Estimating the growth rate of slowly growing marine bacteria from RNA content

In past studies of enteric bacteria such as Escherichia coli, various measures of cellular RNA content have been shown to be strongly correlated with growth rate. We examined this correlation for four marine bacterial isolates. Isolates were grown in chemostats at four or five dilution rates, yielding growth rates that spanned the range typically determined for marine bacterial communities in nature (μ = 0.01 to 0.25 h^-1). All measures of RNA content (RNA cell^-1, RNA:biovolume ratio, RNA:DNA ratio, RNA:DNA:biovolume ratio) were significantly different among isolates. Normalizing RNA content to cell volume substantially reduced, but did not eliminate, these differences. On average, the correlation between μ and the RNA:DNA ratio accounted for 94% of variance when isolates were considered individually. For data pooled across isolates (analogous to an average measurement for a community), the ratio of RNA:DNA μm^-3 (cell volume) accounted for nearly half of variance in μ (r² = 0.47). The maximum RNA:DNA ratio for each isolate was extrapolated from regressions. The regression of (RNA:DNA)/(RNA:DNA)(max) on μ was highly significant (r² = 0.76 for data pooled across four isolates) and virtually identical for three of the four isolates, perhaps reflecting an underlying common relationship between RNA content and growth rate. The dissimilar isolate was the only one derived from sediment. Cellular RNA content is likely to be a useful predictor of growth rate for slowly growing marine bacteria but in practice may be most successful when applied at the level of individual species.