Magnitude and regulation of bacterioplankton respiratory quotient across freshwater environmental gradients

Abstract

Bacterioplankton respiration (BR) may represent the largest single sink of organic carbon in the biosphere and constitutes an important driver of atmospheric carbon dioxide (CO 2 ) emissions from freshwaters. Complete understanding of BR is precluded by the fact that most studies need to assume a respiratory quotient (RQ; mole of CO 2 produced per mole of O 2 consumed) to calculate rates of BR. Many studies have, without clear support, assumed a fixed RQ around 1. Here we present 72 direct measurements of bacterioplankton RQ that we carried out in epilimnetic samples of 52 freshwater sites in Qué bec (Canada), using O 2 and CO 2 optic sensors. The RQs tended to converge around 1.2, but showed large variability (s.d. ¼ 0.45) and significant correlations with major gradients of ecosystem-level, substrate-level and bacterial community-level characteristics. Experiments with natural bacterioplankton using different single substrates suggested that RQ is intimately linked to the elemental composition of the respired compounds. RQs were on average low in net autotrophic systems, where bacteria likely were utilizing mainly reduced substrates, whereas we found evidence that the dominance of highly oxidized substrates, for example, organic acids formed by photo-chemical processes, led to high RQ in the more heterotrophic systems. Further, we suggest that BR contributes to a substantially larger share of freshwater CO 2 emissions than presently believed based on the assumption that RQ is B1. Our study demonstrates that bacterioplankton RQ is not only a practical aspect of BR determination, but also a major ecosystem state variable that provides unique information about aquatic ecosystem functioning.

The efficiency and spectral photon dose dependence of photochemically induced changes to the bioavailability of dissolved organic carbon

“…respired to O 2 consumed (e.g., oxalic acid, a known photochemical product, has an RQ of 4). Berggren et al (2012) found that for a large freshwater data set, RQ was between 0.5 and 2, with a large standard deviation. A strong variation in RQ in irradiated DOM relative to controls, or a change of RQ during the subsequent dark incubation will certainly influence all studies, ours included, that define an RQ a priori for calculating BLP concentrations.…”

Seasonal contribution of terrestrial organic matter and biological oxygen demand to the Baltic Sea from three contrasting river catchments

“…While there is a range of efficiencies with which the microbial community can access DOM (i.e. respiratory quotient RQ; Berggren et al, 2012), for the most part these boreal freshwater RQs fall within a relatively small range (0.5-2) and as such BOD can be used as an effective proxy for the biological reactivity or lability of the DOM (Miller et al, 2002;Asmala et al, 2013). …”

Increases in terrestrially derived carbon stimulate organic carbon processing and CO2 emissions in boreal aquatic ecosystems

“…Boreal biomes contain the highest density of freshwater on Earth, yet this evidence for biologically degradable DOC is scattered and based on isolated components of the continental aquatic network. As a result, the overall susceptibility of DOC to photochemical and biological degradation, and how this susceptibility varies over gradients of increasing terrestrial influence and at scales relevant for continental C budgets, has not been well established 19,[33][34][35][36] .…”