oa Biohopanoid Production in Alphaproteobacteria: Biosynthetic, Chemotaxonomical and Geobiological Implications

Bacteriohopanepolyol derivatives (BHPDs) are extensively used as biomarkers. With the increasing availability of complete bacterial genomes, hopanoid production can be readily predicted by the presence of specific genes.

Genome analysis of >5000 Alphaproteobacteria revealed that ca. 23 % possess the genetic capacity to produce BHPDs, supporting the idea that the Alphaproteobacteria form an important group of BHPD producers in the environment. However, BHPD biosynthesis genes were unevenly distributed between phylogenetic groups. We went on to cultivate 54 strains of Alphaproteobacteria and examine their BHPD profiles. Our results show that the presence of BHPD biosynthetic genes results in actual production of BHPDs. However, the presence of genes encoding proteins that result in methylation of BHPDs does not always lead to the production of methylated BHPDs.

Our results severely complicates the use of BHPDs as biomarkers in environmental and palaeontological studies. Firstly, BHPD concentrations in APB species that do produce hopanoids can vary by two orders of magnitude and are not directly related to a specific phylogenetic group. Secondly, the BHPD biosynthetic gene cluster is not evenly distributed over phylogenetic groups: one cannot predict the production of BHPDs in one species by analyzing a closely related species.