid re-stratification of the prokaryotic diversity and activities in the intermediate layer 5 days after the intense mixing indicated a marked resilience of the communities, apart from the residual deep mixed water patch.

Changes in bacterial community metabolism and composition during the degradation of dissolved organic matter from the jellyfish Aurelia aurita in a Mediterranean coastal lagoon.

Spatial increases and temporal shifts in outbreaks of gelatinous plankton have been observed over the past several decades in many estuarine and coastal ecosystems. The effects of these blooms on marine ecosystem functioning and particularly on the dynamics of the heterotrophic bacteria are still unclear. The response of the bacterial community from a Mediterranean coastal lagoon to the addition of dissolved organic matter (DOM) from the jellyfish Aurelia aurita, corresponding to an enrichment of dissolved organic carbon (DOC) by 1.4, was assessed for 22 days in microcosms (8 l). The high bioavailability of this material led to (i) a rapid mineralization of the DOC and dissolved organic nitrogen from the jellyfish and (ii) the accumulation of high concentrations of ammonium and orthophosphate in the water column. DOM from jellyfish greatly stimulated heterotrophic prokaryotic production and respiration rates during the first 2 days; then, these activities showed a continuous decay until reaching those measured in the control microcosms (lagoon water only) at the end of the experiment. Bacterial growth efficiency remained below 20%, indicating that most of the DOM was respired and a minor part was channeled to biomass production. Changes in bacterial diversity were assessed by tag pyrosequencing of partial bacterial 16S rRNA genes, DNA fingerprints, and a cultivation approach. While bacterial diversity in control microcosms showed little changes during the experiment, the addition of DOM from the jellyfish induced a rapid growth of Pseudoalteromonas and Vibrio species that were isolated. After 9 days, the bacterial community was dominated by Bacteroidetes, which appeared more adapted to metabolize high-molecular-weight DOM. At the end of the experiment, the bacterial community shifted toward a higher proportion of Alphaproteobacteria. Resilience of the bacterial community after the addition of DOM from the jellyfish was higher for metabolic functions than diversity, suggesting that jellyfish blooms can induce durable changes in the bacterial community structure in coastal lagoons.

High contribution of SAR11 to microbial activity in the north west Mediterranean Sea.

We investigated the abundance and activity of SAR11 on a monthly time scale between January 2008 and October 2008 in the oligotrophic NW Mediterranean Sea. Applying MICRO-CARD-FISH, we observed that SAR11 had a large contribution to bulk abundance (37 ± 6% of DAPI-stained cells) and to bulk bacterial heterotrophic production (BHP), as estimated from leucine incorporation (55 ± 15% of DAPI-cells assimilating leucine) in surface waters (5 m) throughout the study period. SAR11 contributed also substantially to the assimilation of glucose, ATP, and a combination of amino acids (44 ± 17%, 37 ± 14%, and 43 ± 12% of DAPI cells assimilating these compounds, respectively), organic compounds that provide either single or combined sources of C, P, and N. Temporal changes in the abundance of SAR11 cells that assimilated leucine, glucose, amino acids, and ATP revealed a pattern consistent with that of substrate-active DAPI cells, suggesting that the activity of SAR11 can explain to a large extent the variability in total cells contributing to the utilization of these compounds. Short-term nutrient enrichment experiments performed on each sampling date revealed a strong co-limitation of at least two of the three elements analyzed (C, N, P), in particular, during summer and early autumn. The in situ abundance of SAR11 cells assimilating leucine appeared to increase with P limitation as determined in the nutrient enrichment experiments (r = 0.81, p = 0.015). Our results demonstrate that SAR11 is an important component of the active bacterial community in the NW Mediterranean Sea. Our observations further indicate that the activity of the bulk bacterial community is linked to the activity of SAR11, possibly due to its adaptation to nutrient limitation.

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

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.