Relationship of bacterial growth efficiency to spatial variation in bacterial activity in the Hudson River

Abstract

Variation in bacterial production (BP) is used as an indicator of bacterial metabolism and carbon processing in the analysis of aquatic ecosystems. The allocation of carbon by bacteria to either BP or respiration (BR), however, is variable and may potentially influence the assessment of carbon cycling by bacteria in ecosystems. We studied 10 transects in the Hudson River estuary where there is a gradient in BP and BR along the flow path of the estuary. We measured BP and BR in filtered sam- ples to derive an estimate of bacterial growth efficiency (BGE) that we could compare with indepen- dent measurements of total BP (BPT) from unfiltered samples to evaluate the relationship of BGE to BP. We further tested the assumption that BGE derived from filtered samples is a good estimator of the ambient BGE on a subset of transects in the upriver section where bacteria dominate respiration. There was good agreement (near 1:1) between respiration measured in unfiltered samples and total BR estimated from BPT and BGE in the filtered fraction (total BR = (BPT/BGE) - BPT). BGE averaged 0.29 but varied from 0.07 to 0.61, and did not explain the general decline in BPT along the river. Rather, BGE was strongly correlated to the residuals of the BPT and specific BPT (i.e. growth) vs. flow path (= river kilometer) relationship, indicating that shifts in bacterial carbon allocation explained local variations in bacterial metabolic activity, and these shifts were superimposed on the larger scale decline in carbon consumption and BP. The pattern in BP along the Hudson River is clearly a combi- nation of changes in consumption as well as in BGE, to the point that the pattern in BPT or growth would be impossible to recreate from any one of these 2 components. We conclude that BGE indicates changes in carbon allocation of bacteria that reflect shifts in relative BR and BP at shorter time and space scales that are distinct from larger overall patterns in consumption and BP.