Marine and Terrestrial Nitrifying Bacteria are Sources of Diverse Bacteriohopanepolyols

Summary Hopanoid lipids and their derivatives, bacteriohopanepolyols, are membrane components of some bacteria that are commonly used as biomarkers for specific bacterial groups or biogeochemical processes in the geologic record ( Newman et al., 2016 ). However, the sources of hopanoids to marine and freshwater environments remain largely unconstrained. Recent marker gene studies suggest widespread capacity for hopanoid biosynthesis in marine bacterioplankton, including nitrifying (i.e., nitrite- or ammonia-oxidizing) bacteria ( Kharbush et al., 2018 ). To explore their hopanoid biosynthetic capacities, we studied the distribution of hopanoid biosynthetic genes in the genomes of cultivated and uncultivated ammonia-oxidizing (AOB) nitrite-oxidizing (NOB) bacteria. We found that hopanoid biosynthesis is common among seven of the nine presently cultivated clades of AOB and NOB. Hopanoid biosynthesis pathways are also conserved among the diverse lineages of AOB and NOB detected in environmental metagenomes. Distinct carbon isotopic signatures of biomass, hopanoids, and fatty acids suggest operation of distinct carbon fixation pathways among nitrifying bacteria. Accordingly, nitrifying bacterial contributions to the geologic record of hopanoids could be estimated by their carbon isotopic compositions. The ubiquity of nitrifying bacteria in the ocean today and the antiquity of this metabolic process suggest the potential for significant contributions to the geologic record of hopanoids.