Benzoyl Chloride Derivatization Advances the Quantification of Dissolved Polar Metabolites on Coral Reefs.

Extracellular chemical cues constitute much of the language of life among marine organisms, from microbes to mammals. Changes in this chemical pool serve as invisible signals of overall ecosystem health and disruption to this finely tuned equilibrium. In coral reefs, the scope and magnitude of the chemicals involved in maintaining reef equilibria are largely unknown. Processes involving small, polar molecules, which form the majority components of labile dissolved organic carbon, are often poorly captured using traditional techniques. We employed chemical derivatization with mass spectrometry-based targeted exometabolomics to quantify polar dissolved phase metabolites on five coral reefs in the U.S. Virgin Islands. We quantified 45 polar exometabolites, demonstrated their spatial variability, and contextualized these findings in terms of geographic and benthic cover differences. By comparing our results to previously published coral reef exometabolomes, we show the novel quantification of 23 metabolites, including central carbon metabolism compounds (e.g., glutamate) and novel metabolites such as homoserine betaine. We highlight the immense potential of chemical derivatization-based exometabolomics for quantifying labile chemical cues on coral reefs and measuring molecular level responses to environmental stressors. Overall, improving our understanding of the composition and dynamics of reef exometabolites is vital for effective ecosystem monitoring and management strategies.

Metabolite diversity among representatives of divergent Prochlorococcus ecotypes.

IMPORTANCE: Approximately half of the annual carbon fixation on Earth occurs in the surface ocean through the photosynthetic activities of phytoplankton such as the ubiquitous picocyanobacterium Prochlorococcus. Ecologically distinct subpopulations (or ecotypes) of Prochlorococcus are central conduits of organic substrates into the ocean microbiome, thus playing important roles in surface ocean production. We measured the chemical profile of three cultured ecotype strains, observing striking differences among them that have implications for the likely chemical impact of Prochlorococcus subpopulations on their surroundings in the wild. Subpopulations differ in abundance along gradients of temperature, light, and nutrient concentrations, suggesting that these chemical differences could affect carbon cycling in different ocean strata and should be considered in models of Prochlorococcus physiology and marine carbon dynamics.

Continuous proxy measurements reveal large mercury fluxes from glacial and forested watersheds in Alaska.

In this study, a stream from a glacially dominated watershed and one from a wetland, temperate forest dominated watershed in southeast Alaska were continuously monitored for turbidity and fluorescence from dissolved organic matter (FDOM) while grab samples for unfiltered (UTHg), particulate (PTHg), and filtered mercury (FTHg) where taken over three 4-day periods (May snowmelt, July glacial melt, and September rainy season) during 2010. Strong correlations were found between FDOM and UTHg concentrations in the wetland, temperate forest watershed (r2=0.81), while turbidity and UTHg were highly correlated in the glacially dominated watershed (r2=0.82). Both of these parameters (FDOM and turbidity) showed stronger correlations than concentration-discharge relationships for UTHg (r2=0.55 for glacial stream, r2=0.38 for wetland/forest stream), thus allowing for a more precise determination of temporal variability in UTHg concentrations and fluxes. The association of mercury with particles and dissolved organic matter (DOM) appears to depend on the watershed characteristics, such as physical weathering and biogeochemical processes regulating mercury transport. Thus employing watershed-specific proxies for UTHg (such as FDOM and turbidity) can be effective for quantifying mercury export from watersheds with variable landcover. The UTHg concentration in the forest/wetland stream was consistently higher than in the glacial stream, in which most of the mercury was associated with particles; however, due to the high specific discharge from the glacial stream during the melt season, the watershed area normalized flux of mercury from the glacial stream was 3-6 times greater than the wetland/forest stream for the three sampling campaigns. The annual specific flux for the glacial watershed was 19.9gUTHgkm-2y-1, which is higher than any non-mining impacted stream measured to date. This finding indicates that glacial watersheds of southeast Alaska may be important conduits of total mercury to the Gulf of Alaska.

A global ocean inventory of anthropogenic mercury based on water column measurements.

Mercury is a toxic, bioaccumulating trace metal whose emissions to the environment have increased significantly as a result of anthropogenic activities such as mining and fossil fuel combustion. Several recent models have estimated that these emissions have increased the oceanic mercury inventory by 36-1,313 million moles since the 1500s. Such predictions have remained largely untested owing to a lack of appropriate historical data and natural archives. Here we report oceanographic measurements of total dissolved mercury and related parameters from several recent expeditions to the Atlantic, Pacific, Southern and Arctic oceans. We find that deep North Atlantic waters and most intermediate waters are anomalously enriched in mercury relative to the deep waters of the South Atlantic, Southern and Pacific oceans, probably as a result of the incorporation of anthropogenic mercury. We estimate the total amount of anthropogenic mercury present in the global ocean to be 290 ± 80 million moles, with almost two-thirds residing in water shallower than a thousand metres. Our findings suggest that anthropogenic perturbations to the global mercury cycle have led to an approximately 150 per cent increase in the amount of mercury in thermocline waters and have tripled the mercury content of surface waters compared to pre-anthropogenic conditions. This information may aid our understanding of the processes and the depths at which inorganic mercury species are converted into toxic methyl mercury and subsequently bioaccumulated in marine food webs.

Mercury speciation and mobilization in a wastewater-contaminated groundwater plume.

We measured the concentration and speciation of mercury (Hg) in groundwater down-gradient from the site of wastewater infiltration beds operated by the Massachusetts Military Reservation, western Cape Cod, Massachusetts. Total mercury concentrations in oxic, mildly acidic, uncontaminated groundwater are 0.5-1 pM, and aquifer sediments have 0.5-1 ppb mercury. The plume of impacted groundwater created by the wastewater disposal is still evident, although inputs ceased in 1995, as indicated by anoxia extending at least 3 km down-gradient from the disposal site. Solutes indicative of a progression of anaerobic metabolisms are observed vertically and horizontally within the plume, with elevated nitrate concentrations and nitrate reduction surrounding a region with elevated iron concentrations indicating iron reduction. Mercury concentrations up to 800 pM were observed in shallow groundwater directly under the former infiltration beds, but concentrations decreased with depth and with distance down-gradient. Mercury speciation showed significant connections to the redox and metabolic state of the groundwater, with relatively little methylated Hg within the iron reducing sector of the plume, and dominance of this form within the higher nitrate/ammonium zone. Furthermore, substantial reduction of Hg(II) to Hg(0) within the core of the anoxic zone was observed when iron reduction was evident. These trends not only provide insight into the biogeochemical factors controlling the interplay of Hg species in natural waters, but also support hypotheses that anoxia and eutrophication in groundwater facilitate the mobilization of natural and anthropogenic Hg from watersheds/aquifers, which can be transported down-gradient to freshwaters and the coastal zone.