Impact of photochemical degradation of DOC on bacterial respiratory quotient in aquatic ecosystems

Many studies assume a respiratory quotient (RQ = CO2 produced per O2 consumed, by moles) of ~1 to calculate bacterial respiration from measured O2 consumption rates. However, the theoretical value of RQ varies with the elemental composition of the compound being decomposed. Photo-oxidation of DOC results in compounds of oxygen-rich organic acids which, theoretically, should result in elevated RQ. We performed organic acid analysis and used optic gas-pressure sensors to monitor the bacterial RQ in irradiated and non-irradiated water from five lakes, plus in Leonardite-extracted humic acid solutions. In irradiated samples, the RQs generally were significantly higher than in the non-irradiated samples. Additionally, we found that enrichment with inorganic nutrients (N + P) consistently increased the RQs in bioassays based on humic acid extracts. This study shows that bacterial RQ varies depending on the state of oxidation of the DOC and the access to nutrients in the water. Our results imply that RQ can be systematically higher than 1 when the bacterial metabolism is to a large extent based on photo-chemically produced substrates.