Low Microbial Respiration of Leucine at Ambient Oceanic Concentration in the Mixed Layer of the Central Atlantic Ocean

Bacterioplankton are the primary consumers of dissolved organic matter in the ocean, thus the quantification of bacterioplankton production (BP) is essential to our understanding of carbon cycling in the largest ecosystems on Earth. We compared BP, measured as the rate of ¹⁴C-leucine (¹⁴C-Leu) or ³H-leucine (³H-Leu) uptake at close to saturating concentration (20 nM), with ambient Leu uptake estimated from dilution bioassays. The latter uses ³H-Leu additions at a range of concentrations close to ambient to estimate ambient Leu uptake rates, in addition to bioavailability and turnover rates. We hypothesised that saturation with Leu would lead to its respiration as a carbon source, thereby not truly representing ambient BP. Seawater samples were collected from the photic zone (22-170 m) in mesotrophic and oligotrophic regions along a transect through the central Atlantic Ocean. Respiration as a proportion of total consumption (uptake + respiration) of close to ambient (0.4 nM) and close to saturating (20 nM) ¹⁴C-Leu additions were compared. Leu uptake rates measured using saturating ³H-Leu additions were generally comparable with ambient rates estimated by dilution bioassays; however, saturating additions may overestimate uptake at low rates and underestimate uptake at high rates. The proportion of total Leu uptake that was respired was 3-fold higher for 20 nM ¹⁴C-Leu additions than 0.4 nM ¹⁴C-Leu additions (15±8% and 5±4%, respectively). Consequently, microbial efficiency of Leu assimilation - an indicator of bacterioplankton growth efficiency - was significantly higher at close to ambient ¹⁴C-Leu additions than at close to saturating concentrations (95±4% and 85±8%, respectively). Thus, saturation of open Atlantic Ocean bacterioplankton with Leu, or other molecules indicative of microbial metabolism, leads to the measurement of a response to nutrient addition, rather than an ambient measurement.