Carbon Fixation by Photosynthetic Mats Along a Temperature Gradient in a Tengchong Hot Spring

Abstract Geothermal hot springs, widely distributed on Earth, are important geological greenhouse gas sources which contribute large amounts of CO 2 emission to the atmosphere. Exploring microbial carbon fixation in these springs is vital to fully understand the carbon budget in these terrestrial aquatic systems. In this study, carbon fixation rate by photosynthetic mats was determined with a 13 C‐labeling approach along a gradient of temperature (~69– ~75 °C) in a geothermal pool at Rehai Park in Tengchong, Yunnan Province, China. Light experiments were performed to determine carbon fixation rate by both photosynthesis and chemosynthesis, while dark experiments were performed by chemosynthesis. The results showed that the highest carbon fixation rate (4,637 μg C/g TOC/hr) was obtained in light at the lowest temperature (~69 °C) in the afternoon (1:00 to 6:00 p.m.), most of which (96.7%) was contributed by photosynthesis. The lowest carbon fixation rate (138 μg C/g TOC/hr) was measured at the highest temperature (~75 °C), where chemosynthesis dominated over photosynthesis in carbon fixation. At these rates, the photosynthetic mats would fix 80% of CO 2 emission during the daytime. At the end of daytime, the photosynthetic mats respired 34–70% of newly fixed organic carbon in 1 hr. Interestingly, both the carbon fixation rate and respiration rate decreased exponentially with increasing temperature, which may be ascribed to a temperature dependence of both microbial community composition and enzymatic activity. In summary, our study revealed that biological carbon fixation by photosynthetic mats is highly temperature dependent and significantly affects carbon cycling in hot springs.