Ecosystems

Lakes are a significant source of atmospheric methane, although methane-oxidizing bacteria consume most methane diffusing upward from anoxic sediments. Diverse methane-oxidizing bacteria form an effective methane filter in the water column of stratified lakes, yet, niche partitioning of different methane-oxidizing bacteria along the oxygen–methane counter gradient remains poorly understood. In our study, we reveal vertical distribution patterns of active methane-oxidizing bacteria along the oxygen–methane counter gradient of four lakes, based on amplicon sequencing analysis of 16S rRNA and pmoA genes, and 16S rRNA and pmoA transcripts, and potential methane oxidation rates. Differential distribution patterns indicated that ecologically different methane-oxidizing bacteria occupied the methane-deficient and oxygen-deficient part above and below the oxygen–methane interface. The interface sometimes harbored additional taxa. Within the dominant Methylococcales, an uncultivated taxon (CABC2E06) occurred mainly under methane-deficient conditions, whereas Crenothrix-related taxa preferred oxygen-deficient conditions. Candidatus Methylomirabilis limnetica (NC10 phylum) abundantly populated the oxygen-deficient part in two of four lakes. We reason that the methane filter in lakes is structured and that methane-oxidizing bacteria may rely on niche-specific adaptations for methane oxidation along the oxygen–methane counter gradient. Niche partitioning of methane-oxidizing bacteria might support greater overall resource consumption, contributing to the high effectivity of the lacustrine methane filter.

Global-scale N-oxide contamination of groundwater within aquifers occurs due to the widespread use of N-bearing fertilizers and chemicals, threatening both human and environmental health. However, the conversion of these pollutants in active nitrogen (N) cycling processes in the subsurface biosphere still remains unclear. This study investigates the global occurrence of anaerobic ammonium oxidation (anammox) in aquifers, where anammox was found to be turned on and off between saturated and unsaturated soil horizons, and contributed 36.8–79.5% to N loss in saturated soil horizons, the remainder being due to denitrification which has traditionally been considered the main pathway for removal of N-pollutants from aquifers. Although anammox activity was undetectable in the unsaturated soil horizons, it could potentially be activated by contact with ascending groundwater. High-throughput pyrosequencing analysis identified Candidatus Brocadia anammoxidans as being the most abundant anammox bacterium in the saturated soils investigated. However, the anammox bacterial abundance was determined by the relative richness of Candidatus Jettenia asiatica. Isotopic pairing experiments revealed that coupling anammox with ammonium oxidation and respiratory ammonification enabled the formation of a revised N cycle in aquifer systems, in which respiratory ammonification acted as an important coordinator. Anammox can therefore contribute substantially to aquifer N cycling and its role in remediation of aquifers contaminated with N-oxides may be of global importance.

Remote sensing of solar-induced chlorophyll fluorescence (SIF) is a rapidly advancing front in terrestrial vegetation science, with emerging capability in space-based methodologies and diverse application prospects. Although remote sensing of SIF – especially from space – is seen as a contemporary new specialty for terrestrial plants, it is founded upon a multi-decadal history of research, applications, and sensor developments in active and passive sensing of chlorophyll fluorescence. Current technical capabilities allow SIF to be measured across a range of biological, spatial, and temporal scales. As an optical signal, SIF may be assessed remotely using high-resolution spectral sensors in tandem with state-of-the-art algorithms to distinguish the emission from reflected and/or scattered ambient light. Because the red to far-red SIF emission is detectable non-invasively, it may be sampled repeatedly to acquire spatio-temporally explicit information about photosynthetic light responses and steady-state behaviour in vegetation. Progress in this field is accelerating with innovative sensor developments, retrieval methods, and modelling advances. This review distills the historical and current developments spanning the last several decades. It highlights SIF heritage and complementarity within the broader field of fluorescence science, the maturation of physiological and radiative transfer modelling, SIF signal retrieval strategies, techniques for field and airborne sensing, advances in satellite-based systems, and applications of these capabilities in evaluation of photosynthesis and stress effects. Progress, challenges, and future directions are considered for this unique avenue of remote sensing.

Marine emissions of dimethyl sulfide (DMS) to the atmosphere play a fundamental role in the global sulfur (S) cycle and have important consequences for the Earth’s radiative balance. In the ocean, DMS is mainly produced by marine algae and bacteria via cleavage of the precursor compound dimethylsulfoniopropionate (DMSP). Here, we studied the reaction between DMS and the strong oxidant hypobromous acid (HOBr), which is also produced by marine algae. Further, reactions between DMS oxidation products and HOBr were studied. The second-order rate constants were determined in competition kinetic experiments using sulfite as a competitor. In addition, we developed a new HPLC-ICP-MS/MS method to identify and quantify the oxidation products of DMS and related compounds. We found that HOBr reacts very fast with DMS to dimethyl sulfoxide (DMSO), with a second-order rate constant of 1.6 × 10⁹ M^–1 s^–1, while the subsequent oxidation of DMSO to dimethyl sulfone (DMSO₂) is much slower (0.4 M^–1 s^–1). Concentrations of DMSP, DMSO₂, and methanesulfonic acid (MSA) did not decrease when exposed to excess concentrations of HOBr, implying that these S-containing compounds are not or only slightly reactive toward HOBr. A quantitative comparison of known DMS sinks shows that HOBr may be an important, hitherto neglected sink for marine DMS that needs to be considered in ocean–atmosphere chemistry models.

Photodegradation processes play an important role in releasing elements tied up in biologically refractory forms in the environment, and are increasingly recognized as important contributors to biogeochemical cycles. While complete photooxidation of dissolved organic carbon (to CO₂), and dissolved organic phosphorous (to PO₄^3-) has been documented, the analogous photoproduction of sulfate from dissolved organic sulfur (DOS) has not yet been reported. Recent high-resolution mass spectrometry studies showed a selective loss of organic sulfur during photodegradation of dissolved organic matter, which was hypothesized to result in the production of sulfate. Here, we provide evidence of ubiquitous production of sulfate, methanesulfonic acid (MSA) and methanesulfinic acid (MSIA) during photodegradation of DOM samples from a wide range of natural terrestrial environments. We show that photochemical production of sulfate is generally more efficient than the production of MSA and MSIA, as well as volatile S-containing compounds (i.e., CS₂ and COS). We also identify possible molecular precursors for sulfate and MSA, and we demonstrate that a wide range of relevant classes of DOS compounds (in terms of S oxidation state and molecular structure) can liberate sulfate upon photosensitized degradation. This work suggests that photochemistry may play a more significant role in the aquatic and atmospheric fate of DOS than currently believed.