Invariable selection of compounds from organic matter by stream microbes

Organic carbon (OC) in rivers is one of the most rapidly recycled carbon pools. However, there is no consensus on the mechanisms that determine which compounds are remineralized. We studied the radiocarbon 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 ∆¹⁴C of respiratory CO₂ increased with the ∆¹⁴C 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 (R² = 0.43). 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 ∆¹⁴C-CO₂ as a function of ∆¹⁴C-DOC was significant (R² = 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 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.