Novel metabolisms relevant to short-chain alkane fate in the deep ocean

Petroleum inputs to the marine environment have received significant attention due to petroleum’s inherent toxicity to marine life and because discharge sometimes occurs through catastrophic and preventable anthropogenic events. These scientific efforts have led to significant advances toward understanding the behavior of liquid phase oil in the ocean, with a focus on the long-chain alkanes and multicyclic compounds. However, the fate of water-soluble volatile hydrocarbons in the environment—a class that is abundant in petroleum reservoirs and many crude and refined products—is poorly understood. To address this gap in knowledge, we conducted a series of at-sea incubations with the water-soluble hydrocarbon pentane using water collected from multiple sites within the deep Gulf of Mexico. By monitoring respiration remotely using optical oxygen sensors, we were able to observe and sample microbial blooms that occurred in response to pentane enrichments. Blooms are characterized by a rapid oxygen decline (0.25 µM oxygen loss per hour), a 10-fold increase in bacterioplankton abundance, nutrient drawdown, and a shift in microbial community composition where one taxa comprises >60% of the total bacterial community. We observed that bloom occurrence, timing, and identity of responding hydrocarbon degraders was related to exposure to natural oil seepage, specifically the prolific seep region in the northwestern Gulf of Mexico. An uncultivated clade of Cycloclasticusoften responds within these blooms, and metagenomic assembled genomes reveal key insights into the metabolic capabilities that enable pentane consumption. Cycloclasticusgenomes encode multiple divergent copies of the three subunits that comprise the particulate hydrocarbon monooxygenase (pHMO) enzyme. Analysis of the gene neighborhood surrounding these pHMO loci reveals the presence of a homologous uncharacterized subunit that may play a key role in the function of this enzyme on short-chain alkanes. High sequence similarity to Cycloclasticusthat naturally responded during the Deepwater Horizon Event (DWH) and to pHMOs transcriptionally active at that time suggest that Cycloclasticus consumption of pentane could have played a significant role in the microbial response during DWH.