Methylphosphonate-driven methane formation and its link to primary production in the oligotrophic North Atlantic

Cold-Temperate Mountainous Freshwater Produces Methane by Algal Metabolism

We reported important environmental drivers of dissolved CH4 concentrations (d-CH4) in nutrient-limited mountainous freshwater in a cold-temperate region and explored the potential for multiple known oxic CH4 production pathways. Field investigation revealed consistent supersaturated d-CH4 in surface water (relative to the theoretical value of d-CH4 at atmospheric equilibrium), with significant seasonal variations. Statistical analysis highlighted the direct impact of algal dynamics and the indirect effect of temperature and nutrients on d-CH4. Further lab-scale incubation demonstrated that CH4 production decreased by 55.25 to 93.65% with algae removal, while it increased 4 to 10 times with methylphosphonate (MPn) amendment. These findings argued that CH4 produced from algal metabolism related to MPn had a high potential for supersaturated d-CH4. It also verified the pivotal role of cyanobacteria in this mechanism, with temperature and light acting as regulatory factors. Through highlighting the role of algae for CH4 characteristics in cold-temperate mountainous freshwater and proposing the potential of oxic CH4 production through MPn metabolism in nutrient-limited lakes, this study enriches comprehension of aquatic CH4 cycle and warns about the importance of preserving environmental balance in freshwater with minimal human disturbance.

Benthic methane fluxes and oxidation over the Western Indian Shelf: No evidence of pelagic methanotrophic denitrification

Despite its pelagic supersaturation and wide occurrence of gas-charged pockets in the sediments, methane occurs at unusually low concentrations in the shelf waters off Western Indian coast, even during euxinic events, compared to other anoxic coastal systems of the world. To understand the reason and benthic biogeochemistry of CH4, we measured benthic CH4 flux rates through whole-core incubations, and carried out CH4-spiked 15N-labeled incubations of the suboxic/anoxic shelf waters during 2011-2013. We observed very low rates of benthic CH4 influx or efflux (-0.23 to 0.37 μmol m-2 d-1) during normoxia, and low to moderate rates of benthic CH4 efflux (2.45-74.89 μmol m-2 d-1) during seasonal anoxia. However, high rates of the potential benthic CH4 efflux (23.82-154.25 μmol m-2 d-1) implied massive benthic CH4 oxidation. The diffusing CH4 was oxidized within the sediments up to 99-100% during normoxia and up to 51-89% during anoxia. The benthic-released CH4 was further oxidized in the water column aerobically at the rate of 0.84 μmol m-2 d-1 during normoxia, and anaerobically at the rate of 0.16 μmol m-2 d-1 during anoxia. However, we did not find any evidence for NOx- (NO3- and/or NO2-)-dependent anaerobic CH4 oxidation (N-DAMO) i.e. methanotrophic denitrification in the shelf waters during anoxia, which suggested the potential role of Fe3+ and Mn4+ and/or SO42- as the alternative oxidants in the anaerobic CH4 oxidation necessitating further research. The total annual benthic CH4- flux rate and oxidation rate from the entire innershelf were estimated to be 9.45 Gg y-1 and 30.71 Gg y-1, respectively. Our study revealed that benthic- and pelagic anaerobic CH4 oxidation combined together reduce the water column CH4 concentration by 78-98%. Therefore the shelf sediments act as an effective CH4 filter by substantially reducing the benthic CH4 flux and ultimately the sea-air CH4 emission.

Proteomic evidence for aerobic methane production in groundwater by methylotrophic Methylotenera

Members of Methylotenera are signature denitrifiers and methylotrophs commonly found together with methanotrophic bacteria in lakes and freshwater sediments. Here, we show that three distinct Methylotenera ecotypes were abundant in methane-rich groundwaters recharged during the Pleistocene. Just like in surface water biomes, groundwater Methylotenera often co-occurred with methane-oxidizing bacteria, even though they were generally unable to denitrify. One abundant Methylotenera ecotype expressed a pathway for aerobic methane production from methylphosphonate. This phosphate-acquisition strategy was recently found to contribute to methane production in the oligotrophic, oxic upper ocean. Gene organization, phylogeny, and 3D protein structure of the key enzyme, carbon-phosphorus lyase subunit PhnJ, were consistent with a role in phosphate uptake. We conclude that phosphate may be a limiting nutrient in productive, methane-rich aquifers, and that methylphosphonate degradation might contribute to groundwater methane production.

Strong Subseasonal Variability of Oxic Methane Production Challenges Methane Budgeting in Freshwater Lakes

Methane (CH4) accumulation in the well-oxygenated lake epilimnion enhances the diffusive atmospheric CH4 emission. Both lateral transport and in situ oxic methane production (OMP) have been suggested as potential sources. While the latter has been recently supported by increasing evidence, quantifying the exact contribution of OMP to atmospheric emissions remains challenging. Based on a large high-resolution field data set collected during 2019-2020 in the deep stratified Lake Stechlin and on three-dimensional hydrodynamic modeling, we improved existing CH4 budgets by resolving each component of the mass balance model at a seasonal scale and therefore better constrained the residual OMP. All terms in our model showed a large temporal variability at scales from intraday to seasonal, and the modeled OMP was most sensitive to the surface CH4 flux estimates. Future efforts are needed to reduce the uncertainties in estimating OMP rates using the mass balance approach by increasing the frequency of atmospheric CH4 flux measurements.

Correlation of methane production with physiological traits in Trichodesmium IMS 101 grown with methylphosphonate at different temperatures

The diazotrophic cyanobacterium Trichodesmium has been recognized as a potentially significant contributor to aerobic methane generation via several mechanisms including the utilization of methylphophonate (MPn) as a source of phosphorus. Currently, there is no information about how environmental factors regulate methane production by Trichodesmium. Here, we grew Trichodesmium IMS101 at five temperatures ranging from 16 to 31°C, and found that its methane production rates increased with rising temperatures to peak (1.028 ± 0.040 nmol CH4 μmol POC-1 day-1) at 27°C, and then declined. Its specific growth rate changed from 0.03 ± 0.01 d-1 to 0.34 ± 0.02 d-1, with the optimal growth temperature identified between 27 and 31°C. Within the tested temperature range the Q10 for the methane production rate was 4.6 ± 0.7, indicating a high sensitivity to thermal changes. In parallel, the methane production rates showed robust positive correlations with the assimilation rates of carbon, nitrogen, and phosphorus, resulting in the methane production quotients (molar ratio of carbon, nitrogen, or phosphorus assimilated to methane produced) of 227-494 for carbon, 40-128 for nitrogen, and 1.8-3.4 for phosphorus within the tested temperature range. Based on the experimental data, we estimated that the methane released from Trichodesmium can offset about 1% of its CO2 mitigation effects.

Mixotrophy in cyanobacteria

Cyanobacteria evolved the oxygenic photosynthesis to generate organic matter from CO2 and sunlight, and they were responsible for the production of oxygen in the Earth's atmosphere. This made them a model for photosynthetic organisms, since they are easier to study than higher plants. Early studies suggested that only a minority among cyanobacteria might assimilate organic compounds, being considered mostly autotrophic for decades. However, compelling evidence from marine and freshwater cyanobacteria, including toxic strains, in the laboratory and in the field, has been obtained in the last decades: by using physiological and omics approaches, mixotrophy has been found to be a more widespread feature than initially believed. Furthermore, dominant clades of marine cyanobacteria can take up organic compounds, and mixotrophy is critical for their survival in deep waters with very low light. Hence, mixotrophy seems to be an essential trait in the metabolism of most cyanobacteria, which can be exploited for biotechnological purposes.