Seasonality of rare bacterial taxa in Radiales E2 Gijon/Xixon time-series station

Rare microbial taxa are increasingly recognized to play key ecological roles, but basic knowledge of their spatio-temporal dynamics is lacking. In a time-series study in coastal waters we identified the largest number of bacterial taxa with significant seasonality to date (83 taxa). Among these seasonal taxa, 24 cycled between rare and abundant, but most of them were always-rare (59 taxa), with maximum abundances predominately found in winter. We demonstrate that recurrent seasonal patterns in marine bacterioplankton are largely driven by lineages that never account >1% of total community abundance. Such dynamics characterized by seasonal growth and loss opens new perspectives in the ecology of rare marine bacteria, hitherto characterized by dormancy and episodic boom and bust, as envisioned by the seed-bank hypothesis. This is the first evidence of temporal reoccurrence of marine rare bacteria, which may include “hidden” key players for the functioning of marine ecosystems.