Reactivity, fate and functional roles of dissolved organic matter in anoxic inland waters

Microbial carbon oxidation in seawater below the hypoxic threshold

Global oxygen minimum zones (OMZs) often reach hypoxia but seldom reach anoxia. Recently it was reported that Michaelis Menten constants (Km) of oxidative enzymes are orders of magnitude higher than respiratory Km values, and in the Hypoxic Barrier Hypothesis it was proposed that, in ecosystems experiencing falling oxygen, oxygenase enzyme activities become oxygen-limited long before respiration. We conducted a mesocosm experiment with a phytoplankton bloom as an organic carbon source and controlled dissolved oxygen (DO) concentrations in the dark to determine whether hypoxia slows carbon oxidation and oxygen decline. Total oxygen utilization (TOU) in hypoxic treatment (ca. 7.1 µM O2) was 21.7% lower than the oxic treatment (ca. 245.1 µM O2) over the first 43 days of the experiment. In addition, following the restoration of fully oxic conditions to the hypoxic treatment, TOU accelerated, demonstrating that oxidative processes are sensitive to DO concentrations found in large volumes of the ocean. Microbial amplicon-based community composition diverged between oxic treatments, indicating a specialized microbiome that included Thioglobaceae (SUP05 Gammaproteobacteria), OM190 (Planctomycetota), ABY1 (Patescibacteria), and SAR86 subclade D2472, thrived in the hypoxic treatment, while the genus Candidatus Actinomarina and SAR11 alphaproteobacteria were sharply inhibited. Our findings support the hypothesis that oxygenase kinetics might slow the progression of ocean deoxygenation in oxygen-poor regions and be a factor in the evolution of microbial taxa adapted to hypoxic environments.

The chemical succession in anoxic lake waters as source of molecular diversity of organic matter

The aquatic networks that connect soils with oceans receive each year 5.1 Pg of terrestrial carbon to transport, bury and process. Stagnant sections of aquatic networks often become anoxic. Mineral surfaces attract specific components of organic carbon, which are released under anoxic conditions to the pool of dissolved organic matter (DOM). The impact of the anoxic release on DOM molecular composition and reactivity in inland waters is unknown. Here, we report concurrent release of iron and DOM in anoxic bottom waters of northern lakes, removing DOM from the protection of iron oxides and remobilizing previously buried carbon to the water column. The deprotected DOM appears to be highly reactive, terrestrially derived and molecularly distinct, generating an ambient DOM pool that relieves energetic constraints that are often assumed to limit carbon turnover in anoxic waters. The Fe-to-C stoichiometry during anoxic mobilization differs from that after oxic precipitation, suggesting that up to 21% of buried OM escapes a lake-internal release-precipitation cycle, and can instead be exported downstream. Although anoxic habitats are transient and comprise relatively small volumes of water on the landscape scale, our results show that they may play a major role in structuring the reactivity and molecular composition of DOM transiting through aquatic networks and reaching the oceans.

Suboxic DOM is bioavailable to surface prokaryotes in a simulated overturn of an oxygen minimum zone, Devil's Hole, Bermuda

Oxygen minimum zones (OMZs) are expanding due to increased sea surface temperatures, subsequent increased oxygen demand through respiration, reduced oxygen solubility, and thermal stratification driven in part by anthropogenic climate change. Devil's Hole, Bermuda is a model ecosystem to study OMZ microbial biogeochemistry because the formation and subsequent overturn of the suboxic zone occur annually. During thermally driven stratification, suboxic conditions develop, with organic matter and nutrients accumulating at depth. In this study, the bioavailability of the accumulated dissolved organic carbon (DOC) and the microbial community response to reoxygenation of suboxic waters was assessed using a simulated overturn experiment. The surface inoculated prokaryotic community responded to the deep (formerly suboxic) 0.2 μm filtrate with cell densities increasing 2.5-fold over 6 days while removing 5 μmol L-1 of DOC. After 12 days, the surface community began to shift, and DOC quality became less diagenetically altered along with an increase in SAR202, a Chloroflexi that can degrade recalcitrant dissolved organic matter (DOM). Labile DOC production after 12 days coincided with an increase of Nitrosopumilales, a chemoautotrophic ammonia oxidizing archaea (AOA) that converts ammonia to nitrite based on the ammonia monooxygenase (amoA) gene copy number and nutrient data. In comparison, the inoculation of the deep anaerobic prokaryotic community into surface 0.2 μm filtrate demonstrated a die-off of 25.5% of the initial inoculum community followed by a 1.5-fold increase in cell densities over 6 days. Within 2 days, the prokaryotic community shifted from a Chlorobiales dominated assemblage to a surface-like heterotrophic community devoid of Chlorobiales. The DOM quality changed to less diagenetically altered material and coincided with an increase in the ribulose-1,5-bisphosphate carboxylase/oxygenase form I (cbbL) gene number followed by an influx of labile DOM. Upon reoxygenation, the deep DOM that accumulated under suboxic conditions is bioavailable to surface prokaryotes that utilize the accumulated DOC initially before switching to a community that can both produce labile DOM via chemoautotrophy and degrade the more recalcitrant DOM.

Dynamics of organic matter in the changing environment of a stratified marine lake over two decades

The marine lake (Rogoznica Lake), which fluctuates between stratified and holomictic conditions, is a unique environment on the eastern Adriatic coast affected by environmental changes. These changes are reflected in the warming of the water column, the apparent deoxygenation of the epilimnion, and the accumulation of organic matter (OM), toxic sulfide, and ammonium in the anoxic hypolimnion. Since the early 1990s, the volume of anoxic water has increased as the chemocline has moved to the surface water layer. A trend toward enrichment of refractory dissolved organic carbon (DOC) was observed in the anoxic hypolimnion, while a decreasing trend was observed in the oxic epilimnion in the spring DOC. At the same time, the most reactive surface-active fraction of DOC showed the opposite trend. In addition, there is evidence of accumulation of particulate organic carbon (POC) in the water column, followed by an increase in the fraction of POC in total organic carbon (TOC). On a multi-year scale (1996-2020), this work presents a unique time series of the dynamics of OM in the stratified marine system, showing a significant change in its quantity and quality due to climate and environmental variability. DOC-normalized surfactant activity is shown to be a good indicator of environmental change.

Oxygen availability driven trends in DOM molecular composition and reactivity in a seasonally stratified fjord

Ocean deoxygenation could potentially trigger substantial changes in the composition and reactivity of dissolved organic matter (DOM) pool, which plays an important role in the global carbon cycle. To evaluate links between DOM dynamics and oxygen availability, we investigated the DOM composition under varying levels of oxygen in a seasonally hypoxic fjord through a monthly time-series over two years. We used ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to characterize DOM on a molecular level. We find a clear trend both in diversity and molecular composition of the DOM along the oxygen gradient. As oxygen decreased, the chemodiversity was significantly increased, along with accumulation of relatively high-molecular-weight, reduced and unsaturated compounds enriched with carboxyl-group structures, which were also thermodynamically less favorable to biodegradation. Our results suggested that oxygen depletion selectively protected otherwise bioavailable compounds from decomposition and may promote the accumulation of a larger recalcitrant DOM pool in the global ocean, which could provide negative feedback to the ocean carbon sequestration and climate change.

A new conceptual framework for the transformation of groundwater dissolved organic matter

Groundwater comprises 95% of the liquid fresh water on Earth and contains a diverse mix of dissolved organic matter (DOM) molecules which play a significant role in the global carbon cycle. Currently, the storage times and degradation pathways of groundwater DOM are unclear, preventing an accurate estimate of groundwater carbon sources and sinks for global carbon budgets. Here we reveal the transformations of DOM in aging groundwater using ultra-high resolution mass spectrometry combined with radiocarbon dating. Long-term anoxia and a lack of photodegradation leads to the removal of oxidised DOM and a build-up of both reduced photodegradable formulae and aerobically biolabile formulae with a strong microbial signal. This contrasts with the degradation pathway of DOM in oxic marine, river, and lake systems. Our findings suggest that processes such as groundwater extraction and subterranean groundwater discharge to oceans could result in up to 13 Tg of highly photolabile and aerobically biolabile groundwater dissolved organic carbon released to surface environments per year, where it can be rapidly degraded. These findings highlight the importance of considering groundwater DOM in global carbon budgets.

Dissolved organic matter concentration, molecular composition, and functional groups in contrasting management practices of peatlands

About 91,300 ha of peatlands has been rewetted in western Europe since the mid-1990s. Still, it is unknown how long-term rewetting alters the dissolved organic matter (DOM) concentration, molecular composition, and functional groups. We examined these DOM characteristics in three peatland types subjected to 47- to 231-yr drainage and 18- to 24-yr rewetting to address this knowledge gap. Cold water-extractable DOM was characterized by pyrolysis field ionization mass spectrometry (Py-FIMS) and X-ray absorption near-edge structure (XANES) spectroscopy. The dissolved organic carbon (DOC) concentration in the rewetted forest peatland was 2.7 times higher than in the drained forest peatland. However, rewetting decreased the DOC concentrations by 1.5 and 4 times in the coastal peatland and percolation mire, respectively, compared with their respective drained peatlands at the topsoil horizons. The Py-FIMS analysis revealed that all nine DOM compound classes' relative abundances differed between the rewetted and drained forest peatland with the lower relative abundances of the labile DOM compound classes in the rewetted forest peatlands. However, most DOM compound classes' relative abundances were similar between the rewetted and drained coastal peatlands and percolation mires. The XANES also revealed nine carbon and seven nitrogen functional groups with no apparent differences between the two contrasting management practices. The influence of drainage and rewetting on DOC concentration and molecular composition depends on peatland type, drainage period, rewetting intensity, and peat degradation status that should be considered in future research for understanding DOM transformation and transportation from degraded and restored peatland ecosystems.

The Stratified Distribution of Dissolved Organic Matter in an AMD Lake Revealed by Multi-sample Evaluation Procedure

As a typical extreme environment, acid mine drainage (AMD) has been extensively studied for its biogeochemical cycle, but little is known about the quality of dissolved organic matter (DOM) in AMD. In this study, DOM molecules in an AMD lake were detected with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and the change of DOM molecules in the stratified water column was analyzed with a multi-sample evaluation procedure. The results demonstrate that DOM quality is highly stratified and can be linked with severe biogeochemical gradients. In the surface layer, DOM molecules can be distinguished by low quantities and intensities, as well as potential photodegradation products. Oxygen-poor and oxygen-rich molecules alternately dominate the chemocline, which can be explained by the redox-dependent adsorption/desorption of DOM on metastable secondary minerals. A rich and abundant DOM pool with a high proportion of heteroatoms exists at the bottom which can be significantly influenced by material exchange with sediments. These findings emphasize the active role of DOM in extreme AMD environments and expand the understanding of the carbon cycle in the hydrosphere.

The role of humic substances in mitigating greenhouse gases emissions: Current knowledge and research gaps

Humic substances (HS) constitute a highly transformed fraction of natural organic matter (NOM) with a heterogeneous structure, which is rich in electron-transferring functional moieties. Because of this feature, HS display a versatile reactivity with a diversity of environmentally relevant organic and inorganic compounds either by abiotic or microbial processes. Consequently, extensive research has been conducted related to the potential of HS to drive relevant processes in bio-engineered systems, as well as in the biogeochemical cycling of key elements in natural environments. Nevertheless, the increase in the number of reports examining the relationship between HS and the microorganisms related to the production and consumption of greenhouse gases (GHG), the main drivers of global warming, has just emerged in the last years. In this paper, we discuss the importance of HS, and their analogous redox-active organic molecules (RAOM), on controlling the emission of three of the most relevant GHG due to their tight relationship with microbial activity, their abundance on the Earth's atmosphere, and their important global warming potentials: carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). The current knowledge gaps concerning the microbial component, on-site occurrence, and environmental constraints affecting these HS-mediated processes are provided. Furthermore, strategies involving the metabolic traits that GHG-consuming/HS-reducing and -oxidizing microbes display for the development of environmental engineered processes are also discussed.

Reactivity, fate and functional roles of dissolved organic matter in anoxic inland waters

The transit of organic matter (OM) through the aquatic compartment of its global cycle has been intensively studied, traditionally with a focus on the processing and degradation of its dissolved fraction (dissolved organic matter, DOM). Because this is so intimately related to oxidation, the notion tenaciously persists that where oxygen is absent, DOM turnover is markedly slowed. In this Opinion Piece, we outline how diverse processes shape, transform and degrade DOM also in anoxic aquatic environments, and we focus here on inland waters as a particular case study. A suite of biogeochemical DOM functions that have received comparatively little attention may only be expressed in anoxic conditions and may result in enhanced biogeochemical roles of these deoxygenated habitats on a network scale.