Metagenomics uncovers a new group of low GC and ultra-small marine Actinobacteria

We describe a deep-branching lineage of marine Actinobacteria with very low GC content (33%) and the smallest free living cells described yet (cell volume ca. 0.013 μm(3)), even smaller than the cosmopolitan marine photoheterotroph, 'Candidatus Pelagibacter ubique'. These microbes are highly related to 16S rRNA sequences retrieved by PCR from the Pacific and Atlantic oceans 20 years ago. Metagenomic fosmids allowed a virtual genome reconstruction that also indicated very small genomes below 1 Mb. A new kind of rhodopsin was detected indicating a photoheterotrophic lifestyle. They are estimated to be ~4% of the total numbers of cells found at the site studied (the Mediterranean deep chlorophyll maximum) and similar numbers were estimated in all tropical and temperate photic zone metagenomes available. Their geographic distribution mirrors that of picocyanobacteria and there appears to be an association between these microbial groups. A new sub-class, 'Candidatus Actinomarinidae' is proposed to designate these microbes.