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 correl ated with growth rate. We examined this correlation for four marine ba cterial isolates. Isolates were grown in chemostats at four or five di lution rates, yielding growth rates that spanned the range typically d etermined for marine bacterial communities in nature (mu = 0.01 to 0.2 5 h-1). All measures of RNA content (RNA cell-1, RNA:biovolume ratio, RNA:DNA ratio, RNA:DNA:biovolume ratio) were significantly different a mong isolates. Normalizing RNA content to cell volume substantially re duced, but did not eliminate, these differences. On average, the corre lation between mu and the RNA:DNA ratio accounted for 94% of variance when isolates were considered individually. For data pooled across iso lates (analogous to an average measurement for a community), the ratio of RNA:DNA mum-3 (cell volume) accounted for nearly half of variance in mu (r2 = 0.47). The maximum RNA:DNA ratio for each isolate was extr apolated from regressions. The regression of (RNA-DNA)/(RNA:DNA)max on mu was highly significant (r2 = 0.76 for data pooled across four isol ates) and virtually identical for three of the four isolates, perhaps reflecting an underlying common relationship between RNA content and g rowth rate. The dissimilar isolate was the only one derived from sedim ent. Cellular RNA content is likely to be a useful predictor of growth rate for slowly growing marine bacteria but in practice may be most s uccessful when applied at the level of individual species.