Tiny photosynthetic ocean bacteria named Prochlorococcus cling to large drifting particles across the world oceans. Scientists long assumed these abundant microbes lived almost entirely as solitary, free-floating cells in surface waters. Instead, vast numbers of the cells fasten themselves to suspended clumps of organic matter that pull them down toward the dark sea floor.
As net biological growth and dissolved inorganic carbon increase in productive marine zones, more bacterial cells join these dense clumps. The microscopic bacteria anchor to the solid material like passengers boarding a ferry. Because the combined debris is heavy, gravity pulls the loaded aggregates down through the water column toward deep layers. This physical descent transports carbon from the sunlit surface directly to a depth of 150 meters.
Researchers analyzed filter-separated water samples collected during research cruises across the Pacific Ocean, Atlantic Ocean, and Mediterranean Sea. The team added internal standards to calculate exact cell numbers and built a mathematical model to convert relative gene readings into absolute counts. When applied to global expedition data, the model confirmed that cells in size fractions greater than 1.6 micrometers make up a substantial share of total ocean populations.
Marine scientists can now recalculate absolute cell numbers of microbes stored inside archived ocean genetic databases. The resulting equations will allow oceanographers to estimate how much atmospheric carbon these drifting bacterial clumps export into deep marine ecosystems.