The SAR11 clade or group of species is the smallest and most abundant of marine organisms. Each bacterium is so small, some 500,000 of them can be found in a single millimeter of seawater. Unsurprisingly, SAR11 have the smallest genomes of any free-living microorganism.
SAR11 makes up a quarter of the marine microorganisms in the oceans’ surface layers on average and its population rises and falls with seasonal shifts from summer to winter just as levels of organic carbon in the water vary during the same cycle. The bacteria are a major consumer of organic carbon, which moves from the upper ocean layers where enough light penetrates to allow green plants to grow down to the deeper waters where it can be sequestered, and SAR11 produce the nutrients that allow algae to daily supply half the oxygen that becomes part the Earth’s atmosphere. Three SAR11 bacterial genomes have already been sequenced; sequencing several more contributes significant information about the marine carbon cycle. The work is relevant to the DOE’s mission to reduce and stabilize the levels of greenhouse gases in the atmosphere through methods such as carbon sequestration.
Studying SAR11 doesn’t just give researchers a chance to learn about the clade’s role in the marine carbon cycle but the interactions between diverse microbial communities as well. Early studies showed several ecotypes of SAR11 bacteria live in ocean surface layer: those who are more suited to the nutrient-abundant conditions of spring; the more oligotrophic bacteria who thrive in the summer waters with a wide range of nutrient levels; bacteria who have adapted to freshwater conditions; and those that live in more brackish environments.
Principal Investigators: Stephen Giovannoni, Oregon State University and Michael Rappé, Hawaii Institute of Marine Biology
Program: CSP 2009