Sedimentary supply of humic-like fluorescent dissolved organic matter and its implication for chemoautotrophic microbial activity in the Izu-Ogasawara Trench

Microbial community structure in the hadal water is reported to be different from that in the upper abyssal water. However, the mechanism governing the difference has not been fully understood. In this study, we investigate the vertical distributions of humic-like fluorescent dissolved organic matter (FDOMH), chemoautotrophic production, apparent oxygen utilization (AOU), and N* in the Izu-Ogasawara Trench. In the upper abyssal waters (< 6000 m), FDOMH has a significantly positive correlation with AOU; FDOMH deviates from the relationship and increases with depth without involving the increment of AOU in the hadal waters. This suggests that FDOMH is transferred from the sediments to the hadal waters through pore water, while the FDOMH is produced in situ in the upper abyssal waters. Chemoautotrophic production and N* increases and decreases with depth in the hadal waters, respectively. This corroborates the effluxes of dissolved substances, including dissolved organic matter and electron donors from sediments, which fuels the heterotrophic/chemoautotrophic microbial communities in the hadal waters. A simple box model analysis reveals that the funnel-like trench topography facilitates the increase in dissolved substances with depth in the hadal waters, which might contribute to the unique microbiological community structure in these waters.

Does Nitrate Enrichment Accelerate Organic Matter Turnover in Subterranean Estuaries?

Due to the widespread pollution of coastal groundwaters with fertilizers, submarine groundwater discharge (SGD) is often thought to be a large dissolved inorganic nitrogen (DIN) source to the ocean. Whether this N is autochthonous or allochthonous to the subterranean estuary (STE), the availability of large quantities of DIN can nevertheless interact with the cycling of other elements, such as carbon (C). In previous studies, we documented the discharge of large quantities of freshwater and NO3– from the mouth of an STE into the Ria Formosa lagoon (SW Iberian Peninsula). For the period covered in this study (2009–2011), the same STE site was dominated by recirculating seawater due to a prolonged fall in piezometric head in the coupled coastal aquifers. Total SGD rates remained similarly high, peaking at 144 cm day–1 at the lower intertidal during fall. We observed a progressive increase of NO3– availability within the STE associated with the recovery of piezometric head inland. Interestingly, during this period, the highest SGD-derived dissolved organic C and DIN fluxes (112 ± 53 and 10 ± 3 mmol m–2 day–1, respectively) originated in the lower intertidal. NO3– enrichment in the STE influences the benthic reactivity of fluorescent dissolved organic matter (FDOM): when seawater recirculation drives STE dynamics, only small changes in the benthic distribution of recalcitrant humic-like FDOM are observed (from −2.57 ± 1.14 to 1.24 ± 0.19 10–3 R.U. “bulk” sediment h–1) in the absence of DIN. However, when DIN is available, these recalcitrant fractions of FDOM are actively generated (from 1.32 ± 0.15 to 11.56 ± 3.39 10–3 R.U. “bulk” sediment h–1), accompanied by the production of labile protein-like FDOM. The results agree with previous studies conducted with flow-through reactor experiments at the same site and suggest that DIN enrichment in the STE enhances the metabolic turnover of sedimentary organic matter up to the point of discharge to surface waters. DIN pollution of coastal aquifers may therefore promote a contraction of the residence time of particulate organic C within the STE, driving carbon from continental storage into the sea.

Quantifying fluvial carbon losses from lowland peatland ecosystems across a drainage-impact spectrum

&lt;p&gt;The export of Dissolved Organic Carbon (DOC) and evasion of carbon dioxide (CO&lt;sub&gt;2&lt;/sub&gt;) from inland waters is increasingly being recognized as a key part of the terrestrial carbon (C) cycle, with recent global estimates suggesting that the magnitude of the aquatic CO&lt;sub&gt;2&lt;/sub&gt; conduit is equivalent to global Net Ecosystem Productivity (2.0 Gt C yr-1; Tranvik et al., 2009). However, a major weakness in the carbon balance estimation of terrestrial ecosystems, such as peatlands, is the poor quantification of DOC and CO&lt;sub&gt;2&lt;/sub&gt; evasion fluxes associated with drainage waters. This has implications for conservation, land-use management and climate change mitigation. Whilst intact peatland systems typically sequester carbon, drainage reverts peatlands to being C sources due, primarily, to the degradation of organic peat soil. This study examines the export of C in fluvial pathways from relatively intact catchments to those that are heavily drained, and also from peatland sites undergoing restoration works. This research is being carried out parallel other linked studies that are quantifying the carbon gaseous emissions from directly from the different bogs in order to determine the comparative net carbon budgets.&lt;/p&gt;&lt;p&gt;This study will focus on three raised bog sites in the midlands of Ireland: one in near natural condition (Clara bog), one significantly drained and degraded due to peat extraction (Garryduff) and one undergoing rehabilitation following many years of peat extraction (Cavemount). Flumes and sondes, with fluorescent dissolved organic matter (fDOM), temperature/conductivity and turbidity sensors, have been installed on the sites. The fDOM measurements will be correlated to grab samples taken every two weeks to give half hour proxy measurements for DOC.&lt;/p&gt;&lt;p&gt;Preliminary results suggest that DOC flux from the heavily drained and mined peatland site is some 295 times higher than that from the catchment with minimal interference. In addition to this, drainage waters are super-saturated in CO&lt;sub&gt;2&lt;/sub&gt; and rapidly evades back to the atmosphere resulting in an additional C loss. Thus, C losses in the drainage systems of peatland catchment areas are significantly under-reported and a significant source of C in countries with significant peat land cover such as Ireland. This research is thereby addressing the magnitude of C losses in fluvial pathways, the associated effects on ecosystem biodiversity and the effectiveness of restoration activities on mitigating against net C loss in degraded systems.&lt;/p&gt;