Methane production in eutrophic Lake Plusssee: seasonal change, temperature effect and metabolic processes in the profundal sediment

Rates of CH4 production in the profundal sediment of Lake PluBsee were high er during the summer than during fall, and fairly well explained the CH4 co ncentration that accumulated in the hypolymnion during summer stratificatio n. Oxidation of CH4 at the oxycline played only a marginal role in the seas onal CH4 cycle. Although the sediment temperature (4 degreesC) did not chan ge with season, CH4 production rates could be stimulated by incubation at i ncreased temperature (25 degreesC). At both 4 degreesC and 25 degreesC, the rmodynamic conditions in the sediment allowed exergonic production of CH4 f rom either acetate, H-2/CO2 or methanol. Inhibition of methanogenesis by 2- bromoethane sulfonic acid (BES) resulted in the accumulation of acetate, bu t was only detectable at 25 degreesC. Inhibition of acetoclastic methanogen esis by methyl fluoride resulted in partial inhibition of CH4 and was only detectable at 25 degreesC. Incorporation of radioactive bicarbonate into CH 4 was also only detectable at 25 degreesC and then accounted for about 25-4 2% of total CH4 production. [2-C-14]acetate was converted to (CH4)-C-14 wit h turnover times of about 24 and 97h at 25 degreesC and 4 degreesC, respect ively, but accounted for only 4-9% of total CH4 production. The turnover ti mes of radioactive methanol were even larger (about 96 at 25 degreesC and 1 53 h at 4 degreesC) and accounted for <1% of total CH4 production. Hence, t hese processes could account for a larger percentage of CH4 production only if taking place in sediment microniches that did not fully equilibrate wit h the pore water. Production of CH4 at 25<degrees>C was stimulated by addit ion of pectin, acetate, methanol or H-2, but at 4 degreesC, it was only sti mulated by H-2 The stable carbon isotopic composition of CH4 and CO2 in the water column and in incubated sediment slurries indicated that CH4 product ion was dominated by acetoclastic methanogenesis, with H-2/CO2-dependent me thanogenesis contributing more at 25 degreesC than 4 degreesC. In conclusio n, the data are consistent with the assumption that CH4 production at in-si tu temperature (4 degreesC) was due to acetoclastic methanogenesis tightly coupled to acetate production, whereas it, was due to both acetoclastic and H-2/CO2-dependent methanogenesis at 25 degreesC. This assumption is also c onsistent with the observation that the sediment fermented added glucose or pectin to acetate as a major product.