Source and supply of terrestrial organic matter affects aquatic microbial metabolism

Abstract

Aquatic ecosystems are connected to their surrounding watersheds through inputs of terrestrial-derived dissolved organic matter (DOM). The assimilation of this allochthonous resource by recipient bacterioplankton has consequences for food webs and the biogeochemistry of aquatic ecosystems. We used laboratory batch experiments to examine how variation in the source and supply (i.e. concentration) of DOM affects the productivity, respiration and growth efficiency of heterotrophic lake bacterioplankton. We created 6 different DOM sources from soils beneath nearmonotypic tree stands in a temperate deciduous-coniferous forest. We then exposed freshwater microcosms containing a natural microbial community to a 1100 µM supply gradient of each DOM source. Bacterial productivity (BP) and bacterial respiration (BR) increased linearly over the broad gradient, on average consuming 7% of the standing pool of dissolved organic carbon (DOC). Bacterial metabolism was also influenced by the chemical composition of the DOM source. Carbon-specific productivity declined exponentially with an increase in the carbon:phosphorus (C:P) ratio of the different DOM sources, consistent with the predictions of ecological stoichiometry. Together, our shortterm laboratory experiments quantitatively describe the metabolic responses of freshwater bacterioplankton to variation in the supply of terrestrial-derived DOM. Furthermore, our results suggest that dissolved organic phosphorus (DOP) content, which may be linked to the identity of terrestrial vegetation, is indicative of DOM quality and influences the productivity of freshwater bacterioplankton.KEY WORDS: Allochthonous · Bacteria · DOC · DOM · Ecosystem · Plankton · Stoichiometry · Subsidy Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 39: [107][108][109][110][111][112][113][114][115][116][117][118][119] 2005 decoupled from local primary productivity. One potential explanation for this decoupling is that bacterial carbon demand is subsidized by terrestrial-derived DOM.Terrestrial ecosystems export large quantities of DOM to inland, estuarine and coastal marine waterbodies (Goni et al. 1997, Findlay et al. 1998, Neff & Asner 2001. However, terrestrial-derived DOM has traditionally been considered a poor-quality resource for aquatic bacteria because it is relatively old (Raymond & Bauer 2001) and comprises humic compounds with low nutritional content (McKnight & Aiken 1998). Nevertheless, a few lines of evidence indicate that bacterial metabolism may be supported to varying degrees by inputs of terrestrial DOM. First, numerous laboratory studies have directly demonstrated that aquatic bacteria grow on terrestrial fractions of DOM (Tranvik 1988, Bano et al. 1997, Moran & Hodson 1994. Second, as lake DOM concentrations increase, community respiration tends to exceed local primary productivity (Hanson et al. 2003), suggesting that allochthonous carbon may be consumed by heterotrophic bacteria. Third, experimental isotope-enrichment of...

Efficient aquatic bacterial metabolism of dissolved low-molecular-weight compounds from terrestrial sources

“…Various compounds of CA, AA and CH were depleted during the 2-week incubation (Figure 1), simultaneous with a decrease in average BGE (mean of all six incubations) from 0.28 to 0.17 (Table 1). The BGE was within the range reported for bacterial use of DOM at different degrees of degradation, in which BGE on fresh DOM from forest soil solutions can be as high as 0.40-0.50 (Lennon and Pfaff, 2005;Berggren et al, 2007) and BGE on DOM that has been aged for years usually is lower than 0.10 (Eiler et al, 2003). We argue that CA, AA and CH were preferentially taken up and incorporated into bacterial cells with high efficiency, leaving a more recalcitrant DOM pool to be metabolized with lower efficiency at the end of the incubations.…”

Experimental warming increases CO2 saturation in a shallow prairie pond

“…In terms of carbon cycling within a lake, supersaturation generally indicates net heterotrophy, where organic carbon from external sources (allochthonous) or from pools of stored carbon (i.e. sediment organic matter or DOC accumulated in the water column) is respired and released to the atmosphere at a greater rate than autochthonous carbon uptake and storage (Carpenter et al 2005;Cole et al 2000Cole et al , 2002del Giorgio and Peters 1993;Duarte and Prairie 2005;Lennon and Pfaff 2005). Undersaturation indicates net autotrophy where more autochthonously generated carbon is stored in the sediments, or water column, than is respired and released to the atmosphere (Flanagan et al 2006;Hanson et al 2004;Schindler et al 1997).…”

Aquatic heterotrophic bacteria have highly flexible phosphorus content and biomass stoichiometry

“…Strict homeostasis of assemblage C:N:P biomass leads to the prediction that the ratio of regenerated C:P increases dramatically with increasing resource C:P (Sterner, 1990), but flexible biomass stoichiometry allows tight coupling and negative feedback between bacterial biomass stoichiometry and resource stoichiometry, facilitating the inherent resilience of ecosystems to nutrient perturbations (Scheffer et al, 2001). It is increasingly recognized that much of the organic matter metabolized in rivers and lakes originates in terrestrial ecosystems where C:P and N:P ratios can be much higher than for organic matter originating in aquatic ecosystems (Lennon and Pfaff, 2005). The observations in this study of extreme flexibility in bacterial biomass stoichiometry are consistent with observations of higher and more variable biomass C:P and and open circles denote data from Experiment 2.…”