Dissolved organic carbon (DOC) in coastal waters is integral to biogeochemical cycling, but global and regional drivers of DOC are still uncertain. In this study we explored spatial and temporal differences in DOC concentrations and stocks across the global coastal ocean, and how these relate to temperature and salinity. We estimated a global median coastal DOC stock of 3.15 Pg C (interquartile range (IQR) = 0.85 Pg C), with median DOC concentrations being 2.2 times higher than in open ocean surface waters. Globally and seasonally, salinity was the main driver of DOC with concentrations correlated negatively with salinity, without a clear relationship to temperature. DOC concentrations and stocks varied with region and season and this pattern is likely driven by riverine inputs of DOC and nutrients that stimulate coastal phytoplankton production. Temporally, high DOC concentrations occurred mainly in months with high freshwater input, with some exceptions such as in Eastern Boundary Current margins where peaks are related to primary production stimulated by nutrients upwelled from the adjacent ocean. No spatial trend between DOC and temperature was apparent, but many regions (19 out of 25) had aligned peaks of seasonal temperature and DOC, related to increased phytoplankton production and vertical stratification at high temperatures. Links of coastal DOC with salinity and temperature highlight the potential for anthropogenic impacts to alter coastal DOC concentration and composition, and thereby ecosystem status.

Measurements of dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP) concentrations are used to characterize the dissolved organic matter (DOM) pool and are important components of biogeochemical cycling in the coastal ocean. Here, we present the first edition of a global database (CoastDOM v1; available at https://doi.org/10.1594/PANGAEA.964012, Lønborg et al., 2023) compiling previously published and unpublished measurements of DOC, DON, and DOP in coastal waters. These data are complemented by hydrographic data such as temperature and salinity and, to the extent possible, other biogeochemical variables (e.g. chlorophyll a, inorganic nutrients) and the inorganic carbon system (e.g. dissolved inorganic carbon and total alkalinity). Overall, CoastDOM v1 includes observations of concentrations from all continents. However, most data were collected in the Northern Hemisphere, with a clear gap in DOM measurements from the Southern Hemisphere. The data included were collected from 1978 to 2022 and consist of 62 338 data points for DOC, 20 356 for DON, and 13 533 for DOP. The number of measurements decreases progressively in the sequence DOC > DON > DOP, reflecting both differences in the maturity of the analytical methods and the greater focus on carbon cycling by the aquatic science community. The global database shows that the average DOC concentration in coastal waters (average ± standard deviation (SD): 182±314 µmol C L−1; median: 103 µmol C L−1) is 13-fold higher than the average coastal DON concentration (13.6±30.4 µmol N L−1; median: 8.0 µmol N L−1), which is itself 39-fold higher than the average coastal DOP concentration (0.34±1.11 µmol P L−1; median: 0.18 µmol P L−1). This dataset will be useful for identifying global spatial and temporal patterns in DOM and will help facilitate the reuse of DOC, DON, and DOP data in studies aimed at better characterizing local biogeochemical processes; closing nutrient budgets; estimating carbon, nitrogen, and phosphorous pools; and establishing a baseline for modelling future changes in coastal waters.

Landslides of peat have been recorded throughout Britain and Ireland for centuries. Whilst these events are not uncommon, land degradation can amplify their magnitude and frequency and, crucially, their immediate impacts are rarely documented. A 20,000 m3 bogflow event that occurred on land undergoing development in the Irish border area in November 2020 was monitored at high frequency in the major receiving river system (384 km2). Samples collected every seven hours over a 28 day period at a site 37 km downstream were analysed for suspended sediment (SS), particulate organic carbon (POC) and dissolved organic carbon (DOC and UV-derived fractions), synchronous with hydrometeorological data and turbidity. There was no impact of the bogflow on DOC concentrations or loads. However, concentrations of SS and POC in the first samples after the bogflow were 825 mg/L and 346 mg C/L, respectively, and fish kill was estimated at 100 %. Analysis of detrended SS and POC loads suggested the main impacts of the bogflow on water quality lasted just eight days. Over this period, an additional 1318 t of SS and 608 t of POC were transported as far as the monitoring point, equating to 325 % more SS and 925 % more POC than would have been expected otherwise under the same river flow conditions. The carbon loss and water quality impacts were short lived, but nevertheless severe, and highlight the vulnerability of peatlands and the risks when these environments are inappropriately managed.

Aquatic habitats are particularly susceptible to chemical pollution from domestic, agricultural, and industrial sources. Antimicrobials are commonly used in medical and industrial environments to reduce harmful bacteria and biofilms. This has led to the rapid increase in the prevalence of antimicrobial resistant (AMR) genes. Alternate remedies to fight pathogenic bacteria and biofilms are in development including synthetic and biological surfactants such as sodium dodecyl sulphate (SDS) and rhamnolipids respectively. In the aquatic environment these surfactants are present as pollutants with potential to affect biofilm formation and AMR gene occurrence; however, there is limited research showing the actual environmental impact of such exposure. We tested the effects of rhamnolipid and SDS on natural aquatic biofilms in a freshwater stream in Northern Ireland. We grew biofilms on contaminant exposure substrata deployed within the stream over four weeks, and then carried out shotgun sequencing to determine microbial community composition, through 16s rRNA analyses (64,678 classifiable reads identified), and AMR gene occurrence (81 instances of AMR genes over 9 AMR gene classes) through a metagenomic analysis. There were no significant changes in community composition within all systems; however, biofilm exposed to rhamnolipid had a greater number of unique taxa as compared to our SDS treatments and controls. AMR gene prevalence was higher in surfactant-treated biofilms, with biofilm exposed to rhamnolipids having the highest presence of AMR genes and classes compared to the control or SDS treatments, in which genes encoding for rifampin resistance were detected. Our results suggest that the presence of rhamnolipid, and to a lesser extent SDS, encourages an increase in the prevalence of AMR genes in biofilms produced in mixed use water bodies.

Surfactants are used to control microbial biofilms in industrial and medical settings. Their known toxicity on aquatic biota, and their longevity in the environment, has encouraged research on biodegradable alternatives such as rhamnolipids. While previous research has investigated the effects of biological surfactants on single species biofilms, there remains a lack of information regarding the effects of synthetic and biological surfactants in freshwater ecosystems. We conducted a mesocosm experiment to test how the surfactant sodium dodecyl sulfate (SDS) and the biological surfactant rhamnolipid altered community composition and metabolic activity of freshwater biofilms. Biofilms were cultured in the flumes using lake water from Lake Lunz in Austria, under high (300 ppm) and low (150 ppm) concentrations of either surfactant over a four-week period. Our results show that both surfactants significantly affected microbial diversity. Up to 36% of microbial operational taxonomic units were lost after surfactant exposure. Rhamnolipid exposure also increased the production of the extracellular enzymes, leucine aminopeptidase, and glucosidase, while SDS exposure reduced leucine aminopeptidase and glucosidase. This study demonstrates that exposure of freshwater biofilms to chemical and biological surfactants caused a reduction of microbial diversity and changes in biofilm metabolism, exemplified by shifts in extracellular enzyme activities. Key points • Microbial biofilm diversity decreased significantly after surfactant exposure. • Exposure to either surfactant altered extracellular enzyme activity. • Overall metabolic activity was not altered, suggesting functional redundancy.

Microbial biofilms have co-evolved with grazing animals, such as gastropods, to develop mutually beneficial relationships. Although microbial biofilms demonstrate resilience and resistance to chemical exposure, pre-existing relationships can be negatively affected by chemical input. In this study, we determined how the grazer, Littorina littorea (common periwinkle sea snail), and a biological surfactant (rhamnolipid) interact on a phototrophic marine biofilm. Biofilms were cultured in 32 twenty-liter buckets at the Queen's University Marine Laboratory in Portaferry, Northern Ireland on clay tiles that were either exposed to 150 ppm of a rhamnolipid solution or that had no chemical exposure. L. littorea were added into half of the buckets, and biofilms were developed over 14 days. Biofilms exposed to grazing alone demonstrated high tolerance to the disturbance, while those growing on rhamnolipid-exposed substrate demonstrated resistance but experienced slight declines in carbon and stoichiometric ratios. However, when exposed to both, biofilms had significant decreases in stoichiometry and declined in productivity and respiration. This is problematic, as continuing marine pollution increases the likelihood that biofilms will be exposed to combinations of stressors and disturbances. Loss of biofilm productivity within these areas could lead to the loss of an important food source and nutrient cycler within the marine ecosystem.

Zwerschke et al. (2021) offer an insight into the potential for carbon sequestration and storage within the new benthic habitats exposed through glacial retreat in Antarctica. The paper provides a description of spatial trends in the carbon stocks of the benthic macrofauna and sediments. In this respect, the paper provides an interesting insight into potential carbon storage following glacial retreat in a number of Antarctic Fjords. I am sceptical about the potential for these fjords to provide a significant feedback against climate change. The study suggests that carbon storage in Antarctic fjords has the potential to sequester 56,909 ± 29,833 tonnes of C per year, following glacial retreat. This would equate to between 0.05% and 0.19% of the estimated global seafloor carbon burial rates of between 29.4 and 117 million t C per year (Bauer et al., 2013; Cai, 2011). The authors, however, fail to consider the wider context of carbon storage in marine sediments and ‘blue carbon’ habitats, such as saltmarsh, mangroves, seagrass beds, etc., or the global impacts of Antarctic glacier and sea ice loss. Given that conservative estimates of Antarctic sea ice loss will cause global sea levels to rise by 4.5–5 cm by the end of this century (Rignot et al., 2019), it is difficult to rationalize small gains in sediment carbon storage in Antarctica against the accelerating global loss of vegetated marine systems (e.g. Fagherazzi et al., 2019; Williams et al., 1999). In addition, Barnes (2017) highlights the fact that carbon storage in Antarctica's coastal sediments is limited by disturbance from Ice-berg–seabed interactions, similar to the effects of trawling on seabed carbon storage in other systems. As such, it seems strange to me that the authors are so keen to herald their results as a potential feedback mechanism against climate change, without consideration of the potentially catastrophic impacts of Antarctic glacier and sea ice loss. I am also concerned about the use of ash-free dry mass as a proxy for the carbon content of both the sediments and microbenthic fauna analysed. In both cases, Zwerschke et al. (2021) applied mathematical conversions to derive estimates of faunal carbon content and sediment total organic content (TOC). The gold standard for this work is through Organic Elemental Analysis to directly quantify the percentage of carbon in a sample. The use of AFDM as a proxy is acceptable but introduces errors into the estimates of carbon storage. These errors are compounded by the authors’ conversion of faunal AFDM to carbon content and used a fixed assumption that skeletal material (carbonates) accounted for 13% of the ash weight (following Salonen et al., 1976). Why not carry out an acid digestion of the ash and estimate carbonate content by mass loss? This would at least allow differences in the skeletal material present in macrofaunal groups from different sites to be included within the wider assessment of carbon storage. Likewise, there are established methods for acid digestion of marine sediment to quantify the relative organic and inorganic carbon content (e.g. Hedges & Stern, 1984). Ultimately, I feel it would be more appropriate to present the data as Ash-Free Dry Mass, which does not diminish the observations in any way. Furthermore, the authors do not quantify microbial processes in the sediment, and assume no remineralization of carbon in anoxic sediments. There are a wide range of microbial metabolic pathways which support carbon remineralization in anoxic marine sediments, including denitrification, manganese (IV) reduction, Iron (III) reduction, sulphur reduction and methanogenesis (Jorgensen & Boudreau, 2001). As such, the authors do not provide a complete assessment of the carbon storage potential of the Antarctic fjords, nor do they provide any insight into the diagenetic processes that influence the residence time of carbon within marine sediments. This makes it difficult to fully assess their global significance in terms of climate feedback mechanisms. Given these issues, I believe there is a danger of overstating the significance of carbon storage in deglaciated Antarctic fjords as a negative climate change feedback. As scientists we have an ethical duty to report our results without bias or unnecessary fanfare. The paper would not have suffered from a more conservative approach, reporting ash-free dry mass values and discussing the potential for carbon storage and the potential processes which may control this in a rapidly changing polar system. I worry that this paper extends its interpretation of the data too far, without directly measuring carbon concentrations, considering the biogeochemical processes that govern carbon preservation, or the wider impacts of Antarctic deglaciation. That said I do believe it makes valuable contribution to our understanding of the potential for carbon storage within polar benthic ecosystems. I would like to acknowledge Nadescha Zweschke and her co-authors for producing an interesting and thought-provoking paper. I declare no conflict of interest related to this work.

Pharmaceutical compounds such as the non-steroidal anti-inflammatory drug ibuprofen and the artificial estrogen 17α-ethynylestradiol (EE2) are contaminants of emerging concern in freshwater systems. Globally, human pharmaceutical use is growing by around ~ 3% per year; yet, we know little about how interactions between different pharmaceuticals may affect aquatic ecosystems. Here, we test how interactions between ibuprofen and EE2 affect the growth and respiration of streambed biofilms. We used contaminant exposure experiments to quantify how these compounds affected biofilm growth (biomass), respiration, net primary production (NPP) and gross primary production (GPP), both individually and in combination. We found no effects of either ibuprofen or EE2 on biofilm biomass (using ash-free dry mass as a proxy) or gross primary production. Ibuprofen significantly reduced biofilm respiration and altered NPP. Concomitant exposure to EE2, however, counteracted the inhibitory effects of ibuprofen upon biofilm respiration. Our study, thus, demonstrates that interactions between pharmaceuticals in the environment may have complex effects upon microbial contributions to aquatic ecosystem functioning.

Here we present the results from a shotgun sequencing effort on foot tissues from a deep-sea hydrothermal vent endemic limpet. We present the complete mitochondrial genome of the hydrothermal vent endemic gastropod Peltospira smaragdina (Gastropoda, Peltospiridae) for the first time. This species is characteristic of circa-Azores hydrothermal vent ecoregion and provides a candidate environmental DNA (eDNA) indicator of active hydrothermal vent sites. The results also suggest that the epilithic biofilm on the newly discovered Moytirra hydrothermal vents is dominated by Sulfurimonas –like microbes and corresponds with similar studies on hydrothermal hosted microbial communities. The association between Peltospira and Sulfurimonas is presented as potentially a holobiontic relationship, with both the snail and the microbial biofilm. We highlight the efficacy of using non-traditional sampling to develop a broader ecosystem understanding.

1. Ocean warming and the loss of larger (often predatory) fauna are major threats to seabed (benthic) ecosystem functioning. Yet we know little about the combined effects of warming and faunal species loss upon the marine carbon cycle. 2. Using stable-isotope pulse-chase experiments, we tested how faunal species loss affects microbial carbon sequestration and retention in intertidal sediments, under both ambient and predicted future warming conditions (ambient + 2°C), using the shore crab Carcinus maenas as a model predator. We traced the fixation and retention of a fixed dose of 13 C-labelled sodium bicarbonate within sediment organic matter and microbial biomass. 3. Carcinus presence was associated with higher total organic carbon concentration within the mesocosm sediments. Temperature had no significant effect upon sediment total organic carbon concentrations. Temperature and Carcinus presence had no significant effect on polar lipid fatty acids (PLFAs) concentrations within the sediment, which is a proxy for microbial biomass. 4. Carcinus presence increased retention of 13 C-labelled carbon within the sediment organic matter pool under future warming conditions. Retention of the 13 C-label within the microbial PLFAs decreased significantly under future-warming conditions. 5. Changes in the relative abundance of PLFAs revealed increased contribution of microeukaryotes to the microbial community under ambient conditions, in the absence of Carcinus . PLFA profiles revealed significant changes in 13 C-label retention within the bacteria and microeukaryotes, driven by interactions between Carcinus presence and temperature. species ocean warming a pronounced effect upon marine carbon Experiments were conducted over 4 weeks (28 days). A single male crab (carapace width = 3.47 [± 0.14] cm, wet biomass = 9.66 [± 2.21] g) placed into five ambient and five warmed mesocosms 7 days after the incubations commenced; a 24 mg (0.1 g.m -2 ) dose of 13 C-labelled sodium bicarbonate was sprayed directly onto the sediment surface of all mesocosms on day 21. The experiments were then incubated for a further 5 days and then destructively sampled to quantify 13 C-incorporation and retention in microbial biomass and the sediment organic matter pool, following Middelburg et al. (2000). Changes in the biomass and spatial variability of the sediment microphytobenthos (MPB) were measured within each mesocosm using a BBE Moldänke BenthoTorch [BBE Moldänke Gmbh, Schwentinental, Germany] to quantify surficial chlorophyll a (Chl a ) concentrations after 7 (Week 0), 14 (Week 1), 21 (Week 2) and 28 (Week 3) days (Kahlert & McKie 2014). At each interval five replicate measurements of the surficial Chl a were made, within each mesocosm. Mean Chl a concentrations within each mesocosm were estimated as a proxy for MPB biomass, with the coefficient of variation of the Chl a measurements within each mesocosm providing a metric for MPB patchiness within each mesocosm. The study shows that the presence of the mobile epibenthic predator, Carcinus maenas, mediated the retention of newly fixed ( 13 C-labelled) carbon within intertidal sediments, with faunal impacts altered by changes to the ambient temperature. MPB biomass and patchiness, (proxies for assessing the productive potential of coastal sediments, Hicks et al. 2009; Kahlert & McKie 2014), were both initially affected by the introduction of Carcinus to the mesocosms. Where Carcinus were present, the sediment surface was visibly disturbed with track marks, and appeared loss cohesive in texture. Alongside this disturbance the MPB biomass increased and MPB patchiness decreased over the subsequent two weeks, suggesting that faunal reworking of the sediment had little long term effects upon the MPB. This is surprising given that faunal disturbance tends to limit the accumulation of MPB biomass or fresh algal phytodetritus at the sediment surface (Canuel et al. 2007; Spivak et al. 2007; Hicks et al. 2009; Jeffreys et al. 2011; Fanjul et al. 2015). However, benthic diatoms can switch between autotrophy at the sediment surface and heterotrophic fermentation within oxygen and light limited sub-surface sediments (Bourke et al. 2016). We postulate that faunal reworking of the sediments provides a mechanism for the transfer of active MPB cells between the sediment surface and deeper sediment layers, which allows MPB biomass to recover from the initial disturbance event. Our study builds upon observations of cascading effects of crab predation upon sediment organic matter composition in coastal sediments and provides a direct test of how faunal species loss affects sediment carbon sequestration under both present climatic conditions and predicted future-warming conditions. Our study demonstrates that both faunal presence and temperature are important regulators for organic matter retention within coastal sediments. Faunal presence was the primary driver of changes in ‘old’ organic carbon within the mesocosm, whilst strong interactions between faunal presence and temperature determine the fate of ‘recently-fixed’ organic matter. Bulk analysis of the TOC and total PLFAs was, however, limited in its scope to identify the interacting effects of Carcinus presence and temperature. Compound-specific analysis of the PLFA profiles, allowed us, to discern how faunal presence stimulated 13 C-incorporation by heterotrophic microbes under the predicted-future warming treatment, and suppressed the accumulation of microeukaryote (fungal) biomass under ambient conditions. These effects are clearly driven by changes in faunal bioturbation and the rate-limiting effects of temperature upon microbial activity.

Foraminifera are an important faunal element of the benthos in oxygen-depleted settings such as Oxygen Minimum Zones (OMZs) where they can play a relevant role in the processing of phytodetritus. We investigated the uptake of phytodetritus (labeled with 13C and 15N) by cal-careous foraminifera in the 0-1 cm sediment horizon under different oxygen concentrations within the OMZ in the eastern Arabian Sea. The in situ tracer experiments were carried out along a depth transect on the Indian margin over a period of 4 to 10 days. The uptake of phy-todetrital carbon within 4 days by all investigated species shows that phytodetritus is a rele-vant food source for foraminifera in OMZ sediments. The decrease of total carbon uptake from 540 to 1100 m suggests a higher demand for carbon by species in the low-oxygen core region of the OMZ or less food competition with macrofauna. Especially Uvigerinids showed high uptake of phytodetrital carbon at the lowest oxygenated site. Variation in the ratio of phytodetrital carbon to nitrogen between species and sites indicates that foraminiferal carbon and nitrogen use can be decoupled and different nutritional demands are found between spe-cies. Lower ratio of phytodetrital carbon and nitrogen at 540 m could hint for greater demand or storage of food-based nitrogen, ingestion or hosting of bacteria under almost anoxic condi-tions. Shifts in the foraminiferal assemblage structure (controlled by oxygen or food availabil-ity) and in the presence of other benthic organisms account for observed changes in the pro-cessing of phytodetritus in the different OMZ habitats. Foraminifera dominate the short-term processing of phytodetritus in the OMZ core but are less important in the lower OMZ bounda-ry region of the Indian margin as biological interactions and species distribution of foraminif-era change with depth and oxygen levels.

Cold-water corals form prominent reef ecosystems along ocean margins that depend on suspended resources produced in surface waters. In this study, we investigated food processing of C and N labelled bacteria and algae by the 10 cold-water coral Lophelia pertusa. Coral respiration, tissue incorporation of C and N and metabolic-derived C incorporation into the skeleton were traced following the additions of different food concentrations (100, 300, 1300 μg C l) and two ratios of suspended bacterial and algal biomass (1:1, 3:1). Respiration and tissue incorporation by L. pertusa increased markedly following exposure to higher food concentrations. The net growth efficiency of L. pertusa was low (0.08±0.03), which is consistent with their slow growth rates. The contribution of algae and bacteria to total coral assimilation was proportional to 15 the food mixture in the two lowest food concentrations, but algae were preferred over bacteria as food source at the highest food concentration. Similarly, the stoichiometric uptake of C and N was coupled in the low food treatment, but was uncoupled in the high food treatment and indicated a comparatively higher uptake or retention of bacterial carbon as compared to algal nitrogen. We argue that behavioural responses for these small-sized food particles, such as tentacle behaviour, mucus trapping and physiological processing, are more likely to explain the observed food selectivity as 20 compared to physical-mechanical considerations. A comparison of the experimental food conditions to natural organic carbon concentrations above CWC reefs suggests that L. pertusa is well adapted to exploit temporal pulses of high organic matter concentrations in the bottom water caused by internal waves and downwelling events.

In fluvial ecosystemsmineral erosion, carbon (C), and nitrogen (N) fluxes are linked via organomineral complexation, where dissolved organic molecules bind to mineral surfaces. Biofilms and suspended aggregates represent major aquatic microbial lifestyles whose relative importance changes predictably through fluvial networks. We tested how organomineral sorption affects aquatic microbial metabolism, using organomineral particles containing a mix of C-13, N-15-labeled amino acids. We traced C-13 and N-15 retention within biofilm and suspended aggregate biomass and its mineralization. Organomineral complexation restricted C and N retention within biofilms and aggregates and also their mineralization. This reduced the efficiency with which biofilms mineralize C and N by 30% and 6%. By contrast, organominerals reduced the C and N mineralization efficiency of suspended aggregates by 41% and 93%. Our findings show how organomineral complexation affects microbial C: N stoichiometry, potentially altering the biogeochemical fate of C and N within fluvial ecosystems.

Benthic (streambed) biofilms metabolize a substantial fraction of particulate organic matter and nutrient inputs to streams. These microbial communities comprise a significant proportion of overall biomass in headwater streams, and they present a primary control on the transformation and export of labile organic carbon. Biofilm growth has been linked to enhanced fine particle deposition and retention, a feedback that confers a distinct advantage for the acquisition and utilization of energy sources. We quantified the influence of biofilm structure on fine particle deposition and resuspension in experimental stream mesocosms. Biofilms were grown in identical 3 m recirculating flumes over periods of 18–47 days to obtain a range of biofilm characteristics. Fluorescent, 8 µm particles were introduced to each flume, and their concentrations in the water column were monitored over a 30 min period. We measured particle concentrations using a flow cytometer and mesoscale (10 µm to 1 cm) biofilm structure using optical coherence tomography. Particle deposition‐resuspension dynamics were determined by fitting results to a stochastic mobile‐immobile model, which showed that retention timescales for particles within the biofilm‐covered streambeds followed a power‐law residence time distribution. Particle retention times increased with biofilm areal coverage, biofilm roughness, and mean biofilm height. Our findings suggest that biofilm structural parameters are key predictors of particle retention in streams and rivers.

Sorption of organic molecules to mineral surfaces is an important control upon the aquatic carbon (C) cycle. Organo-mineral interactions are known to regulate the transport and burial of C within inland waters, yet the mechanisms that underlie these processes are poorly constrained. Streamwater contains a complex and dynamic mix of dissolved organic compounds that coexists with a range of organic and inorganic particles and microorganisms. To test how microbial metabolism and organo-mineral complexation alter amino acid and organic carbon fluxes we experimented with (13)C-labelled amino acids and two common clay minerals (kaolinite and montmorillonite). The addition of (13)C-labelled amino acids stimulated increased microbial activity. Amino acids were preferentially mineralized by the microbial community, concomitant with the leaching of other (non-labelled) dissolved organic molecules that were removed from solution by clay-mediated processes. We propose that microbial processes mediate the formation of organo-mineral particles in streamwater, with potential implications for the biochemical composition of organic matter transported through and buried within fluvial environments.