Energy limitation of oceanic bacteria reflected in consistent patterns in bacterial growth efficiency, leucine processing and the conversion factor

Abstract

Oceanographers and ocean modelers require estimates of the amount of carbon that heterotrophic bacterial assemblages process. These estimates are commonly obtained from measures of bacterial production based on the incorporation of either leucine or thymidine. The accuracy of these estimates depends on empirical carbon-to-leucine conversion factors (LCF). On the other hand, bacterial respiration, another key aspect of microbial metabolism, is linked to bacterial production through bacterial growth efficiency (BGE), which has been shown to relate to bacterial physiological condition. Here we will first present a review of published and unpublished values of LCF. More than 200 individual measurements in all types of systems that suggests that this parameter varies systematically from coastal to open ocean communities, and that this pattern is similar to that observed for BGE. We also show data collected on a series of cruises in the Pacific and the Atlantic Ocean, and in the mediterranean Blanes Bay Microbial Observatory in which we empirically explored the connections between LCF and BGE and the consequences of their variability. In general terms we found that the LCFs were positively correlated to BGE, and negatively to the proportion of leucine respired and to the ratio leucine-thymidine, suggesting that even though some of the general variability might be methodological, the patterns in bacterial growth, BGE and LCF all reflect the basic physiological response of bacteria to energy limitation