Assessing the relevance of nucleic acid content as an indicator of marine bacterial activity

Abstract

Current flow cytometry techniques allow the rapid estimation of the abundance of 2 distinct groups of heterotrophic bacteria, characterized by their relative nucleic acid content. High nucleic acid (HNA) bacteria are, at least in coastal environments, usually regarded as more active than the low nucleic acid (LNA) group. We tested the effects of substrate supply and bacterial cell size on the relationship between bacterial activity and the abundance of HNA bacteria by simultaneous measurements of LNA and HNA cell distributions, chlorophyll a and 3H-leucine uptake rates in temperate shelf waters of the northern Iberian Peninsula. We considered 3 zones based on hydrological properties. Significant correlations were found between bacterial activity (range 0.1 to 80 pmol Leu l–1 h–1) and both total and relative (range 28 to 84%) HNA cell abundance for pooled data, but the ready use of HNA bacterial abundance as a proxy for activity in natural systems was questioned by the low percentage of variance explained (16%). However, a detailed regional study of bottom-up effects revealed that the strength of this relationship increased significantly when bacteria were apparently controlled by phytoplankton substrate supply. Moreover, the relationship between mean biomass (overall range 12.4 to 21.2 fg C cell–1) and abundance-activity correlation coefficients in the 3 zones (r = 0.94, p = 0.005, n = 6) suggests that only at large cell sizes can we expect bacterial activity and production to be reasonably predicted by the abundance of HNA cells.