Large-scale variability in surface bacterial carbon demand and growth efficiency in the subtropical northeast Atlantic Ocean

We present surface estimates of bacterial respiration, bacterial heterotrophic production (BHP), and bacterial growth efficiency (BGE), and their relationship with nutrient availability, along a trophic gradient from coastal upwelling waters to the open-ocean waters of the eastern North Atlantic. Bacterial respiration generally ranged between 10 and 30 μg C L^_1 d^-1 and was relatively unaffected by nutrient enrichment. In contrast, BHP showed higher variability (more than one order-of-magnitude range) and was affected by carbon and/or phosphorus additions in different regions. Empirical bacterial carbon-to-leucine (Leu) conversion factors (CFs) (range, 0.02-1.29 kg C mol Leu^-1) decreased from the coast to the open ocean, largely influencing the BHP estimates in oligotrophic waters. We found high percentages of Leu respiration in oceanic waters (average 68% of Leu taken up by bacteria), possibly related to the low CFs found offshore. Empirical CFs were highly correlated to BGE (Pearson correlation coefficient r = 0.86, n = 12, p < 0.0004, log-log transformed), which varied between 1% in offshore waters and 56% in the upwelling waters. Empirical CFs could be critical not only for accurately constraining BHP, but probably also for predicting BGE in oceanic waters.