235 / 2025-04-15 16:33:30

Submerged plants and microbiome enhance carbon sink in karst aquatic ecosystems

karst,carbon sink,submerged plants,microorganism,dissolved oxygen

Theme 2: Geobiology of Modern Habitable Earth > Session 2a: Microbe-mediated Carbon Cycling on Modern Earth

The carbonate rock represents the largest inorganic carbon reservoir on Earth. The dissolution of it in water is reversible and rapid, which was an important link affecting the carbon cycle, required further research on karst and controlling mechanism to quantify the source and sink fluxes accurately, contributing to the global carbon budget balance, and addressing the global climate change scientifically.

In aquatic environment, the low diffusion rate of CO₂ often limits the availability of inorganic carbon for the photosynthesis of submerged plants. To address this challenge, many submerged plants have evolved inorganic carbon concentrating mechanisms to alleviate carbon deficiency caused by insufficient carbon source in water. Karst water, characterized by slight alkaline settings, primarily contains inorganic carbon in the form of HCO₃^-, serving as a vital carbon source to promote photosynthetic activity and facilitate the conversion of inorganic carbon into organic carbon, thereby significantly stabilizing the karst carbon sink.

This study focused on a typical karst basin in southwestern China and conducted the following work. Firstly, sediments analysis revealed the pH levels were positively correlated with particle size but negatively correlated with total organic carbon (TOC) and total nitrogen (TN), with sediment carbon and nitrogen sharing consistent sources. Biomass was found to influence sediment physicochemical properties, particularly the C/N ration. Secondly, using the dissolved oxygen (DO) method, we quantified the carbon sink flux mediated by submerged plants and microorganisms. The results demonstrate that submerged plant photosynthesis is the primary driver of diurnal hydrogeochemical variations, inhibiting the degassing effect of water-air interface, and presenting a high autotrophy. The ratio of photosynthesis to respiration is 1.53, with a 0.35 mmol/L^-1day^-1 average net ecosystem productivity of submerged plants and microorganisms. The average biomass of this river section was 74.18 kg/m², and the amount of inorganic carbon utilized and sequestrated is around 0.62 mg. Compared with the DO method, the result calculated by hydrochemical equilibrium only accounts for 34.29% of the carbon sink fixed by submerged plants, which greatly underestimates the amount of karst carbon sink produced by submerged plants. The discovery provides a new way for accurately assessing carbon sink and global carbon cycle.

Further research will focus on identifying critical microbial communities in karst aquatic environments and their different feedback on DO and other karst hydrogeochemistry dynamics, to distinguish the different mechanisms controlling by submerged plants and microorganisms affecting carbon cycle.