Dark carbon fixation in stream carbon cycling

Abstract

Headwater streams are often characterized by turbulence, organic matter inputs from terrestrial systems, net heterotrophy, and the microbial loop supplying carbon and energy for consumers. However, ecological models overlook dark carbon fixation (DCF), the light‐independent inorganic carbon uptake, mainly based on chemosynthesis, using energy yields from redox reactions. The quantification of microbial biomass production, including DCF, heterotrophic production (HP), gross primary production (GPP), and ecosystem respiration (ER) in lotic aquatic systems, has long yet to be addressed. Here, we investigate HP and DCF in water, sediment, and litter in addition to GPP and ER from streams in pristine rainforests in three distinct sub‐basins of the Amazon River, assessing the variety of turbid, black, and clear waters. We observed mean (min–max) values of microbial biomass production of about 0.1 (0.02–1.2), 3.2 (0.8–14.1), and 0.1 (0.02–0.5) mg C m−2 h−1 in water, sediment, and litter samples, in which DCF : HP showed mean (min–max) values of 0.5 (0.2–2), 0.02 (0.001–0.07), and 0.2 (0.001–0.5). Hence, measurements yielded DCF of similar magnitude as HP in water and litter but significantly lower in sediment, indicating that DCF supplied more carbon to planktonic and litter microbes than in top sediments of streams. Literature comparisons show similar DCF and GPP, both being lower than ER in streams. Finally, we found stream DCF higher than in lentic systems, suggesting that flow and turbulence may accelerate chemosynthesis.

Water stagnancy and wastewater input enhance primary productivity in an engineered river system

“…In this study, primary productivity varied along the continuum of Sabarmati and Mahi with lower rates in the upstream reaches of the Sabarmati as compared to its downstream reaches (Figure 2). These rates of primary productivity were similar to the rates (dark carbon fixation) estimated in streams using 14 C labeled tracer (Machado-Silva et al, 2023). Considering an euphotic layer of 10 cm and recalculating our rates to compare with literature data, the primary productivity varied from 0.22 to 147.53 mg C m −2 d −1 , which was comparable to the range of values of stream GPP (1-2500 mg C m −2 d −1 ) reported by studies elsewhere (Battin et al, 2008;Bernhardt et al, 2022;Gagne-Maynard et al, 2017;Neely & Wetzel, 1995;Odum, 1956;Wissmar et al, 1981).…”

“…Rivers play a significant role in the global carbon (C) cycle, as they act as active reactors of organic matter and conduits of greenhouse gases to the atmosphere (Aufdenkampe et al, 2011;Cole et al, 2007;Drake et al, 2018). Rivers as bioreactors are sustained by the availability and degradation of organic matter (Machado-Silva et al, 2023). Apart from allochthonous inputs, in situ primary productivity is a major contributor of organic matter in rivers by fixing inorganic C into biomass.…”