PNNL

Abstract

Interactions between autotrophs and heterotrophs are central to carbon exchanges across trophic levels in essentially all ecosystems. Metabolite exchange is a frequent mechanism for this transfer and is key for distributing freshly fixed carbon (C) between spatially structured ecosystems, such as microbial mats. Despite the importance of carbon exchange for community function, the timescales at which fixed C is transferred from autotrophs to heterotrophs is poorly understood. We employed a stable isotope tracer combined with spatially resolved isotope analysis to quantify photoautotrophic uptake of bicarbonate and track subsequent exchange of this freshly fixed C across a vertical depth gradient coinciding with photon availability. We observed dynamics over a diel cycle whereby C spatial mobility was highest during periods of active photoautotrophy. Parallel mat incubations with 13C-labeled organic substrates (acetate and glucose) had much less vertical exchange of C when compared to bicarbonate incubations. Stable isotope proteomic analysis revealed rapid C exchange between cyanobacterial and heterotrophic community members based on 13C incorporated into identified proteins during the day but a decrease in the distribution of this isotope signature at night. We observed strong diel control on the spatial exchange of freshly fixed C within tightly interacting mat communities suggesting a rapid redistribution, both spatially and taxonomically, primarily during daylight periods.