Members of the marine Roseobacter lineage have been characterized as ecological generalists, suggesting that there will be challenges in assigning well-delineated ecological roles and biogeochemical functions to the taxon. To address this issue, genome sequences of 32 Roseobacter isolates were analyzed for patterns in genome characteristics, gene inventory, and individual gene/pathway distribution using three predictive frameworks: phylogenetic relatedness, lifestyle strategy and environmental origin of the isolate. For the first framework, a phylogeny containing five deeply branching clades was obtained from a concatenation of 70 conserved single-copy genes. Somewhat surprisingly, phylogenetic tree topology was not the best model for organizing genome characteristics or distribution patterns of individual genes/pathways, although it provided some predictive power. The lifestyle framework, established by grouping isolates according to evidence for heterotrophy, photoheterotrophy or autotrophy, explained more of the gene repertoire in this lineage. The environment framework had a weak predictive power for the overall genome content of each strain, but explained the distribution of several individual genes/pathways, including those related to phosphorus acquisition, chemotaxis and aromatic compound degradation. Unassembled sequences in the Global Ocean Sampling metagenomic data independently verified this global-scale geographical signal in some Roseobacter genes. The primary findings emerging from this comparative genome analysis are that members of the lineage cannot be easily collapsed into just a few ecologically differentiated clusters (that is, there are almost as many clusters as isolates); the strongest framework for predicting genome content is trophic strategy, but no single framework gives robust predictions; and previously unknown homologs to genes for H₂ oxidation, proteorhodopsin-based phototrophy, xanthorhodpsin-based phototrophy, and CO₂ fixation by Form IC ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) expand the possible mechanisms for energy and carbon acquisition in this remarkably versatile bacterial lineage.

Dimethylsulfoniopropionate (DMSP) accounts for most of the organic sulfur fluxes from primary to secondary producers in marine microbial food webs. Incubations of natural communities and axenic cultures with radio-labeled DMSP showed that dominant phytoplankton groups of the ocean, the unicellular cyanobacteria Prochlorococcus and Synechococcus and diatoms, as well as heterotrophic bacteria take up and assimilate DMSP sulfur, thus diverting a proportion of plankton-produced organic sulfur from emission into the atmosphere.

Stratified sulfurous lakes are appropriate environments for studying the links between composition and functionality in microbial communities and are potentially modern analogs of anoxic conditions prevailing in the ancient ocean. We explored these aspects in the Lake Banyoles karstic area (NE Spain) through metagenomics and in silico reconstruction of carbon, nitrogen and sulfur metabolic pathways that were tightly coupled through a few bacterial groups. The potential for nitrogen fixation and denitrification was detected in both autotrophs and heterotrophs, with a major role for nitrogen and carbon fixations in Chlorobiaceae. Campylobacterales accounted for a large percentage of denitrification genes, while Gallionellales were putatively involved in denitrification, iron oxidation and carbon fixation and may have a major role in the biogeochemistry of the iron cycle. Bacteroidales were also abundant and showed potential for dissimilatory nitrate reduction to ammonium. The very low abundance of genes for nitrification, the minor presence of anammox genes, the high potential for nitrogen fixation and mineralization and the potential for chemotrophic CO₂ fixation and CO oxidation all provide potential clues on the anoxic zones functioning. We observed higher gene abundance of ammonia-oxidizing bacteria than ammonia-oxidizing archaea that may have a geochemical and evolutionary link related to the dominance of Fe in these environments. Overall, these results offer a more detailed perspective on the microbial ecology of anoxic environments and may help to develop new geochemical proxies to infer biology and chemistry interactions in ancient ecosystems.

Summary: The ecological significance of the marine bacterial populations distinguishable by flow cytometry on the basis of the fluorescence (FL) of their nucleic acid (NA) content and proxies of cell size (such as side scatter, SSC) remains largely unknown. Some studies have suggested that cells with high NA (HNA) content and high SSC (HS) represent the active members of the community, while the low NA (LNA) cells are inactive members of the same phylogenetic groups. But group‐specific activity measurements and phylogenetic assignment after cell sorting have suggested this is not be the case, particularly in open‐ocean communities. To test the extent to which the different NA subgroups are similar, and consequently the extent to which they likely have similar ecological and biogeochemical roles in the environment, we analysed the phylogenetic composition of three populations after cell sorting [high NA‐high SC (HNA‐HS), high NA‐low SC (HNA‐LS), low NA (LNA)] by 454 pyrosequencing in two contrasting periods of the year in NW Mediterranean coastal waters (BBMO, Blanes Bay Microbial Observatory) where these three populations have recurrent seasonal patterns. Statistical analyses showed that summer and winter samples were significantly different and, importantly, the sorted populations within a sample were composed of different taxa. The majority of taxa were associated with one NA fraction only, and the degree of overlap (i.e. OTUs present simultaneously in 2 fractions) between HNA and LNA and between summer and winter communities was very small. Rhodobacterales , SAR116 and Bacteroidetes contributed primarily to the HNA fraction, whereas other groups such as SAR11 and SAR86 contributed largely to the LNA fractions. Gammaproteobacteria other than SAR86 showed less preference for one particular NA fraction. An increase in diversity was observed from the LNA to the HNA‐HS fraction for both sample dates. Our results suggest that, in Blanes Bay, flow cytometric signatures of natural communities track their phylogenetic composition.