Impact of photo-chemical processing of dissolved organic carbon on the bacterial respiratory quotient in aquatic ecosystems

Abstract: Dissolved organic carbon (DOC) enters inland waters from the surrounding terrestrial ecosystems, and is assimilated by bacterioplankton, mineralizing it to carbon dioxide (CO2). This bacterial respiration (BR) is an important part of carbon processing in freshwater, but it is also a process of global significance; in oceans and inland waters combined, BR is probably the largest single sink of organic carbon on Earth. Many studies assume a respiratory quotient (RQ = CO2 produced per O2 consumed, by moles) of ~1 to calculate planktonic bacterial respiration rates from measured O2 consumption rates. However, the theoretical value of RQ varies with the elemental composition of the compound being decomposed. If the O-content of the compound is high, less oxygen is needed from the surroundings and the RQ increases. Photo-chemical oxidation of DOC with ultraviolet (UV) light results in oxygen-rich organic acids which, theoretically, should lead to elevated RQ. In this study samples of both UV light irradiated and non-irradiated water were incubated in the laboratory and the bacterial RQ was monitored with optic gas-pressure sensors. The water samples used were from the humic lake Övre Björntjärnen in the north of Sweden and Leonardite-extracted humic acid solutions. In irradiated samples, the RQs frequently exceeded 1 and generally were significantly higher than in the non-irradiated samples. Additionally, enrichment with inorganic nutrients (N+P) to humic acids extract consistently increased the RQs in bioassays. In non-irradiated humic acid solutions, both nutrient enriched and non-enriched, RQ was lower than 1. This study shows that bacterial RQ varies depending on the state of oxidation of the DOC and the access to nutrients in the water. The 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. The use of an assumed RQ of 1 may both underestimate and overestimate bacterial respiration in aquatic ecosystems.