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Elevated sulfate levels in eutrophic lakes have been observed to induce the release of endogenous phosphorus. While previous studies have predominantly examined its impact on iron-bound phosphorus (FeP), the influence on organic phosphorus (OP), a crucial active phosphorus component in sediments, remains poorly understood. In this study, mesocosms were established with lactate supplementation and varying sulfate concentrations to explore sulfate reduction and its impacts on phosphorus mobilization in freshwater sediments. Lactate addition induced hypoxia and provided substrates, thereby stimulating sulfate reduction with a decline of sulfate levels, an increase of sulfide concentrations, and fluctuations of sulfate-reducing bacteria. Meanwhile, concentrations of total dissolved phosphorus and phosphate were dramatically promoted during lactate decomposition, with a higher sulfate concentration associated with greater phosphorus elevation, correlating with the decrease of total phosphorus in sediment. The increase in phosphorus of the overlying water was partly attributed to FeP release from the sediment, confirmed by a decrease in its sediment content. FeP release was ascribed to dissimilatory reduction of iron oxides or chemical reduction mediated by sulfides in anoxic sediments, leading to the desorption and subsequent release of phosphorus. Evidences included the proliferation of iron-reducing bacteria, a decrease in Fe(II) concentrations in sediment pore- water, and the continuous accumulation of solid iron sulfides in surface sediments. Furthermore, OP mineralization in sediment also contributed to the increase in phosphorus in water columns, confirmed by a reduction in its content and the abundance of fermentation bacteria in surface sediment. Notably, the decrease in OP content accounted for >80 % of the total phosphorus reduction in surface sediment in the end. Thus, sulfur cycling plays a critical role in iron and phosphorus cycling, significantly stimulating not only the mobilization of FeP but also OP in sediments, with OP mineralization potentially being the primary contributor to endogenous phosphorus release., Competing Interests: Declaration of competing interest The manuscript has no conflict of interest including any financial, personal or other relationships with other people or organizations., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Although cyanobacteria blooms lead to an increase in methane (CH 4 ) emissions in eutrophic lakes have been intensively studied, the methane production pathways and driving mechanisms of the associated CH 4 emissions are still unclear. In this study, the hypereutrophic Lake Taihu, which has extreme cyanobacteria accumulation, was selected to test hypothesis of a potential methylotrophic CH 4 production pathway. Field observation displayed that the CH 4 emission flux from the area with cyanobacteria accumulation was 867.01 μg m -2 ·min -1 , much higher than the flux of 3.44 μg m -2 ·min -1 in the non-cyanobacteria accumulation area. The corresponding abundance of methane-producing archaea (MPA) in the cyanobacteria-concentrated area was 77.33% higher than that in the non-concentrated area via RT-qPCR technologies. Synchronously, sediments from these areas were incubated in anaerobic bottles, and results exhibited the high CH 4 emission potential of the cyanobacteria concentrated area versus the non-concentrated area (1199.26 vs. 205.76 μmol/L) and more active biological processes (CO 2 emission, 2072.8 vs. -714.62 μmol/L). We also found evidence for the methylotrophic methane producing pathway, which contributed to the high CH 4 emission flux from the cyanobacteria accumulation area. Firstly, cyanobacteria decomposition provided the prerequisite of abundant methyl thioether substances, including DMS, DMDS, and DMTS. Results showed that the content of methyl thioethers increased with the biomass of cyanobacteria, and the released DMS, DMDS, and DMTS was up to 96.35, 3.22 and 13.61 μg/L, respectively, in the highly concentrated 25000 g/cm 3 cyanobacteria treatment. Then, cyanobacteria decomposition created anaerobic microenvironments (DO 0.06 mg/L and Eh -304.8Mv) for methylotrophic methane production. Lastly, the relative abundance of Methanosarcinales was increased from 7.67% at the initial stage to 36.02% at the final stage within a sediment treatment with 10 mmol/L N(CH 3 ) 3 . Quantitatively, the proportion of the methylotrophic methane production pathway was as high as 32.58%. This finding is crucial for accurately evaluating the methane emission flux, and evaluating future management strategies of eutrophic lakes., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Sulfur reduction in freshwater ecosystems has previously been considered as negligible because of often very low sulfate concentrations and generally low sulfate reducing capacity in freshwater sediments. In this study, seasonal variations on three types of sediments from central lake, dredged and algae accumulated areas in a eutrophic lake in China, Lake Taihu, were investigated. The high temperature in summer and the accumulation of algae are conducive to the reduction processes in freshwater lake sediments. Iron reduction was observed as the major anaerobic process in all types of sediments, while sulfate reduction was weak in central and dredged lake areas. However, strong sulfate reduction with increase of sulfate reducing bacteria and sulfides generation (119.5 ± 0.2 μmol L -1 ) was found in surface sediments in algae accumulated areas. Based on the results of Fe reduction rate and the quantity of Fe reducing bacteria, extensive sulfate reduction in algae accumulated sediments inhibited the microbial Fe reduction, and the ΣS 2- -mediated chemical Fe reduction (SCIR) dominated instead. Iron was principally stored in the sediments as Fe sulfide compounds, which weakened the rebinding of phosphorus and stimulated phosphorus mobilization. Therefore, attention should be paid to the alteration of Fe cycling and phosphorus mobility caused by the SCIR in algae accumulated sediments and the consequent effects on the eutrophication of freshwater lakes., Competing Interests: Declaration of competing interest The authors declare that they have no competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020. Published by Elsevier B.V.)

Except for excessive nutrient input and climate warming, the rapidly rising SO 42- concentration is considered as a crucial contributor to the eutrophication in shallow lakes; however, the driving process and mechanism are still far from clear. In this study, we constructed a series of microcosms with initial SO 42- concentrations of 0, 30, 60, 90, 120, and 150 mg L -1 to simulate the rapid SO 42- increase in Lake Taihu, China, subjected to cyanobacteria blooms. Results showed that the sulfate reduction rate was stimulated by the increase in initial SO 42- concentrations and cyanobacteria-derived organic matter, with the maximal sulfate reduction rate of 39.68 mg (L d) -1 in the treatment of 150 mg L -1 SO 42- concentration. During the sulfate reduction, the produced maximal ∑ S 2- concentration in the overlying water and acid volatile sulfate (AVS) in the sediments were 3.15 mg L -1 and 11.11 mg kg -1 , respectively, and both of them were positively correlated with initial SO 42- concentrations (R2=0.97 ; R2= 0.92). The increasing abundance of sulfate reduction bacteria (SRB) was also linearly correlated with initial SO 42- concentrations (R2=0.96), ranging from 6.65×107 to 1.97×108 copies g -1. However, the Fe 2+ concentrations displayed a negative correlation with initial SO 42- concentrations, and the final Fe 2+ concentrations were 9.68, 7.07, 6.5, 5.57, 4.42, and 3.46 mg L -1 , respectively. As a result, the released total phosphorus (TP) in the overlying water, to promote the eutrophication, was up to 1.4 mg L -1 in the treatment of 150 mg L -1 SO 42- concentration. Therefore, it is necessary to consider the effect of rapidly increasing SO 42- concentrations on the release of endogenous phosphorus and the eutrophication in lakes. [ABSTRACT FROM AUTHOR]