Invariable selection of compounds from organic matter by stream microbes

Organic carbon (OC) in rivers is one of the most rapidly recycled carbon pools. A significant proportion of OC is remineralized and contributes to the globally relevant CO 2 emissions of river networks. However, there is no consensus on the mechanisms that determine which compounds are remineralized. Previous studies found that OC older than the mean age of the source was decomposed by aquatic microorganisms. Here, we study the radiocarbon (Δ 14 C) age of dissolved OC (DOC) that is decomposed in laboratory experiments across a range of stream bulk DOC ages. Stream DOC was collected from small forested catchments under summer dry flow, average flow and storm flow conditions. The decomposed DOC was neither consistently older nor younger than the original stream DOC. The Δ 14 C of respiratory CO 2 increased with the Δ 14 C of stream DOC ( P = 0.006, N = 16). However, the slope of the regression was small (0.20 ± 0.06) and the dependence was weak ( R 2 = 0.43). Moreover, the age range of respired DOC (modern to 950 years BP) was considerably narrower than the age range of stream DOC (modern to 3880 years BP). In further experiments, we used leachates of catchment soil from 0-8 cm and 8–20 cm depth and a 1:1 mixture of the two depths as initial DOC. Again, the increase in Δ 14 C-CO 2 as a function of Δ 14 C-DOC was significant ( R 2 = 0.74, P = 0.028, N = 6), but the slope was small (0.13 ± 0.04) and the age range of respired DOC was narrow (modern to 280 years BP) compared to initial leachate DOC (600 to 3400 years BP). Fourier-transform ion cyclotron resonance mass spectrometry showed that the dissolved organic matter (DOM) starting material from the deeper soil layer was characterized by smaller, more reduced (i.e. aliphatic) molecules as compared to the DOM from the shallower soil layer. However, similar (small, unsaturated, oxygen-rich) CHO molecules were consumed regardless of DOM source. The narrow age ranges of respired DOC suggest that intrinsic chemical quality sets the limits for which compounds can be utilized under given geochemical conditions. However, strategies of microorganisms to optimize growth (optimal foraging) may modulate their specific substrate choice, as indicated by the dependence of the age of respired OC on the age composition of the original DOC. Analysis of published decomposition experiments showed that in 60% of measurements the decomposed OC was older than the original river or lake OC, excluding studies with peat-derived OC or OC affected by permafrost thaw. However, due to the freshwater reservoir effect (photosynthetic assimilation of 14 C-old inorganic carbon), existing data provide no consistent evidence that either younger or older OC is preferentially decomposed.