Pelagibacter, a genus of marine bacteria, is a significant yet largely unseen inhabitant of the ocean. Though microscopic, these microbes collectively influence the global ocean, Earth’s largest ecosystem. Their widespread presence and metabolic activities are important for oceanic health and global biogeochemical cycles. Studying Pelagibacter offers insights into how life adapts to nutrient-poor environments and sustains planetary processes.
An Ocean-Dwelling Giant in Miniature
Pelagibacter, specifically the species Candidatus Pelagibacter communis (also known as Pelagibacter ubique), thrives in the surface waters of oceans worldwide. This bacterium is a member of the SAR11 clade, a group within the Alphaproteobacteria, which was first identified in the Sargasso Sea in 1990 through ribosomal RNA gene analysis. Despite its minute size, its sheer abundance makes it a dominant force in marine environments.
Estimates suggest that Pelagibacter and its relatives constitute approximately 25% of all microbial plankton cells in the ocean, and in temperate ocean surface waters during summer, they can account for nearly half of all microbial cells. The total global abundance of this group is estimated to be around 2 x 10^28 individual cells, making them potentially the most numerous organisms on Earth. This prevalence highlights their widespread distribution and ecological success in marine systems.
Orchestrating Global Cycles
Pelagibacter plays an important role in regulating Earth’s biogeochemical cycles, particularly the carbon and sulfur cycles. These bacteria are heterotrophic, meaning they obtain energy by consuming organic matter. They efficiently process dissolved organic carbon (DOC), which includes compounds like amino acids and sugars released by marine organisms.
Pelagibacterales are estimated to oxidize between 5% and 22% of all photosynthetically produced organic carbon to carbon dioxide (CO2) daily. This activity recycles organic carbon, influencing its residence time in the ocean and shaping the composition of dissolved organic pools. The consumption of DOC by Pelagibacter also releases nutrients that algae require for growth.
Beyond carbon, Pelagibacter is an important player in the sulfur cycle, notably through its interaction with dimethylsulfoniopropionate (DMSP). Marine phytoplankton produce DMSP annually as a byproduct of photosynthesis. Pelagibacter can assimilate DMSP for its sulfur needs or convert it into dimethyl sulfide (DMS) and methanethiol (MeSH) through metabolic pathways. DMS is a climate-active gas that, when released into the atmosphere, contributes to cloud formation, reflecting sunlight and potentially moderating global temperatures.
The Secrets to Its Success
The prevalence of Pelagibacter stems from adaptations that allow it to thrive in the nutrient-scarce conditions of the open ocean. One distinctive feature is its small, streamlined genome, which contains only the genes necessary for survival. With approximately 1,354 genes, Candidatus Pelagibacter communis has one of the smallest genomes among free-living organisms, lacking non-essential DNA like pseudogenes, introns, or transposons. This genetic minimalism significantly reduces the energy required for cell replication, providing an advantage in low-nutrient environments.
Pelagibacter’s metabolism is highly efficient and tailored for acquiring scarce nutrients. It possesses complete biosynthetic pathways for all 20 amino acids and most cofactors, even with its reduced gene set. Its small cell size, typically 0.4 to 0.7 micrometers, also provides a high surface-to-volume ratio, which enhances nutrient acquisition from dilute environments. The bacterium’s ability to rapidly replicate, despite its slow doubling time of about 29 hours, under these low-nutrient conditions further contributes to its ecological dominance. This minimalist approach, characterized by shedding non-essential genes and focusing on efficient resource utilization, is central to Pelagibacter’s success.