Abstract
Rhizobiales and related orders of Alphaproteobacteria comprise several genera of nodule-inducing symbiotic bacteria associated with plant roots. Here we describe the genome and the metabolic network of “Candidatus Phaeomarinobacter ectocarpi” Ec32, a member of a new candidate genus closely related to Rhizobiales and found in association with cultures of the filamentous brown algal model Ectocarpus. The “Ca. P. ectocarpi” genome encodes numerous metabolic pathways that may be relevant for this bacterium to interact with algae. Notably, it possesses a large set of glycoside hydrolases and transporters, which may serve to process and assimilate algal metabolites. It also harbors several proteins likely to be involved in the synthesis of algal hormones such as auxins and cytokinins, as well as the vitamins pyridoxine, biotin, and thiamine. As of today, “Ca. P. ectocarpi” has not been successfully cultured, and identical 16S rDNA sequences have been found exclusively associated with Ectocarpus. However, related sequences (≥97% identity) have also been detected free-living and in a Fucus vesiculosus microbiome barcoding project, indicating that the candidate genus “Phaeomarinobacter” may comprise several species, which may colonize different niches.
Highlights
Most eukaryotes are known to live in association with bacteria and have established mutualistic relationships with several of them
Despite the fact that we have not been able to culture this bacterium, for which we propose the name “Candidatus Phaeomarinobacter ectocarpi,” we found it to be frequently associated with brown algae, and the analysis of its genome, as well as the reconstruction of its metabolic network, enabled us to form several hypotheses about the biology of this organism and the interactions it may have with Ectocarpus
The existence of new specific enzymes or other derived pathways to synthesize auxin in E. siliculosus cannot be excluded, our analyses show that auxin synthesis may occur by “Ca. P. ectocarpi” or synergistically between E. siliculosus and the bacterium, assuming that intermediates can be exchanged between both organisms
Summary
Most eukaryotes are known to live in association with bacteria and have established mutualistic relationships with several of them. The importance of mutualistic relationships between algae and bacterial biofilms is well-established (Bartsch et al, 2008; Wahl et al, 2012), yet studies considering the algal-bacterial holobiont are still rare (Dittami et al, 2014). This is despite the fact that macroalgae, and especially large kelp-forest forming brown algal species, constitute important structural elements of coastal ecosystems. They provide shelter and breeding grounds for fish species and marine invertebrates (Santelices, 2007), but are of direct economic interest, e.g., as a source of alginates or for the production of biofuels (Wei et al, 2013)
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