Bacterial utilization of different size classes of dissolved organic matter

Abstract

Bacterial utilization of high‐molecular‐weight (HMW; > 1 kDa) and low‐molecular‐weight (LMW; <1 kDa) dissolved organic C (DOC) was investigated in freshwater and marine systems by measuring dissolved oxygen consumption, bacterial abundance, and bacterial production in size‐fractionated samples. Tangential‐flow ultrafiltration was used to separate HMW and LMW DOC. More than 80% of the DOC in Amazon River samples was recovered in the HMW fraction, whereas most marine DOC (up to 70%) was of LMW. Bacterial growth efficiencies were consistently higher in the LMW fractions (16‐66%) than in the HMW fractions (8‐39%), indicating compositional differences in the two size fractions. In all experiments, measured rates of bacterial growth and respiration in HMW incubations were higher than those in LMW incubations. Carbon‐normalized bacterial DOC utilization rates were 1.4–4‐fold greater in the HMW fractions than in the LMW fractions, and a greater proportion (0.7–22.5%) of HMW DOC was utilized per day than LMW DOC (0.5–6.6%). All bacterial growth and respiration measurements indicated that HMW DOC was utilized to a greater extent than LMW DOC in all environments investigated. The traditional model of DOM degradation, stating that LMW compounds are most bioreactive, does not appear to apply to the bulk of natural DOM. Rather, the data and results from independent studies suggest a new conceptual model whereby the bioreactivity of organic matter decreases along a continuum of size (from large to small) and diagenetic state (from fresh to old). This size‐reactivity continuum model suggests that the bulk of HMW DOM is more bioreactive and less diagenetically altered than the bulk of LMW DOM.