Coherent patterns in bacterial growth, growth efficiency, and leucine metabolism along a northeastern Pacific inshore‐offshore transect

Abstract

We investigated the patterns in bacterial growth, production, respiration, growth efficiency (BGE), and bacterial leucine respiration and C‐to‐leucine yield (i.e., conversion factor [CF]) along a transect off the coast of Oregon. Plankton respiration along the transect averaged 1.15 ± 0.16 mg C L−1 h−1, peaking in the coastal upwelling region. The respiration in the filtered fraction, which was dominated by bacterial biomass, accounted for 79% of the total respiration. The different approaches that we used converged to an average BGE of 13% ± 1%, with peaks of over 20% in the more productive coastal areas and values declining to below 5% toward the oligotrophic gyre waters. There was overall coherence between the various aspects of bacterial C metabolism: communities with low BGE also tended to have low growth rates and high leucine‐to‐thymidine incorporation ratios. The patterns in BGE were mirrored at the single compound level, and in the most oligotrophic sites, bacteria tended to quickly respire a large fraction (20–75%) of the leucine that was taken up and had the lowest C‐to‐leucine yield, suggesting that the patterns in bulk BGE and growth also apply to individual substrates. Bacterial growth was a function of both C consumption and BGE; these two aspects of bacterial C metabolism do not necessarily covary, and they are regulated differently. The patterns in C consumption, growth, BGE, and leucine metabolism all reflect the basic physiological response of bacteria to energy limitation due to high maintenance costs associated with life in oligotrophy.