Marine biofilms: cyanobacteria factories for the global oceans

Marine biofilms were newly revealed as a giant microbial diversity pool for global oceans. However, the cyanobacterial diversity in marine biofilms within the upper seawater column and its ecological and evolutionary implications remains undetermined. Here, we reconstructed a full picture of modern marine cyanobacteria habitats by re-analysing 9.3 terabyte metagenomic datasets and 2648 metagenome-assembled genomes (MAGs). The abundance of cyanobacteria lineages exclusively detected in marine biofilms were up to 9-fold higher than those in seawater at similar sample size. Analyses revealed that cyanobacteria in marine biofilms are specialists with strong geographical and environmental constraints on their genome and functional adaption, which in stark contrast to the generalistic features of seawater-derived cyanobacteria. Molecular dating suggests the important diversifications in biofilm-forming cyanobacteria appear to coincide with the Great Oxidation Event (GOE), “boring billion” middle Proterozoic, and the Neoproterozoic Oxidation Event (NOE). These new insights suggest that marine biofilms are large and important cyanobacterial factories for the global oceans.