Growth rates of abundant marine bacterial clades in pure cultures and in the Delaware estuary

Ratios of 16S rRNA to 16S rRNA genes (rRNA:rDNA) have been used to assess the contribution of bacterial taxa to total community growth and carbon cycling. However, interpretations of rRNA:rDNA ratios is based upon a limited number of studies with rapidly growing bacteria. The most abundant bacteria in the oceans probably grow more slowly than those bacteria whose rRNA:rDNA versus growth rate relationships are known. To understand how rRNA:rDNA varies in abundant marine bacteria, I used quantitative PCR and reverse transcriptase quantitative PCR to measure rRNA:rDNA in bacteria known to be abundant in coastal Delaware waters and elsewhere. Four marine isolates were examined including Ca. Pelagibacter ubique HTCC1062, a coastal isolate of SAR11, the most abundant bacterial clade in the oceans. In culture, there were significant relationships between rRNA:rDNA and growth rate for some strains but not for others. The rRNA:rDNA ratios determined along a transect in the Delaware estuary suggested that oligotrophic bacteria grew up to ten-fold faster than copiotrophic bacteria in the same communities. I find that rRNA:rDNA ratios can be useful for estimating growth rates in some bacterial taxa and that knowledge of the rRNA:rDNA versus growth rate relationship for a given taxon can enhance interpretations of rRNA:rDNA data from natural communities.