Elucidating stream bacteria utilizing terrestrial dissolved organic matter

Terrestrial dissolved organic matter (tDOM) is susceptible to microbiological and photolytic oxidations and contributes significantly to the energy flow in aquatic ecosystems. However, bacterial species actively utilizing this tDOM are still not determined. We here elucidated the microbial groups actively utilizing tDOM. We characterized sediment microbial biomass and community structure using phospholipid phosphate and phospholipid fatty acids (PLFAs) analysis, respectively, and identified metabolically active members using PLFA stable-isotope-probing. Prokaryotes comprised 61% of the streambed microbial community consisting of aerobic, facultative anaerobic and anaerobic bacteria while microeukaryotes comprised the remaining 39%. Sediments were incubated in re-circulating mesocosm chambers amended with leachate from composted ¹³C-labelled tulip poplar (Liriodendron tulipifera L.) tree-tissues and examined for ¹³C incorporation into microbial PLFAs. The structure of stream sediment microbial communities prior to and after mesocosm incubation, in both the presence and absence of ¹³C-labeled DOM, showed no significant differences and indicated our mesocosm-based experimental design as sufficiently robust to investigate the utilization of ¹³C-DOM by sediment microbial communities. After 48 h of incubation, bacterial fatty acids i15:0, a15:0, 16:0, 16:1ω9, 18:1ω9c, 18:1ω7c, 10me16 and cy19:0 showed increased abundance of ¹³C. This identified the aerobic, facultative anaerobic and anaerobic bacteria as actively utilizing the ¹³C-labeled DOM. A single dark 48 h incubation showed incorporation into both bacterial and microeukaryotic fatty acids (20:4ω6, 20:5ω3) suggesting that microeukaryotic predators consumed bacteria that utilized ¹³C-labeled DOM. Hence, our data support the hypothesis that streamwater tDOM is utilized by stream bacteria, and substantially contributes to the energy flow in aquatic ecosystems.