Dissolved organic matter cycling revealed from the molecular level in three coastal bays of China

Macrophyte Restoration Promotes Lake Microbial Carbon Pump to Enhance Aquatic Carbon Sequestration

Macrophyte-based lake restoration has successfully transitioned lakes from turbid conditions dominated by phytoplankton to a more natural, clear state; however, its impact on microbial carbon pump-mediated dissolved organic carbon (DOM) storage and greenhouse gas (GHG) emissions in the aquatic ecosystem remains largely unexplored. Through a year-long field study, we conducted a comparative analysis of two alternative habitats within the same lake-restored and unrestored areas. Results demonstrated that restoration not only substantially decreases nutrient levels and algal blooms-evidenced by over 50% reductions in nitrogen, phosphorus, and chlorophyll a-but also significantly increases the accumulation of recalcitrant DOM. This is characterized by rises of 9.52% in highly unsaturated compounds, 8.68% in carboxyl-rich alicyclic molecules, 37.54% polycyclic condensed aromatics and polyphenols, and 20.21% in SUVA254. Additionallly, key microbial taxa with potent carbon pump functions-primarily Gammaproteobacteria, Alphaproteobacteria, and Actinobacteria-are enriched in restored areas. Structural equation modeling (SEM) further elucidated the complex interrelationships within more pristine lake ecosystems: macrophytes and elevated dissolved oxygen (DO) concentrations enhance carbon sequestration via microbial carbon pump pathways, while the restoration significantly mitigates methane emissions caused by eutrophication. These findings highlight an extra function of aquatic macrophyte restoration, offering valuable insights into microbial processes for future restoration efforts aimed at promoting sustainable aquatic ecosystems and mitigating global warming.

Seasonal effects of fish, seaweed and abalone cultures on dissolved organic matter and carbon sequestration potential in Sansha Bay, China

Coastal bays serve as undeniable dissolved organic matter (DOM) reactors and the role of prevalent mariculture in DOM cycling deserves investigation. This study, based on four seasonal field samplings and a laboratory incubation experiment, examined the source and seasonal dynamics of DOM and fluorescent dissolved organic matter (FDOM) in the seawater of fish (Larimichthys crocea, LC), seaweed (Gracilaria lemaneiformis, GL) and abalone (Haliotis sp., HA) culturing zones in Sansha Bay, China. Using three-dimensional fluorescence spectroscopy coupled with parallel factor analysis (EEMs-PARAFAC), three fluorescent components were identified, i.e. protein-like C1, protein-like C2, and humic-like C3. Our results showed that mariculture activities dominated the DOM pool by seasonal generating abundant DOM with lower aromaticity and humification degrees. Accounting for 40-95 % of total fluorescent components, C1 (Ex/Em = 300/340 nm) was regarded the same as D1 (Ex/Em = 300/335 nm) identified in a 180-day degradation experiments of G. lemaneiformis detritus, indicating that the cultured seaweed modulated DOM through the seasonal production of C1. In addition, the incubation experiment revealed that 0.7 % of the total carbon content of seaweed detritus could be preserved as recalcitrant dissolved organic carbon (RDOC). However, fish culture appeared to contribute to liable DOC and protein-like C2, exerting a substantial impact on DOM during winter but making a negligible contribution to carbon sequestration, while abalone culture might promote the potential export and sequestration of seaweed-derived carbon to the ocean. Our results highlight the influences of mariculture activities, especially seaweed culture, in shaping DOM pool in coastal bays. These findings can provide reference for future studies on the carbon accounting of mariculture.

Molecular composition and characteristics of Sediment-adsorbed Dissolved Organic Matter (SDOM) along the coast of China

Sediment-adsorbed Dissolved Organic Matter (SDOM) in coast plays a crucial role in the terrestrial and marine carbon cycle processes of the global environment. However, understanding the transport dynamics of SDOM along the coast of China, particularly its interactions with sediments, remains elusive. In this study, we analyzed the δ13C and δ15N stable isotopic compositions, as well as the molecular characteristics of SDOM collected from coastal areas spanning the Bohai Sea (BS), Yellow Sea (YS), East China Sea (ECS), and South China Sea (SCS), by using isotope ratio mass spectrometry and Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS). We identified the predominant sources of carbon and nitrogen in coastal sediments, revealing terrigenous origins for most C and N, while anthropogenic sources dominated in the SCS. Spatial variations in SDOM chemodiversity were observed, with diverse molecular components influenced by distinct environmental factors and sediment sources. Notably, lignins and saturated compounds (such as proteins/amino sugars) were the predominant molecular compounds detected in coastal SDOM. Through Mantel tests and Spearman's correlation analysis, we elucidated the significant influence of spatial environmental factors (temperature, DO, salinity, and depth) and sediment sources on SDOM molecular chemodiversity. These findings contribute to a more comprehensive understanding of the carbon cycle dynamics along the Chinese coast.