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In the littoral zone, at the land–water interface of lakes, the areal productivity of aquatic vegetation rivals that of rainforests, resulting in a potentially very high carbon (C) turnover. Whereas tidal wetlands at the land–ocean interface are included in global C budgets, lake littoral zones are currently not accounted for, despite the total shoreline of lakes being estimated at four times longer than that of the global ocean. Here we quantify the littoral net atmospheric C sink using mass balance and a model of C export from the littoral to the pelagic zone. We argue that ignoring littoral C turnover in lakes potentially results in biased estimates of continental C cycling. In our global estimate, we show that the estimated global C balance of lakes may reverse from a net C source to a net C sink (that is, net C burial > net C outgassing). In addition, a large part of the C outgassed in the pelagic might originate from the littoral, implying that previous estimates of terrestrial C inputs to inland waters were too high. We argue that quantifying and modelling lake littoral C fluxes are essential to more accurately estimate the feedback between the continents and climate.

There is substantial variation in estimates of the respiratory quotient (RQ), i.e., molar ratio of produced CO₂ and consumed O₂ during microbial mineralization of organic matter (OM). While several studies have examined RQ's controlling factors in terrestrial or aquatic ecosystems, there are no broader cross-ecosystem comparisons, and there is a lack of general understanding of the extrinsic (environmental) and intrinsic (organic matter composition) controls on RQ. In this study, we examine RQ across a broad range of environments, including soils, aquatic sediments, lake and coastal water. We measured CO₂ production and O₂ consumption using membrane inlet mass spectrometry (MIMS). We also assessed the microbial metabolic profiles using BIOLOG EcoPlates and determined the energy content of the natural OM with bomb calorimetry and its elemental composition. We show that RQ differs significantly between the ecosystem types and strongly deviates from the frequently assumed value of 1. In addition, microbial mineralization across the different studied ecosystems is correlated with the bulk energy content of the OM (kJ g^-1 organic carbon). Finally, RQ was correlated to the metabolic profiles of microorganisms, as estimated based on BIOLOG EcoPlates. We argue that an increased use of cross-ecosystem experimental studies will enhance the understanding of the factors controlling carbon cycling.

Soil protists are increasingly recognized as key players in organic matter turnover, yet their role as direct decomposers (i.e., saprotrophs) remains underexplored compared to that of bacteria and fungi. Here, we synthesize ecological, physiological, and genomic evidence to highlight the potential of protists to actively decompose organic matter and influence soil carbon cycling. We distinguish two saprotrophic strategies within protists—lysotrophic (extracellular) and phagotrophic (intracellular)—with the latter being unique to protists among microbial decomposers. By directly ingesting particulate or dissolved organic matter, phagotrophic saprotrophic protists may bypass constraints associated with extracellular decomposition, potentially providing an advantage in breaking down recalcitrant substrates. In contrast, lysotrophic saprotrophy in protists involves the secretion of enzymes, similar to bacterial and fungal decomposers. We propose that integrating protist saprotrophy into conceptual and quantitative models of soil organic matter decomposition could address critical knowledge gaps. This integration involves employing functional genomics and functional ecology methodologies to determine, in vitro, the capacity of protists to function as saprotrophs, elucidate the genetic pathways underpinning saprotrophic activities, and assess, in situ, their direct contributions to organic matter decomposition processes. Ultimately, a clearer view of the organic matter decomposition capacities of soil protists will refine our understanding of microbially driven carbon fluxes.

We have investigated the bacterial decomposition of dissolved organic matter (DOM) extracted from the organic layer of a boreal forest soil and filtered at a pore size of 0.2 mu m. This DOM source has previously been extensively characterized and contains approximately equal amounts by carbon of a colloidal fraction, mainly composed of carbohydrates, and a fraction of molecularly dissolved DOM. Here, extracts were inoculated with soil bacteria and the decomposition of DOM was followed over a period of 2 months, during which it was analyzed with scattering methods and H-1 NMR, and by measuring the concentration of total organic carbon. A comparison was also made with dialyzed extract. Results showed that while the bacteria fully decomposed the molecular fraction within approximately two weeks, the colloidal fraction was stable with no visible decomposition within the 2 months. The results indicate the importance of distinguishing small molecules from colloidal aggregates in decomposition studies, and demonstrate the usefulness of combining scattering methods with ¹H NMR for this purpose.

Dissolved organic matter (DOM) is a complex mixture of thousands of molecular formulas comprised of an unknown number of chemical compounds, the concentration and composition of which are critical to ecosystem function and biogeochemical cycling. Despite its importance, our understanding of the DOM composition is lacking. This is principally due to its molecular complexity, which means that no single method is capable of describing DOM in its entirety. Quantification is typically done by proxy (e.g., relative to carbon content) and does not necessarily match well to compositional data, due to incomplete analytical windows and selectivity of different analytical methods. We present an integrated liquid chromatography (LC)-diode array detector (DAD)-charged aerosol detector (CAD)-mass spectrometry (MS) pipeline designed to both characterize and quantify solid-phase extractable DOM (SPE-DOM) in a single analysis. We applied this method to a set of eight Swedish water bodies sampled in the summer and winter. Chromophoric SPE-DOM was proportionally higher in samples with higher SPE-DOM concentrations but remained relatively consistent between sampling occasions. Ionizable SPE-DOM was relatively consistent across sites but was proportionally higher in summer. Overall, the carbon content of DOM was very consistently similar to 40% across sites in both summer and winter. These findings suggest that SPE-DOM concentration at these sites is driven by (presumably allochthonous) chromophoric inputs, with an increased relative contribution in summer of material that is more ionizable and less chromophoric and may be either autochthonous or selectively enriched from allochthonous sources. Thus, with minimal additional effort, this method provided further compositional insights not attained by any single analysis in isolation.

Photochemical degradation of dissolved organic matter (DOM) has been the subject of numerous studies; however, its regulation along the inland water continuum is still unclear. We aimed to unravel the DOM photoreactivity and concurrent DOM compositional changes across 30 boreal aquatic ecosystems including peat waters, streams, rivers, and lakes distributed along a water residence time (WRT) gradient. Samples were subjected to a standardized exposure of simulated sunlight. We measured the apparent quantum yield (AQY), which corresponds to DOM photomineralization per photon absorbed, and the compositional change in DOM at bulk and individual compound levels in the original samples and after irradiation. AQY increased with the abundance of terrestrially derived DOM and decreased at higher WRT. Additionally, the photochemical changes in both DOM optical properties and molecular composition resembled changes along the natural boreal WRT gradient at low WRT (<3 years). Accordingly, mass spectrometry revealed that the abundance of photolabile and photoproduced molecules decreased with WRT along the boreal aquatic continuum. Our study highlights the tight link between DOM composition and DOM photodegradation. We suggest that photodegradation is an important driver of DOM composition change in waters with low WRT, where DOM is highly photoreactive.

As the permafrost region warms and permafrost soils thaw, vast stores of soil organic carbon (C) become vulnerable to enhanced microbial decomposition and lateral transport into aquatic ecosystems as dissolved organic carbon (DOC). The mobilization of permafrost soil C can drastically alter the net northern permafrost C budget. DOC entering aquatic ecosystems becomes biologically available for degradation as well as other types of aquatic processing. However, it currently remains unclear which landscape characteristics are most relevant to consider in terms of predicting DOC concentrations entering aquatic systems from permafrost regions. Here, we conducted a systematic review of 111 studies relating to, or including, concentrations of DOC in terrestrial permafrost ecosystems in the northern circumpolar region published between 2000 and 2022. We present a new permafrost DOC dataset consisting of 2845 DOC concentrations, collected from the top 3 m in permafrost soils across the northern circumpolar region. Concentrations of DOC ranged from 0.1 to 500 mg L⁻¹ (median = 41 mg L⁻¹) across all permafrost zones, ecoregions, soil types, and thermal horizons. Across the permafrost zones, the highest median DOC concentrations were in the sporadic permafrost zone (101 mg L⁻¹), while lower concentrations were found in the discontinuous (60 mg L⁻¹) and continuous (59 mg L⁻¹) permafrost zones. However, median DOC concentrations varied in these zones across ecosystem type, with the highest median DOC concentrations in each ecosystem type of 66 and 63 mg L⁻¹ found in coastal tundra and permafrost bog ecosystems, respectively. Coastal tundra (130 mg L⁻¹), permafrost bogs (78 mg L⁻¹), and permafrost wetlands (57 mg L⁻¹) had the highest median DOC concentrations in the permafrost lens, representing a potentially long-term store of DOC. Other than in Yedoma ecosystems, DOC concentrations were found to increase following permafrost thaw and were highly constrained by total dissolved nitrogen concentrations. This systematic review highlights how DOC concentrations differ between organic- or mineral-rich deposits across the circumpolar permafrost region and identifies coastal tundra regions as areas of potentially important DOC mobilization. The quantity of permafrost-derived DOC exported laterally to aquatic ecosystems is an important step for predicting its vulnerability to decomposition.

Climate change induced shifts in treeline position, both towards higher altitudes and latitudes induce changes in soil organic matter. Eventually, soil organic matter is transported to alpine and subarctic lakes with yet unknown consequences for dissolved organic matter (DOM) diversity and processing. Here, we experimentally investigate the consequences of treeline shifts by amending subarctic and temperate alpine lake water with soil-derived DOM from above and below the treeline. We use ultra-high resolution mass spectrometry (FT-ICR MS) to track molecular DOM diversity (i.e., chemodiversity), estimate DOM decay and measure bacterial growth efficiency. In both lakes, soil-derived DOM from below the treeline increases lake DOM chemodiversity mainly through the enrichment with polyphenolic and highly unsaturated compounds. These compositional changes are associated with reductions in bulk and compound-level DOM reactivity and reduced bacterial growth efficiency. Our results suggest that treeline advancement has the potential to enrich a large number of lake ecosystems with less biodegradable DOM, affecting bacterial community function and potentially altering the biogeochemical cycling of carbon in lakes at high latitudes and altitudes. Shifts in the treeline may induce changes in organic matter composition of lakes at high altitude and latitude. Here, the authors experimentally unravel effects of soil-derived DOM for lake carbon biogeochemistry and bacterial carbon use efficiency.

The global language of limnology is English, but most of our study objects do not have English names. Here, I compare 57,000 lake names in a lake-rich, non-English speaking country, that is, Sweden, with a previous analysis of 83,000 lakes in the conterminous United States. The diversity of lake name appellations is strikingly different. In the United States, three different appellations (“lake,” “pond,” “reservoir”) apply to 96% of the lakes, whereas in Sweden to account for 93% of the lakes, 76 different appellations and suffixes were required. The etymology of the remaining largely idiosyncratic 4000 lake names is difficult to assess, and of ancient origin. In the United States, lake names with appellations in languages of non-English European colonizers are rare and lakes that include words in indigenous languages almost exclusively also include an English appellation. Contrastingly, in regions of Sweden where Sami, Finnish, and Meänkieli are spoken, lake names are typically fully indigenous, including the appellation. The historical reasons for the differences are discussed. Examples of malpractice in the use of lake names in scientific papers are presented, and suggestions are made for how we better can achieve a good lake-name practice.