Cross-exchange of B-vitamins underpins a mutualistic interaction between Ostreococcus tauri and Dinoroseobacter shibae.

Ostreococcus tauri, a picoeukaryotic alga that contributes significantly to primary production in oligotrophic waters, has a highly streamlined genome, lacking the genetic capacity to grow without the vitamins thiamine (B1) and cobalamin (B12). Here we demonstrate that the B12 and B1 auxotrophy of O. tauri can be alleviated by co-culturing with a heterotrophic bacterial partner Dinoroseobacter shibae, a member of the Rhodobacteraceae family of alpha-proteobacteria, genera of which are frequently found associated with marine algae. D. shibae lacks the complete pathway to synthesise three other B-vitamins: niacin (B3), biotin (B7), and p-aminobenzoic acid (a precursor for folate, B9), and the alga is in turn able to satisfy the reciprocal vitamin requirements of its bacterial partner in a stable long-term co-culture. Bioinformatics searches of 197 representative marine bacteria with sequenced genomes identified just nine species that had a similar combination of traits (ability to make vitamin B12, but missing one or more genes for niacin and biotin biosynthesis enzymes), all of which were from the Rhodobacteraceae. Further analysis of 70 species from this family revealed the majority encoded the B12 pathway, but only half were able to make niacin, and fewer than 13% biotin. These characteristics may have either contributed to or resulted from the tendency of members of this lineage to adopt lifestyles in close association with algae. This study provides a nuanced view of bacterial-phytoplankton interactions, emphasising the complexity of the sources, sinks and dynamic cycling between marine microbes of these important organic micronutrients.

Quantitative proteomics of a B12 -dependent alga grown in coculture with bacteria reveals metabolic tradeoffs required for mutualism.

The unicellular green alga Lobomonas rostrata requires an external supply of vitamin B12 (cobalamin) for growth, which it can obtain in stable laboratory cultures from the soil bacterium Mesorhizobium loti in exchange for photosynthate. We investigated changes in protein expression in the alga that allow it to engage in this mutualism. We used quantitative isobaric tagging (iTRAQ) proteomics to determine the L. rostrata proteome grown axenically with B12 supplementation or in coculture with M. loti. Data are available via ProteomeXchange (PXD005046). Using the related Chlamydomonas reinhardtii as a reference genome, 588 algal proteins could be identified. Enzymes of amino acid biosynthesis were higher in coculture than in axenic culture, and this was reflected in increased amounts of total cellular protein and several free amino acids. A number of heat shock proteins were also elevated. Conversely, photosynthetic proteins and those of chloroplast protein synthesis were significantly lower in L. rostrata cells in coculture. These observations were confirmed by measurement of electron transfer rates in cells grown under the two conditions. The results indicate that, despite the stability of the mutualism, L. rostrata experiences stress in coculture with M. loti, and must adjust its metabolism accordingly.

Exploring mutualistic interactions between microalgae and bacteria in the omics age.

Microalgae undertake a wide range of mutualistic interactions with bacteria. Here we consider how transcriptomic, metagenomic and metabolomic approaches have been combined with microbiological and biochemical analyses to expand our understanding of algal-bacterial interactions. Identification of the major bacterial species associated with algae indicates that specific bacterial groups, particularly the alpha-Proteobacteria, are found more frequently, suggesting that these may have the means to initiate and maintain symbiotic relationships. Nutrient exchange is frequently the basis of algal-bacterial mutualism, and as the compounds involved are characterised, evidence is accumulating that these are complex and specific molecules, offering opportunities for signalling processes and regulation rather than merely passive diffusion. At the same time, it is clear that the interactions are not static, but can be initiated and broken in response to environmental and developmental cues.