Heterotrophic microbial communities use ancient carbon following glacial retreat

Abstract

When glaciers retreat they expose barren substrates that become colonized by organisms, beginning the process of primary succession. Recent studies reveal that heterotrophic microbial communities occur in newly exposed glacial substrates before autotrophic succession begins. This raises questions about how heterotrophic microbial communities function in the absence of carbon inputs from autotrophs. We measured patterns of soil organic matter development and changes in microbial community composition and carbon use along a 150-year chronosequence of a retreating glacier in the Austrian Alps. We found that soil microbial communities of recently deglaciated terrain differed markedly from those of later successional stages, being of lower biomass and higher abundance of bacteria relative to fungi. Moreover, we found that these initial microbial communities used ancient and recalcitrant carbon as an energy source, along with modern carbon. Only after more than 50 years of organic matter accumulation did the soil microbial community change to one supported primarily by modern carbon, most likely from recent plant production. Our findings suggest the existence of an initial stage of heterotrophic microbial community development that precedes autotrophic community assembly and is sustained, in part, by ancient carbon.

Culturable bacteria community development in postglacial soils of Ecology Glacier, King George Island, Antarctica

“…Biogeochemical data were in line with the literature reports on postglacial areas (Bardgett et al 2007;Nemergut et al 2007). However, total organic carbon was lower than in the high Arctic (Schmidt et al 2008).…”

“…The high total carbon contents at point '0' (close to the glacier) were striking, considering the lower levels 100-200 m away from the glacier, where microbial photosynthesis would be expected to provide organic carbon. This might be attributable to the enriching influence of supraglacial meltwater (such as from cryoconite holes) or ancient subglacial sediment carbon (Stibal et al 2006;Bardgett et al 2007). The highest TC and CFU counts were determined in a zone closest to the glacier (point '0').…”

“…Microorganisms are important in nutrient cycling in postglacial environments (Tscherko et al 2003b) but are negatively affected by long periods of snow cover, low temperatures, and accompanying low water availability (Bardgett et al 2007). Other studies, however, indicate that C and N pools increase in the first 20 years of succession, hinting at C and N fixation in these soils (Nemergut et al 2007).…”

Subglacial methanogenesis: A potential climatic amplifier?

“…There are uncertainties regarding the degree to which the noneroded SOC might be degraded. Although there is abundant evidence from other extreme environments that recalcitrant organic carbon compounds can be utilized by microorganisms (marine, proglacial, soils [Bardgett et al, 2007;Leschine, 1995;Ishiwatari and Uzaki, 1987]), these data are lacking from subglacial ecosystems. As a result, we conservatively estimate that 50% of the SOC that is not eroded (167 Pg C) is available to subglacial microbes.…”

“…Degradation of lignocellulose components, however, can be 10-30 times slower than aerobic mineralization rates [Benner and Hodson, 1985] and research on lake sediments shows very long timescales for lignin degradation (half lives for lignin in lake sediments have been calculated as 400 ka) [Ishiwatari and Uzaki, 1987]. The production of methane from fossil carbon buried in tills in the present-day [Simpkins and Parkin, 1993] and observations that microbes in recently deglaciated terrain use ancient carbon [Bardgett et al, 2007] support the degradation of recalcitrant OC. Decomposition rates under persistent cold temperatures, however, will be slower [Hobbie et al, 2000].…”