Research progresses of marine Roseobacter lineage

Marine Roseobacter clade is a phylogenetically coherent,but physiologically diverse group of α-Proteobacteria,with members sharing 89% identity of the 16 S rRNA gene. It contains more than 40 different genera. Roseobacter was found to be abundant in marine environment,especially in coastal and polar oceans,where Roseobacter clade comprises 15% —25% of the total bacterioplankton communities. Roseobacters also exist ubiquitously in the ocean. Based on culture collections,16 S rRNA clone libraries, and single-cell analyses, roseobacters have been identified in most marine environment sampled,such as sea ice,sea floor,hypersaline microbial mats,sponges,sea grasses,and coastal biofilms,etc. whereas,this bacterial group is noticeable absent in analogous freshwater and terrestrial soil environment. Quantitative 16 S rRNA gene analyses show that Roseobacter populations fall off with depth in the ocean,and are often most abundant in bacterial communities associated with marine algae. Roseobacter lineage is the only abundant marine group,whose cultivated representatives are closely related to the not-yet-cultivated environmental clone sequences. Diverse life styles have been found in Roseobacter clade, such as free living, particle associated, or in commensal relationships with marine phytoplankton,invertebrates,and vertebrates. Also,diverse physiological metabolic traits were found in this bacterial group. For example, some roseobacters can generate metabolic energy from light using the ancient purple bacterial mechanism of anaerobic photosynthesis without production of oxygen( anaerobic anoxygenic photosynthesis),while someroseobacters have the physiological metabolic traits of degradation of sulfur compounds or aromatics,oxidation of carbon monoxide,or reduction of trace metals,etc. Thus,it is considered that roseobacters may play important roles in marine carbon and sulfur cycles,as well as the global climate regulation. There are more than forty roseobacters whose whole genome sequences are available in the current public databases. Analysis of these genomes also reflected the metabolic versatility of the Roseobacter lineage. The cellular requirements of roseobacters for nitrogen are largely satisfied by regenerated ammonium and organic compounds( polyamines,allophanate,and urea),while the requirement for carbon sources are satisfied by amino acids,glyoxylate,and aromatic metabolites. Also,a large number of genes are predicted to encode proteins involved in the production,degradation,and efflux of toxins and metabolites,suggesting the potential for interacting with neighboring cells and impacting the routing of organic matter into the microbial loop. Laboratory experiments found that the various bioactive secondary metabolites of roseobacters include antagonist against fish larval bacterial pathogens,antibiotic against marine bacteria and algae,shellfish poison,and the bioactive LuxR- activating acylated homoserine lactones( AHLs),which are a class of signaling molecules involved in bacterial quorum sensing,etc. Recently,the interactions between roseobacters and their phages caused widespread attention. Several host-phage interaction systems of the roseobacters were set up in the laboratory. Laboratory experiments revealed that roseophages play important roles in mediating the physiology and promoting the evolution of roseobacters in marine environment. This review described the recent research progresses of Roseobacter lineage in terms of their ecological distribution,lifestyle,physiological functions,and genome features. Finally we suggested future research directions based on our understanding of the literature and our own work.