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1. Distinguishing the molecular diversity, nutrient content, and energetic potential of exometabolomes produced by macroalgae and reef-building corals

   https://doi.org/10.1073/pnas.2110283119  

   Wegley Kelly, Linda ; Nelson, Craig E. ; Petras, Daniel ; Koester, Irina ; Quinlan, Zachary A. ; Arts, Milou G.I. ; Nothias, Louis-Felix ; Comstock, Jacqueline ; White, Brandie M. ; Hopmans, Ellen C. ; et al ( February 2022 , Proceedings of the National Academy of Sciences)

   Metabolites exuded by primary producers comprise a significant fraction of marine dissolved organic matter, a poorly characterized, heterogenous mixture that dictates microbial metabolism and biogeochemical cycling. We present a foundational untargeted molecular analysis of exudates released by coral reef primary producers using liquid chromatography–tandem mass spectrometry to examine compounds produced by two coral species and three types of algae (macroalgae, turfing microalgae, and crustose coralline algae [CCA]) from Mo’orea, French Polynesia. Of 10,568 distinct ion features recovered from reef and mesocosm waters, 1,667 were exuded by producers; the majority (86%) were organism specific, reflecting a clear divide between coral and algal exometabolomes. These data allowed us to examine two tenets of coral reef ecology at the molecular level. First, stoichiometric analyses show a significantly reduced nominal carbon oxidation state of algal exometabolites than coral exometabolites, illustrating one ecological mechanism by which algal phase shifts engender fundamental changes in the biogeochemistry of reef biomes. Second, coral and algal exometabolomes were differentially enriched in organic macronutrients, revealing a mechanism for reef nutrient-recycling. Coral exometabolomes were enriched in diverse sources of nitrogen and phosphorus, including tyrosine derivatives, oleoyl-taurines, and acyl carnitines. Exometabolites of CCA and turf algae were significantly enriched in nitrogen with distinct signals from polyketide macrolactams and alkaloids, respectively. Macroalgal exometabolomes were dominated by nonnitrogenous compounds, including diverse prenol lipids and steroids. This study provides molecular-level insights into biogeochemical cycling on coral reefs and illustrates how changing benthic cover on reefs influences reef water chemistry with implications for microbial metabolism. 

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2. Molecular investigation of the multi-phase photochemistry of Fe( iii )–citrate in aqueous solution

   https://doi.org/10.1039/D1EM00503K  

   West, Christopher P. ; Morales, Ana C. ; Ryan, Jackson ; Misovich, Maria V. ; Hettiyadura, Anusha P. ; Rivera-Adorno, Felipe ; Tomlin, Jay M. ; Darmody, Andrew ; Linn, Brittany N. ; Lin, Peng ; et al ( February 2023 , Environmental Science: Processes & Impacts)

   Iron (Fe) is ubiquitous in nature and found as Fe II or Fe III in minerals or as dissolved ions Fe 2+ or Fe 3+ in aqueous systems. The interactions of soluble Fe have important implications for fresh water and marine biogeochemical cycles, which have impacts on global terrestrial and atmospheric environments. Upon dissolution of Fe III into natural aquatic systems, organic carboxylic acids efficiently chelate Fe III to form [Fe III –carboxylate] 2+ complexes that undergo a wide range of photochemistry-induced radical reactions. The chemical composition and photochemical transformations of these mixtures are largely unknown, making it challenging to estimate their environmental impact. To investigate the photochemical process of Fe III –carboxylates at the molecular level, we conduct a comprehensive experimental study employing UV-visible spectroscopy, liquid chromatography coupled to photodiode array and high-resolution mass spectrometry detection, and oil immersion flow microscopy. In this study, aqueous solutions of Fe III –citrate were photolyzed under 365 nm light in an experimental setup with an apparent quantum yield of ( φ ) ∼0.02, followed by chemical analyses of reacted mixtures withdrawn at increment time intervals of the experiment. The apparent photochemical reaction kinetics of Fe 3+ –citrates (aq) were expressed as two generalized consecutive reactions of with the experimental rate constants of j 1 ∼ 0.12 min −1 and j 2 ∼ 0.05 min −1 , respectively. Molecular characterization results indicate that R and I consist of both water-soluble organic and Fe–organic species, while P compounds are a mixture of water-soluble and colloidal materials. The latter were identified as Fe–carbonaceous colloids formed at long photolysis times. The carbonaceous content of these colloids was identified as unsaturated organic species with low oxygen content and carbon with a reduced oxidation state, indicative of their plausible radical recombination mechanism under oxygen-deprived conditions typical for the extensively photolyzed mixtures. Based on the molecular characterization results, we discuss the comprehensive reaction mechanism of Fe III –citrate photochemistry and report on the formation of previously unexplored colloidal reaction products, which may contribute to atmospheric and terrestrial light-absorbing materials in aquatic environments. 

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3. Fundamental drivers of dissolved organic matter composition across an Arctic effective precipitation gradient

   https://doi.org/10.1002/lno.11385  

   Kellerman, Anne M. ; Arellano, Ana ; Podgorski, David C. ; Martin, Ellen E. ; Martin, Jonathan B. ; Deuerling, Kelly M. ; Bianchi, Thomas S. ; Spencer, Robert G. M. ( December 2019 , Limnology and Oceanography)

   The standard model for aquatic ecosystems is to link hydrologic connectivity to dissolved organic carbon (DOC) concentration and dissolved organic matter (DOM) composition and, ultimately, reactivity. Studies across effective precipitation gradients have been suggested as models for predicting how carbon cycling will change in Arctic aquatic ecosystems with projected drying (i.e., reduced hydrologic connectivity). To evaluate links between DOM dynamics and hydrologic connectivity, 41 stream samples from Greenland were analyzed across an effective precipitation gradient for DOM optical properties and elemental composition using ultrahigh‐resolution mass spectrometry. Sites with negative effective precipitation and decreased hydrologic connectivity exhibited elevated specific conductivity (SpC) and DOC concentrations as well as DOM composition indicative of decreased hydrologic connectivity, for example, lower aromaticity, assessed using carbon‐specific UV absorbance at 254 nm, decreased relative abundances of polyphenolic and condensed aromatic compounds, and increased relative abundances of highly unsaturated and phenolic compounds. Allochthonous inputs decreased as the summer progressed as exhibited by decreases in aromatic compounds. A decrease in molecular richness and N‐containing compounds coincided with the decrease in allochthonous inputs. DOC concentrations increased over the summer but more slowly than SpC, suggesting degradation processes outweighed combined evapoconcentration and production. The patterns in DOM composition suggest evapoconcentration and photodegradation are dominant controls. However, when hydrologic connectivity was high, regardless of effective precipitation, DOM reflected allochthonous sources such as snowmelt‐fed wetlands. These results highlight the challenges of modeling carbon cycling in aquatic ecosystems across effective precipitation gradients, particularly those with strong seasonality and regional variability in hydrologic inputs.

    

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4. Hydrologic connectivity determines dissolved organic matter biogeochemistry in northern high‐latitude lakes

   https://doi.org/10.1002/lno.11417  

   Johnston, Sarah Ellen ; Striegl, Robert G. ; Bogard, Matthew J. ; Dornblaser, Mark M. ; Butman, David E. ; Kellerman, Anne M. ; Wickland, Kimberly P. ; Podgorski, David C. ; Spencer, Robert G. M. ( February 2020 , Limnology and Oceanography)

   Northern high‐latitude lakes are undergoing climate‐induced changes including shifts in their hydrologic connectivity with terrestrial ecosystems. How this will impact dissolved organic matter (DOM) biogeochemistry remains uncertain. We examined the drivers of DOM composition for lakes in the Yukon Flats Basin in Alaska, an arid region of low relief that is characteristic of over one‐quarter of circumpolar lake area. Utilizing the vascular plant biomarker lignin, chromophoric dissolved organic matter (CDOM), and ultrahigh‐resolution mass spectrometry, we interpreted DOM compositional changes using lake‐water stable isotope (δ¹⁸O‐H₂O) composition as a proxy for lake hydrologic connectivity with the landscape. We observed a relative decrease in CDOM in more hydrologically isolated lakes (enriched δ¹⁸O‐H₂O) without a corresponding decrease in dissolved organic carbon (DOC) concentration. Although DOC and CDOM were weakly correlated, a significant positive relationship between lignin and CDOM (r²= 0.67) demonstrates that optical parameters are useful for estimating lignin concentration and thus vascular plant contribution to lake DOM. Indicators of allochthonous DOM, including lignin carbon normalized yields, CDOM aromaticity proxies, and relative abundances of polyphenolic and condensed aromatic compound classes, were negatively correlated with δ¹⁸O‐H₂O (r² > 0.45), suggesting there is little allochthonous DOM supplied to many of these hydrologically isolated lakes. We conclude that decreased lake hydrologic connectivity, driven by ongoing climate change (i.e., decreased precipitation, warming temperatures), will reduce allochthonous DOM contributions and shift lakes toward lower CDOM systems with ecosystem‐scale ramifications for heat transfer, photochemical reactions, productivity, and ultimately their biogeochemical function.

    

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5. Redox and Nucleophilic Reactions of Naphthoquinones with Small Thiols and Their Effects on Oxidization of H2S to Inorganic and Organic Hydropolysulfides and Thiosulfate

   https://doi.org/10.3390/ijms24087516  

   Olson, Kenneth R. ; Clear, Kasey J. ; Gao, Yan ; Ma, Zhilin ; Cieplik, Nathaniel M. ; Fiume, Alyssa R. ; Gaziano, Dominic J. ; Kasko, Stephen M. ; Luu, Jennifer ; Pfaff, Ella ; et al ( April 2023 , International Journal of Molecular Sciences)

   Naphthoquinone (1,4-NQ) and its derivatives (NQs, juglone, plumbagin, 2-methoxy-1,4-NQ, and menadione) have a variety of therapeutic applications, many of which are attributed to redox cycling and the production of reactive oxygen species (ROS). We previously demonstrated that NQs also oxidize hydrogen sulfide (H2S) to reactive sulfur species (RSS), potentially conveying identical benefits. Here we use RSS-specific fluorophores, mass spectroscopy, EPR and UV-Vis spectrometry, and oxygen-sensitive optodes to examine the effects of thiols and thiol-NQ adducts on H2S-NQ reactions. In the presence of glutathione (GSH) and cysteine (Cys), 1,4-NQ oxidizes H2S to both inorganic and organic hydroper-/hydropolysulfides (R2Sn, R=H, Cys, GSH; n = 2–4) and organic sulfoxides (GSnOH, n = 1, 2). These reactions reduce NQs and consume oxygen via a semiquinone intermediate. NQs are also reduced as they form adducts with GSH, Cys, protein thiols, and amines. Thiol, but not amine, adducts may increase or decrease H2S oxidation in reactions that are both NQ- and thiol-specific. Amine adducts also inhibit the formation of thiol adducts. These results suggest that NQs may react with endogenous thiols, including GSH, Cys, and protein Cys, and that these adducts may affect both thiol reactions as well as RSS production from H2S. 

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