Activity of abundant and rare bacteria in a coastal ocean

TLDR

The surface ocean hosts bacteria ranging from dormant to highly active, yet experimental data on the activity of specific taxa, especially rare ones, are limited. The study aims to examine how bacterial abundance relates to activity by tracking temporal changes in abundance and measuring the 16S rRNA to rDNA ratio of individual taxa. We analyzed the V1–V2 region of 16S rRNA and rDNA by tag pyrosequencing over a 3‑year Delaware coast surface water study and confirmed the rRNA–rDNA relationships for five taxa using quantitative PCR. More than half of the taxa alternated between abundant and rare, about 12 % remained consistently rare, and 20 % exhibited higher rRNA:rDNA ratios when rare; overall, abundance generally tracks activity, but a substantial portion of the rare community remains active with growth rates that decline as abundance increases.

Abstract

The surface layer of the oceans and other aquatic environments contains many bacteria that range in activity, from dormant cells to those with high rates of metabolism. However, little experimental evidence exists about the activity of specific bacterial taxa, especially rare ones. Here we explore the relationship between abundance and activity by documenting changes in abundance over time and by examining the ratio of 16S rRNA to rRNA genes (rDNA) of individual bacterial taxa. The V1–V2 region of 16S rRNA and rDNA was analyzed by tag pyrosequencing in a 3-y study of surface waters off the Delaware coast. Over half of the bacterial taxa actively cycled between abundant and rare, whereas about 12% always remained rare and potentially inactive. There was a significant correlation between the relative abundance of 16S rRNA and the relative abundance of 16S rDNA for most individual taxa. However, 16S rRNA:rDNA ratios were significantly higher in about 20% of the taxa when they were rare than when abundant. Relationships between 16S rRNA and rDNA frequencies were confirmed for five taxa by quantitative PCR. Our findings suggest that though abundance follows activity in the majority of the taxa, a significant portion of the rare community is active, with growth rates that decrease as abundance increases.