Carbon fixation continues through winter
A paper published in Carbon Research reports that microbial inorganic carbon fixation remains active in Qinghai Lake , the largest saline lake in China, during the ice-covered season. The work focuses on how microorganisms assimilate inorganic carbon beneath lake ice, a period that has received far less attention than the summer open-water season despite its importance for annual lake carbon cycling.
The team, led by corresponding author Xiaoyan Li of Beijing Normal University , examined both light-dependent carbon fixation and dark-dependent carbon fixation in under-ice waters. Qinghai Lake, located on the Tibetan Plateau , provides a natural setting for this question because it is a high-altitude saline lake with prolonged seasonal ice cover, low temperatures, and distinctive microbial habitats.
Tracking carbon uptake under ice
To quantify microbial uptake of inorganic carbon, the authors used a ¹³C-bicarbonate labelling approach combined with geochemical measurements and microbial community analyses. Water samples were collected on March 1, 2024, from three representative sites and multiple depths, then incubated in situ beneath the ice in transparent and opaque bottles to distinguish light-driven and dark-driven uptake over a 6-hour period.
Across all samples, total microbial carbon fixation ranged from 4.92 to 13.09 µg C/(L·h). Dark-dependent carbon fixation was significantly greater than light-dependent carbon fixation, contributing on average 61.06 ± 10.95% of total microbial carbon fixation. Dark fixation ranged from 2.82 to 9.21 µg C/(L·h), whereas light fixation ranged from 1.14 to 4.37 µg C/(L·h). These results indicate that dark-dependent carbon fixation is a major winter carbon assimilation pathway in this saline lake.
Opposite depth patterns point to dual controls
The vertical patterns of the two pathways moved in opposite directions. Light-dependent carbon fixation increased with depth, while dark-dependent carbon fixation decreased with depth. The authors interpret this contrast through an environment–microbe–function framework , in which environmental gradients and microbial community structure jointly regulate carbon fixation under ice.
According to the reported correlations, dark-dependent fixation was significantly influenced by temperature and nitrate, while light-dependent fixation was associated with temperature, sulfate, and chlorophyll a. The two pathways were also linked to different microbial genera. Dark fixation correlated with Nannochloropsis, Bacteroides, Pyramimonas, Tetracystis, and Hemiselmis , whereas light fixation was associated with Leptolyngbya, Amphora, Navicula, and Surirella . The authors propose that these distinct associations reflect ecological niche partitioning between winter carbon-fixing communities.
Why the winter signal matters
The findings address a gap in understanding of carbon cycling in alpine saline lakes during frozen periods. The paper argues that strong attenuation of light by ice and snow, together with near-freezing water temperatures, suppresses phototrophic activity and shifts the balance toward dark carbon fixation. In Qinghai Lake, this makes winter microbial processes relevant to the lake’s broader role as a carbon sink.
The authors also place Qinghai Lake in a wider aquatic context, noting that its winter dark carbon fixation rates are higher than those reported for many ice-covered lakes, especially polar systems, while remaining lower than the very high rates observed in some alkaline saline lakes. That intermediate position suggests that under-ice dark fixation in this system is substantial rather than negligible.
Limits and next steps
The paper identifies several limitations. Sampling covered only three sites, deep-water areas greater than 15 m were inaccessible because of unsafe ice conditions, and sequencing targeted cyanobacteria and phytoplankton rather than the full bacterioplankton community. In addition, the observations represent a single time point during the ice-covered season, so temporal changes across winter were not resolved.
Future work proposed by the authors includes broader spatial coverage, deeper-water sampling, and identification of the active microorganisms involved in both dark and light carbon fixation. Such efforts would improve estimates of microbial carbon fixation fluxes in saline lakes and strengthen predictions of how seasonally frozen lake ecosystems respond to ongoing climate change.
Corresponding Author: Xiaoyan Li
Original Source: https://doi.org/10.1007/s44246-026-00291-3
Contributions: All authors contributed to the study conception and design. Data curation, resources, and drafting of the original manuscript were performed by Yixuan Han. Reviewing and editing of the manuscript were performed by Fangzhong Shi. Data curation and resources were performed by Xin Liu, Zhigang Wang and Rui Wang. Conceptualization, resources, and data curation were performed by Xiaoyan Li. All authors read and approved the final manuscript.
Carbon Research
Experimental study
Not applicable
Microbial inorganic carbon fixation characteristics in the largest saline lake of China during the ice-covered period
3-Aug-2026
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.