Variability in the empirical leucine-to-carbon conversion factors along an environmental gradient

Bacterial production is one of the key parameters to evaluate bacterial role in ocean carbon fluxes. Estimation of bacterial production requires the leucine-to-carbon conversion factors that change widely across environments. However, empirical leucine-to-carbon conversion factors (eCFs) are seldom determined in situ because of time consuming and little is known on regulating factors for the eCFs. During May 2015 to January 2016, fourteen dilution experiments were conducted, from the Zhujiang (Pearl River) Estuary to the coast of the northern South China Sea, to determine spatiotemporal variability in the eCFs and its potential controlling factors along an environmental gradient. The eCFs showed clear spatial variations with the highest (1.27-1.69 (kg C)/(mol Leu)) in low salinity waters (salinity < 15), intermediate (1.03-1.25 (kg C)/(mol Leu)) in moderate salinity (salinity of 15-25), and the lowest (0.48-0.85 (kg C)/(mol Leu)) in high salinity waters (salinity > 25). Substrate availability was responsible for spatial variability in the eCFs. In the pristine coastal waters, low eCFs was related to substrate limitation and leucine incorporated was respired to maximize the survival rather than bacterial production. Hence, the eCFs measurement was needed for estimating bacterial production accurately in various marine environments.