Population dynamics of Pelagibacterales clonal lineages: ecological-consortia and frequency modulation

Abstract

Pelagibacterales gMED is the dominant epipelagic genomospecies in the western Mediterranean Sea. We used the O-chain biosynthesis gene clusters, OBCs, as clonal barcodes to analyse strain-level population structure. In total, 385 OBC-defined clonal lineages were tracked across Mediterranean metagenomes spanning 14 years and depths from 5 to 90 m within the photic zone, with between 128 and 336 detected per metagenome. The relative conservation of dominant OBC types across years, seasons, and geographic locations indicated a persistently high and stable clonal diversity. Abundance distributions were log-normal in all metagenomes -- the canonical signature of bet-hedging portfolio dynamics. Co-varying consortia of clonal lineages were detected, identifying ecologically coherent guilds whose relative abundances fluctuate in concert across environmental gradients, particularly depth during the stratified season. Consortia membership showed no correlation with core-genome phylogeny, indicating convergent ecological adaptation independently of ancestry. Our work reveals another layer of complexity below the community level, which together with its modulation, provides the gMED population with a robust and versatile genetic repertoire for the degradation of dissolved organic matter under the fluctuating conditions of the epipelagic ocean. We have named this mechanistic model frequency-modulation (FM).