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Ongoing climate change is making the large pool of organic matter (OM) stored in Arctic permafrost vulnerable to mobilization; thus, garnering a deeper understanding of molecular transformations within the abundant pool of soil OM, specifically humic substances, is crucial. Here we present the first high-resolution mass-spectrometry examination of molecular compositions of humic acid (HA) and fulvic acid (FA) isolated from organic-rich deep yedoma (Pleistocene age ice-rich permafrost) and alas (thermokarst deposit formed during permafrost thaw) cores. The FA fractions were dominated by oxygen-rich unsaturated compounds, whereas the HA fractions were mostly composed of relatively reduced saturated and aromatic moieties. A substantial increase in contribution of both CHO-only and N-containing aliphatic compounds was observed in the HA fractions of the yedoma OM with depth, whereas the alas HA fractions were depleted in aliphatics but enriched with condensed and hydrolyzable tannins. The observed differences in compositional space of the immobile OM stored in the deep yedoma versus alas deposits were consistent with evolution of OM during thermokarst lake genesis, implying intense microbial degradation of N-rich OM released from the yedoma deposits and accumulation of highly degraded, plant-derived OM. The patterns of molecular transformations of OM were apparent in compositional space of the least degraded HA fractions as compared to much more oxidized FA fractions. This shows great promise of molecular exploration of the alkali-extracted OM, comprising up to 50% of the total organic carbon in deep permafrost both for paleoreconstructions and predictions of climate feedback to released OM due to permafrost thaw.

Abstract:A new concept for the mediating action of humic substances (HS) in the contaminated environment is developed. It defines three scenarios of mitigating activity of HS in the system "living cell-ecotoxicant". The first scenario refers to deactivation of ecotoxicants (ET) by HS due to formation of non-toxic and non-bioavailable complexes. It takes place outside of the cell and is defined as "exterior effects". The second scenario refers to deactivation of ET due to HS adsorption onto the cell wall or membrane and is defined as "boundary effects": sorption takes place on the cell surface and implies changes in permeability and structure of the cell membrane. The third scenario refers to amelioration of contaminant toxicity due to activation of systemic resistance to chemical stress. This implies HS participation in immune response activation and is defined as "interior" effects. Viability of this concept was confirmed by the results of detoxification experiments. It was shown that chemical binding ("exterior effects") played a key role in ameliorating toxicity of ecotoxicants (Hg(II) and PAHs) strongly interacting with HS, whereas enhanced immune response ("boundary and interior" effects) was much more operative for a decrease in toxicity of atrazine weakly interacting with HS. The formulated concept provided satisfactory explanations for a vast pool of reported findings of mitigating activity of HS reviewed in the chapter. Few cases of amplified toxicity reported for weakly interacting contaminants in the presence of low molecular weight HS were related to facilitated penetration and follow up dissociation of humiccontaminant complexes in the cell interior. It is concluded that the developed concept can be used as a prospective tool for both predictive modelling of 249I. Twardowska et al. (eds.), and Water Pollution Monitoring, Protection and Remediation, 3-23.