Dynamics of heterotrophic bacteria in temperate coastal waters: Similar net growth but different controls in low and high nucleic acid cells

Heterotrophic bacteria in aquatic environments are almost universally distributed into 2 distinct flow cytometric clusters based on their relative nucleic acid content: low (LNA) and high (HNA). The dynamics and possible regulation processes of both subgroups in southern Bay of Biscay coastal waters were examined by comparing weekly measurements over 2 yr at a shallow environment (L'Arbeyal beach) with monthly samples at 3 continental shelf stations off Xixón (Spain). Similar seasonal variations of temperature and chlorophyll a characterized inshore and offshore waters. Total bacterial abundances were also similar in shallow and shelf waters (from 0.22 to 3.72 × 10⁶ cells ml^-1), with the most remarkable difference being the higher abundance and larger size of HNA bacteria at L'Arbeyal, probably related to greater substrate availability. There, apparent net growth rates estimated from coherent periods (6 to 26 wk) of sustained increase or decrease (-0.02 to 0.04 d^-1) were similar for LNA and HNA cells, although related to different factors. LNA bacteria net growth rates were strongly positively correlated with temperature (r = 0.95, p < 0.01, n = 6) and negatively with chlorophyll a (r = -0.90, p = 0.01, n = 6), supporting the hypothesis that they are independent from phytoplankton, which was recently suggested for this and other coastal sites using different approaches. We also show an opposite relationship between cell size and apparent net growth rates of LNA and HNA bacteria, providing further evidence of fundamentally different ecological roles.