Oxic methane production from methylphosphonate in a large oligotrophic lake: limitation by substrate and organic carbon supply

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.

Epilimnetic oligotrophication increases contribution of oxic methane production to atmospheric methane flux from stratified lakes

Although considerable attention has been paid to the effects of eutrophication on aquatic methane (CH4) emissions to the atmosphere, the ecosystem-level effects of oligotrophication/re-oligotrophication on aquatic CH4 production and subsequent ecological responses remain to be elucidated. It has been hypothesized that dissolved inorganic phosphorus (DIP)-deficient conditions drive the ecosystem to utilize poorly bioavailable organic phosphorus for biomass formation, thereby generating CH4 as a by-product. To test this hypothesis, a mass balance approach was used to estimate in situ oxic methane production (OMP) in an oligotrophic, deep Lake Fuxian. The isotopic signature of dissolved 13C-CH4, the potential substrates for OMP, and the phnJ/phnD genes associated with microbial demethylation of organic phosphorus compounds were analyzed. Our results indicate that CH4 accumulation was maximal in the surface mixed layer (SML, i.e., Epilimnion) during lake stratification, and ∼ 86 % of the total CH4 flux to the atmosphere was due to OMP. Decomposition of methylphosphonate (MPn) by Alphaproteobacteria (genera Sphingomonas and Mesorhizobium) contributed significantly to OMP. Furthermore, water temperature (Temp), chlorophyll a (Chla), and DIP were the most critical predictors of water OMP potential. Meta-analysis of currently available global data showed that OMP had a negative exponential distribution with DIP (OMP = 2.0 e-0.71DIP, R2 = 0.57, p < 0.05). DIP concentrations below a threshold of 3.40 ∼ 9.35 μg P L-1 triggered OMP processes and increased the atmospheric CH4 emissions. Under future warming scenarios, stratification and catchment management induced oligotrophication or re-oligotrophication may systematically affect the biogeochemical cycling of phosphorus and the OMP contribution to CH4 emission in stratified lakes.

Out of sight, but not out of season: Nitrifier distributions and population dynamics in a large oligotrophic lake

Nitrification is an important control on the form and distribution of nitrogen in freshwater ecosystems. However, the seasonality of nitrogen pools and the diversity of organisms catalyzing this process have not been well documented in oligotrophic lakes. Here, we show that nitrogen pools and nitrifying organisms in Flathead Lake are temporally and vertically dynamic, with nitrifiers displaying specific preferences depending on the season. While the ammonia-oxidizing bacteria (AOB) Nitrosomonadaceae and nitrite-oxidizing bacteria (NOB) Nitrotoga dominate at depth in the summer, the ammonia-oxidizing archaea (AOA) Nitrososphaerota and NOB Nitrospirota become abundant in the winter. Given clear seasonality in ammonium, with higher concentrations during the summer, we hypothesize that the succession between these two nitrifying groups may be due to nitrogen affinity, with AOB more competitive when ammonia concentrations are higher and AOA when they are lower. Nitrifiers in Flathead Lake share more than 99% average nucleotide identity with those reported in other North American lakes but are distinct from those in Europe and Asia, indicating a role for geographic isolation as a factor controlling speciation among nitrifiers. Our study shows there are seasonal shifts in nitrogen pools and nitrifying populations, highlighting the dynamic spatial and temporal nature of nitrogen cycling in freshwater ecosystems.