Iron oxides act as an alternative electron acceptor for aerobic methanotrophs in anoxic lake sediments.

Conventional aerobic CH4-oxidizing bacteria (MOB) are frequently detected in anoxic environments, but their survival strategy and ecological contribution are still enigmatic. Here we explore the role of MOB in enrichment cultures under O2 gradients and an iron-rich lake sediment in situ by combining microbiological and geochemical techniques. We found that enriched MOB consortium used ferric oxides as alternative electron acceptors for oxidizing CH4 with the help of riboflavin when O2 was unavailable. Within the MOB consortium, MOB transformed CH4 to low molecular weight organic matter such as acetate for consortium bacteria as a carbon source, while the latter secrete riboflavin to facilitate extracellular electron transfer (EET). Iron reduction coupled to CH4 oxidation mediated by the MOB consortium was also demonstrated in situ, reducing 40.3% of the CH4 emission in the studied lake sediment. Our study indicates how MOBs survive under anoxia and expands the knowledge of this previously overlooked CH4 sink in iron-rich sediments.

Methane distribution patterns along a transect of Lake Fuxian, a deep oligotrophic lake in China.

Freshwater ecosystems are recognized as one of the important natural methane (CH4) sources, but little is known about the emission hotspots and the effects of algal blooms on CH4 production in deep lakes. In this study, carried out from the littoral (S1), pelagic (S2-S4), and the deepest site (S5), water samples from different depths and sediment cores were collected along the transect of Lake Fuxian, a deep monomictic lake to investigate the spatial-temporal variations of CH4. Dissolved methane concentration observed at the oxic metalimnion was 37.5% and 19.5% higher than that those observed at the epilimnion and at the layer between 80 and 100 m depth, respectively. During the overturn period, the vertical distribution of CH4 in the water column was uniform, with an average concentration of 0.031 ± 0.007 µM in S2-S5. Statistical analysis indicated that the CH4 concentration in the water column was significantly higher in S1 than other sites along the transect during both sampling periods. Sediment CH4 concentration and methane production potential (MPP) were also significantly higher in S1 than in other sites. Along the sediment depth, the maximum MPP was observed at 6-8 cm in S1, but it moved up to the surface layer in S2-S5 in both sampling periods. In addition, stable carbon isotope analysis indicated that the surface sediments in the pelagic zone (S2-S5) mainly comprised autochthonous organic matters. In this zone, MPP had a significantly positive correlation with sediment total organic carbon (TOC) (R2 = 0.401, p < 0.01). In summary, we described the spatial and temporal distributions of CH4 in deep Lake Fuxian, littoral zones are CH4 emission hotspots that can contribute to the CH4 accumulation in the oxic metalimnion layer during the stratification period. In the pelagic zone, autochthonous organic matter was transported into the surface sediment after a massive algal bloom, representing another hotspot for CH4 production.

Small-Sized Microplastics Negatively Affect Rotifers: Changes in the Key Life-History Traits and Rotifer-Phaeocystis Population Dynamics.

Most coastal waters are at risk from microplastics, which vary in concentration and size. Rotifers, as important primary consumers linking primary producers and higher trophic consumers, usually coexist with the harmful alga Phaeocystis and microplastics in coastal waters; this coexistence may interfere with rotifer life-history traits and ingestion of Phaeocystis. To evaluate the effects of microplastics on rotifers, we designed a series of experiments concerning rotifer Brachionus plicatilis life-history traits and rotifer-Phaeocystis (predator-prey) population dynamics under different concentrations and sizes of microplastics. The results showed that small-sized microplastics (0.07 µm) at high levels (≥5 µg mL-1) decreased rotifer survival and reproduction, prolonged the time to maturation, and reduced the body size at maturation, whereas large-sized microplastics (0.7 and 7 µm) had no effect on rotifer life-history traits. For rotifer-Phaeocystis population levels, small-sized microplastics (0.07 µm) significantly delayed the elimination of Phaeocystis by rotifers; this is the first study to test the effects of microplastics on predator-prey dynamics. The results of rotifer-Phaeocystis population dynamics are consistent with the changes in the life-history traits of rotifers and further confirm the negative effects of small-sized microplastics (0.07 µm) on rotifers. These findings help to reveal the effect of pollutants on predator-prey population dynamics.