Species-specific methane production in anoxic seagrass sediments is linked to carbon source quality: a microcosm study

Seagrass meadows play a significant role in capturing and storing organic carbon, both from its own primary production and from other adjacent habitats. A high accumulation of organic matter can cause oxygen depletion in the sediment due to aerobic remineralization, leaving space for anaerobic remineralization such as methanogenesis, where methane is produced in the final step. The extent of methane production is likely influenced by the quality of the degrading organic matter. In a long-term microcosm experiment, we studied how organic matter from eight different macrophytes, including seagrass (Zostera marina), reed (Phragmites australis), green macroalgae (Ulva lactuca, Ulva sp.), brown macroalgae (Fucus serratus, Ectocarpales), and red macroalgae (Furcellaria lumbricalis and Polysiphonia spp.), added as a carbon source, affects methane production in anoxic seagrass sediments from the cold-temperate Swedish west coast. Sediment from a unvegetated area with low carbon input was mixed with a small amount of seagrass sediment, functioning as an inoculum of the associated microbial community. The sediment was then placed in glass bottles and each carbon source (dried and homogenized) were added separately to the different bottles and mixed in with the sediment. Seawater was added but leaving enough space for a gas phase. The bottles were kept under anoxic condition with nitrogen flushing and a gas-tight septa. The methane in the gas phase were measured regularly (from twice a week in the beginning to once a month at the end) for nearly two years. The total emission was calculated as the cumulative methane emitted from every measuring point during the experiment. Our preliminary findings revealed that methane emission patterns were highly dependent on the carbon source, with the total cumulative methane emission during the experiment ranging between 0.01 and 47.42 mg, which corresponds to a loss of 0.0009 to 4.5 % of the added organic carbon. The two selected red macroalgae in this study, although of different plants structure (i.e. one filamentous and one more rigid), both generated significantly higher methane emission than all the other carbon sources. Thereby, the origin of carbon source appeared to have a greater influence on methane production than the plant structure. From the selected carbon sources, the treatment with red macroalgae yielded the highest methane levels, while the treatment with green macroalgae yielded the lowest levels. Interestingly, the treatment with organic carbon from Z. marina started to produce methane later than any other treatment and showed relatively low methane emissions (with a cumulative value of 2.70 mg corresponding to 0.26% of the added organic carbon) throughout the study period, highlighting that carbon source composition is crucial for the methane emission levels from seagrass meadow sediments.