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Environmental context: Dissolved organic carbon (DOC) is ubiquitous in freshwater and concentrations are rising universally while pH is decreasing with climate change. This study demonstrates the interrelationships among DOC characterisation techniques and the pH-sensitive aspects of these techniques that were previously not well understood. As DOC regulates important processes within ecosystems, understanding DOC behaviour at altered pH and identifying techniques to effectively evaluate DOC composition are critical requirements for monitoring aquatic ecosystem health. Rationale: Dissolved organic carbon (DOC) is both ubiquitous and heterogeneous in freshwater. Freshwaters are browning universally and pH values are decreasing with climate change. DOCs influence water pH, whereas changes in water pH potentially alter the conformation and function of DOCs. The physicochemical properties of DOCs can be characterised by optical and chemical indices, but the inter-relationships among them, and the effects of low pH, are not well understood. Methodology: We characterised five naturally sourced DOCs, spanning large differences in origin and composition, at pH 7 and 4, using multiple indices: specific absorbance coefficient at 340 nm, molecular weight index, fluorescence index, octanol–water partition coefficient, molecular charge, proton binding index, size-fractionation, and percentage humic-acid-like, percentage fulvic-acid-like and percentage protein-like components by fluorescence-based parallel factor analysis. Results: Many of the indices changed between pH 7 and 4 as reflected in the corresponding absorbance and fluorescence profiles. Generally, apparent aromaticity, apparent molecular weight and molecular charge all decreased with low pH, while lipophilicity increased. Key positive correlations occurred between aromaticity and apparent molecular weight, chemical reactivity and apparent molecular weight, and aromaticity and chemical reactivity, and a negative correlation between lipophilicity and molecular charge. These relationships were pH dependent. Discussion: Our results highlight that physicochemical indices used to characterise DOCs from distinct sources should consider pH and be interpreted carefully. The pH-dependent changes in many of the indices are likely alterations in the conformation, reflected in the optical signatures, rather than changes in the composition of DOCs. In contrast, increased lipophilicity and reduced charge at lower pH are due to actual changes in DOC molecules, resulting from proton binding. The ecological functions of DOCs are dependent on source and will likely change with natural acidification events such as increasing atmospheric CO2. Environmental context. Dissolved organic carbon (DOC) is ubiquitous in freshwater and concentrations are rising universally while pH is decreasing with climate change. This study demonstrates the interrelationships among DOC characterisation techniques and the pH-sensitive aspects of these techniques that were previously not well understood. As DOC regulates important processes within ecosystems, understanding DOC behaviour at altered pH and identifying techniques to effectively evaluate DOC composition are critical requirements for monitoring aquatic ecosystem health. (Image credit: Carolyn Morris.) [ABSTRACT FROM AUTHOR]

Leptospirosis is a reemerging infectious disease in California. Leptospirosis is the most widespread zoonosis throughout the world, though it is infrequently diagnosed in the continental United States. From 1982 to [...]

The objectives of this study are to compare the chemical properties of organic matter (OM) in alkaline extracts of turkey litter compost, source‐separated organic compost, and biosolid pellets, and to validate the use of humic acids (HAs) for analyzing the functionality (i.e., the ability to complex with nutrients) of OM. The humic fractions, including (HAs), fulvic acids (FAs), and alkaline nondissolved fraction (humin), were isolated either with NaOH or Na4P2O7 (PyroP) and the chemical properties were analyzed by solid‐state 13C NMR spectroscopy. The study showed that the proportions of each functional group of the combination of HAs, FAs, and humin were essentially identical to that of their HCl‐treated original materials, except for a lower recovery of O‐alkyl groups. Although the yield of HAs extracted with PyroP was lower than that with NaOH, their spectral characteristics were not significantly different. The functionality of the OM, assuming a correlation with the sum of the proportions of carboxyl and phenolic groups, was higher in both HAs and FAs than that in humin. Based on the proportion and functionality of the HAs in the OM, we conclude that the use of HAs extracted with either NaOH or PyroP is valid for studies of OM functionality. The study also shows that the functionality and stability of OM are highly source dependent. We predict that amendments of the more humified turkey litter compost would have higher potential to increase SOM levels and retain nutrients compared with the source‐separated organic compost and biosolid pellets. [ABSTRACT FROM AUTHOR]