It may seem a bit incongruous that in this age of great advances everywhere in the microbial world, we must pause to learn the names of more and more microbes. The molecular biologists of old (that’s fifty years ago!) needed to recognize only a few names such as E. coli, B. subtilis, and, if broadminded, a few others. (I remember how one of the distinguished founders of the field had written on a corner of his blackboard: E. coli = Gram-negative.) No longer so. Phylogeny, ecology, and the likes make new demands on us. We now have to learn about such creatures as the planctomycetes, acidobacteria, and verrucomicrobia, to rattle off just a few. I worry about my brain capacity, but, on the other hand, am glad that all this leads us to revisit what Lynn Margulis called The Garden of Microbial Delights. I intend to occasionally post very brief taxonomic pieces on selected bacterial groups. I may not be fully attentive to the taxon level and may mix up genera and higher taxa.
Currently, the roseobacters are a well-studied group of marine bacteria, and deservedly so. They make up 25% of the bacterial biomass in some coastal marine waters from the tropics to the poles. Globally they play important roles in the Earth's carbon and sulfur cycles. Their sulfur story is particularly interesting. Here, roseobacters do not act in isolation but they engage in symbioses with members of the phytoplankton. These bacteria convert a compound made in abundance by microalgae, dimethylsulfoniopropionate (DMSP), into the volatile dimethylsulfide (DMS), which participates in cloud formation.
Roseobacters are a diverse lot that are classified within the α-proteobacteria (family Rhodobacteraceae). The group includes some with gas vacuoles, some with holdfasts, and others with their own morphological singularities. They were not well known to science until 1991 when R. litoralis OCh149 and R. denitrificans OCh114, two photosynthesizing strains, both of which produce pink-pigmented bacteriochlorophyll a, were isolated from marine algae. Other roseobacters make colonies of different colors including, brown, pink, yellow, and green.
To give an example of what’s going on in the Roseobacter field, a recent paper discusses the discovery of complex aromatic compounds made by these bacteria, some of which stimulate the growth of their algal partner, Emiliania huxleyi, others that kill it. The toxic substances, called roseobacticides, are produced in response to compounds released by damaged or senescent algae. The authors appropriately refer to these roseobacters as Jekyll and Hyde bacteria. As you can see, the roseobacters present us with cogent examples of important environmental phenomena, distinctive symbiotic relationships, and engrossing aspects of cell physiology.