Controls on the autochthonous production and respiration of organic matter in cryoconite holes on High Arctic glaciers
There is current debate about whether the balance of photosynthesis and respiration has any impact on the net accumulation of organic matter on glacier surfaces. This study assesses controls on rates of net ecosystem production (NEP), respiration, and photosynthesis in cryoconite holes during the main melt season (June-August 2009) on three valley glaciers in Svalbard. Cryoconite thickness and organic matter content explained 87% of the total variation in rates of respiration (in units of volume), and organic matter (but not sediment depth) was a significant (p < 0.05) control on photosynthesis (by volume). The average rates of respiration and gross photosynthesis within the cryoconite holes were overall closely balanced, ranging from net autotrophic to heterotrophic. Sediment depth explained over half the variation of NEP, with net autotrophic rates typical only in sediment < 3 mm thick. The measured rates of NEP were not sufficient to account for the organic matter which has likely accumulated in the cryoconite on timescales of less than decades, suggesting three alternatives for the source of the organic matter. First, the glacier surface may have received windblown allochthonous organic material from surrounding environments. Second, cryoconite may consist of in-washed autochthonous material from the glacier surface which has comparable organic carbon content. Third, much of the organic matter may have accumulated in the hole during a nascent period, when rates of NEP were much higher. The cycling of autochthonous labile carbon produced by phototrophs may sustain a significant proportion of the total in situ microbial activity within cryoconite holes.
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Available from: Martyn Tranter
- "Processes of carbon cycling have long been documented in cryoconite holes (Tranter et al., 2004;Bagshaw et al., 2007;Foreman et al., 2007;Stibal and Tranter, 2007), but the extent of the dependence of photosynthetic organisms concentrated at the surface of the sediment layer on heterotrophic processes deeper in the sediment has not been fully discussed in the literature to date. This inter-dependence may also help explain the link between sediment depth and P:R balance previously observed in short term (6-24 hr) cryoconite incubations (Telling et al., 2012). The heterotrophic activity in the deeper sediment serves to relocate organic matter, fixed as CO 2 , from the deeper layers to the surface of the sediment, and to dissolve carbonate minerals. "
Available from: Andy Hodson
- "One of the greater uncertainties in this context is the so-called bacterial growth efficiency, which Foreman et al (2013) suggest is likely to be as low as 1—2% in a range of ice surface habitats and thus capable of explaining a large proportion of the CO 2 transfer to the atmosphere that is inferred from incubation studies (e.g. Telling et al 2012). However, the empirical approaches used to calculate this crucial parameter might not be appropriate for supraglacial ecosystems. "
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ABSTRACT: The fourteen letters that contributed to this focus issue on cryospheric ecosytems provide an excellent basis for considering the state of the science following a marked increase in research attention since the new millennium. Research letters from the focus issue provide significant insights into the biogeochemical and biological processes associated with snow, glacier ice and glacial sediments. This has been achieved via a significant, empirical effort that has given particular emphasis to glacier surface habitats. However, far less is known about aerobiology, glacial snow covers, supraglacial lakes and sub-ice sedimentary habitats, whose access for sampling and in-situ monitoring remains a great challenge to scientists. Furthermore, the use of models to explore key fluxes, processes and impacts of a changing glacial cryosphere are conspicuous by their absence. As a result, a range of process investigations and modelling studies are required to address the increasing urgency and uncertainty that is associated with understanding the response of cryospheric ecosystems to global change.
Available from: Brent G. Pautler
- "Cryoconite from holes in close proximity to the ice margin contained less biogenic material and displayed greater mineral diversity than cryoconite sampled from the glacier interior (Langford et al., 2011). The thickness of cryoconite debris has also been shown to influence microbial activity and carbon cycling, with net photosynthesis (CO 2 fixation) being favored in holes with thinner debris layers and net heterotrophy (CO 2 respiration) in holes with thicker debris layers (Cook et al., 2010; Telling et al., 2012). Furthermore DNA sequencing of rRNA genes of microbes isolated from Arctic and Antarctic cryoconite has revealed geographically distinct communities including unknown bacterial, eukaryotic and archael taxa (Cameron et al., 2012b). "
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ABSTRACT: Glacier surfaces are reservoirs that contain organic and inorganic debris referred to as cryoconite. Solar heating of this material results in the formation of water-filled depressions that are colonized by a variety of microbes and are hypothesized to play a role in carbon cycling in glacier ecosystems. Recent studies on cryoconite deposits have focused on their contribution to carbon fluxes to determine whether they are a net source or sink for atmospheric CO2. To better understand carbon cycling in these unique ecosystems, the molecular constituents of cryoconite organic matter (COM) require further elucidation. COM samples from four glaciers were analyzed by targeted extraction of plant- and microbial-derived biomarkers in conjunction with non-targeted NMR experiments to determine the COM composition and potential sources. Several molecular proxies were applied to assess COM degradation and microbial activity using samples from Greenland, the Canadian Arctic, and Antarctica. COM from Canadian (John Evans glacier) and Greenlandic (Leverett glacier) locations was more chemically heterogeneous than that from the Antarctic likely due to inputs from higher plants, mosses and Sphagnum as suggested by the solvent-extractable alkyl lipids and sterols and the detection of lignin- and Sphagnum-derived phenols after cupric oxide chemolysis. Solid-state 13C nuclear magnetic resonance (NMR) experiments highlighted the bulk chemical functional groups of COM allowing for a general assessment of its degradation stage from the alkyl/O-alkyl proxy whereas solution-state 1H NMR highlighted both microbial and plant contributions to base-soluble extracts from these COM samples. The dominance of 1H NMR signals from microbial protein/peptides in base-soluble extracts of COM from Antarctica (Joyce glacier and Garwood glacier), phospholipid fatty acid (PLFA) biomarker detection and the absence of plant-derived biomarkers in both the solvent and cupric oxide extracts suggests that this COM is dominated by microbial-derived material. These results indicate that COM carbon composition is dependent on the local glacier environment which may have a profound impact on carbon cycling and sequestration on glacier surfaces.
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