Anaerobic Oxidation of Ethane, Propane, and Butane by Marine Microbes: A Mini Review

Abstract

The deep ocean and its sediments are a continuous source of non-methane short-chain alkanes (SCAs) including ethane, propane, and butane. Their high global warming potential, and contribution to local carbon and sulfur budgets has drawn significant scientific attention. Importantly, microbes can use gaseous alkanes and oxidize them to CO₂, thus acting as effective biofilters. A relative decrease of these gases with a concomitant ¹³C enrichment of propane and n-butane in interstitial waters vs. the source suggests microbial anaerobic oxidation. The reported uncoupling of sulfate-reduction (SR) from anaerobic methane oxidation supports their microbial consumption. To date, strain BuS5 isolated from the sediments of Guaymas Basin, Gulf of California, is the only pure culture that can anaerobically degrade propane and n-butane. This organism belongs to a metabolically diverse cluster within the Deltaproteobacteria called Desulfosarcina/Desulfococcus. Other phylotypes involved in gaseous alkane degradation were identified based on stable-isotope labeling and fluorescence in-situ hybridization. A novel syntrophic association of the archaeal genus, Candidatus Syntrophoarchaeum, and a thermophilic SR bacterium, HotSeep-1 was recently discovered from the Guaymas basin, Gulf of California that can anaerobically oxidize n-butane. Strikingly, metagenomic data and the draft genomes of ca. Syntrophoarchaeum suggest that this organism uses a novel mechanism for n-butane oxidation, distinct from the well-established fumarate addition mechanism. These recent findings indicate that a lot remains to be understood about our understanding of anaerobic SCA degradation. This mini-review summarizes our current understanding of microbial anaerobic SCA degradation, and provides an outlook for future research.

Keywords: Desulfosarcina/Desulfococcus; Gulf of Mexico; anaerobic oxidation; short-chain alkanes; sulfate reduction.

Figure 1

Anaerobic activation of propane at the sub-terminal (A) and terminal (B) carbon atom (marked with stars) via fumarate addition yielding isopropylsuccinate and n-propylsuccinate, respectively. A similar activation mechanism exclusively at the sub-terminal carbon atom is proposed for the anaerobic oxidation of n-butane.

Figure 2

Maximum likelihood tree of translated full-length and partial masD/assA/bssA/nmsA homologs from selected isolates as well as pristine and seepage-impacted metagenomes obtained from GenBank (accession numbers are shown in parentheses). Tree was inferred using the Le_Gascuel_2008 model (Le and Gascuel, 2008) and involved 85 amino acid sequences and a total of 210 positions. All positions with less than 95% site coverage were eliminated. Full-length glycerol dehydratase (dhaB1) from Clostridium butyricum was used as an outgroup. Node circles denote bootstrap value percentages from 100 replicate trees. Scale bar represents 20% estimated sequence divergence. Evolutionary tree was constructed in MEGA7 (Kumar et al., 2016).