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Purpose: We evaluated the influence of acetate and sulphate addition on methanogenesis in sediment layers of cores of different carbon and sulphate concentrations from the littoral region colonized by Typha domingensis Pers. (area 1) and from the limnetic region (area 2) of a tropical coastal lagoon separated from the sea by a sandbar. We hypothesized that area 1 presents a proportionally smaller potential methane production (PMP) on sulphate addition than area 2. We further hypothesized that acetate addition stimulates proportionally greater PMP in area 2 sediment layers than in area 1.

Materials and methods: The PMP rates were measured in sediment samples from three depths (0-2, 2-6, and 6-10 cm) in areas 1 and 2 for 89 days. The sediment incubations were prepared with additions of acetate (1 mM), sulphate (10 mM), and acetate plus sulphate (1 and 10 mM, respectively); the control treatment had no addition of acetate and/or sulphate. We also measured dissolved organic carbon (DOC), carbohydrates, and sulphate in the interstitial water.

Results and discussion: PMP responded differently to acetate and sulphate addition. In general, PMP in area 2 was proportionally more stimulated by the addition of acetate and less inhibited by the addition of sulphate. In the 0-2-cm layer, the addition of acetate did not stimulate PMP in relation to the control in area 1, reaching a rate of 376.11 nmol CH4 g(-1) day(-1). In area 2, PMP was 5-fold higher in relation to the control, reaching 5.4 nmol CH4 g(-1)day(-1). Contrarily, in the 6-10-cm layer, sulphate addition inhibited PMP by 127.8-fold in relation to the control in area 1, reaching 0.6 nmol CH4 g(-1) day(-1). In area 2, PMP decreased 3.2-fold in relation to the control, reaching a value of 3.7 nmol CH4 g(-1) day(-1). These results agreed with the higher amounts of carbohydrates and lower amounts of sulphate in area 1 in the experiment beginning.

Conclusions: Acetate and sulphate are effective PMP regulators in the sediment of two areas of a tropical coastal lagoon. Acetate increased PMP more in area 2, due to the lower sediment carbon availability. In contrast, sulphate decreased PMP more in area 1, due to the lower sulphate availability. The metabolic responses of methanogens and sulphate-reducing bacteria to natural and anthropogenic changes in carbon and sulphate availability are important for understanding the functioning of continental aquatic ecosystems, especially in scenarios of global climate change.