Abstract
Aquatic macrophytes usually show high productivity rate, especially in shallow environments, and may constitute the main source of organic matter to these ecosystems. The coastal lagoons are shallow environments that typically present a broad colonization by aquatic macrophytes. The organic matter derived from aquatic macrophytes consists of detritus and root exudates, from live plants. Methanogens are microorganisms that use labile organic matter (e.g. acetate) in the metabolism, releasing methane (CH4) as an end product. Assessing the influence of aquatic macrophytes on methanogenesis is fundamental to understanding the carbon cycle in shallow environments, such as coastal lagoons. A peculiarity of coastal lagoons that may also influence the methanogenesis is its proximity to the sea, providing the entrance of sulfate in the environment. The methanogenesis can be inhibited by the sulfate reduction when there is sulfate availability sulfate. In this context, we aimed to analyze the methane production in an aquatic macrophyte stand and in the limnetic region of a coastal lagoon, assessing the influence of quantity and quality of organic carbon and sulfate availability on methane production in the sediment profile. We observe that the presence of aquatic macrophytes benefits the methanogenesis, not only by detritus accumulation, but particularly by the release of root exudates from the living plants. The variation in quantity and quality of organic carbon is the main factor that controls the range and shape of the methane production curves. The availability of sulfate presents probably a secondary role, being important when the organic matter is not sufficient for the occurrence of methanogenesis and sulfate reduction simultaneously.
Highlights
Methanogenesis is the primary route of anaerobic decomposition of organic matter in freshwater sediments, accounting for 30% - 80% of anaerobic carbon (C) mineralization [1] [2]
Methane production was significantly higher in the macrophytes stand (Table 2, Figure 2 and Figure 3), indicating a positive effect of the detritus of aquatic macrophytes in methanogenesis
Methane production increased after the 27th day, while in the macrophytes stand the process started after the first day after the sediment incubation
Summary
Methanogenesis is the primary route of anaerobic decomposition of organic matter in freshwater sediments, accounting for 30% - 80% of anaerobic carbon (C) mineralization [1] [2]. In this process, the C is converted in methane (CH4). Many studies show that CH4 production is higher in the uppermost sediment layers, which receive fresh carbon inputs from aquatic macrophytes [6] [7], such as coastal lagoon [8] [9], indicating an interaction between aquatic macrophytes and methanogens, through the availability of substrates derived from plants to the microorganisms; 2) the proximity to the sea, providing the entry of seawater sulfate through sea spray or, more drastically, by natural or anthropogenic sandbar openings [10]. The two processes can occur simultaneously if there is sufficient availability of common substrates [12]
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