The picocyanobacterium Prochlorococcus is responsible for ~10% of annual marine carbon fixation and plays a role in the global carbon budget. While these phototrophs are primarily considered free-living and neutrally buoyant in the euphotic zone, we observe that they can form biofilms on diverse substrates. This trait is conserved across Prochlorococcus ecotypes, and populations continuously transition between planktonic and biofilm states via a non-genetic heritable mechanism. Throughout their growth, cells in biofilms retain a reversible, dynamic attachment state, and measurements of growth, photosynthesis, and carbon exudation rates reveal that cells in biofilms exude more organic carbon than their planktonic counterparts. Estimates of the fraction of Prochlorococcus cells attached to particles in the ocean-obtained through metagenomic analysis of serially-fractionated samples-reveal that a significant adherent population exists throughout the euphotic and mesopelagic zones. This work describes a dimension of Prochlorococcus's ecological niche and suggests a role in carbon export to the deep sea.
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