The radiocarbon signature of microorganisms in the mesopelagic ocean

Abstract

Several lines of evidence indicate that microorganisms in the mesoand bathypelagic ocean are metabolically active and respiring carbon. In addition, growing evidence suggests that archaea are fixing inorganic carbon in this environment. However, direct quantification of the contribution from deep ocean carbon sources to community production in the dark ocean remains a challenge. In this study, carbon flow through the microbial community at 2 depths in the mesopelagic zone of the North Pacific Subtropical Gyre was examined by exploiting the unique radiocarbon signatures (⌬ 14 C) of the 3 major carbon sources in this environment. The radiocarbon content of nucleic acids, a biomarker for viable cells, isolated from size-fractionated particles (0.2-0.5 m and >0.5 m) showed the direct incorporation of carbon delivered by rapidly sinking particles. Most significantly, at the 2 mesopelagic depths examined (670 m and 915 m), carbon derived from in situ autotrophic fixation supported a significant fraction of the free-living microbial community (0.2-0.5 m size fraction), but the contribution of chemoautotrophy varied markedly between the 2 depths. Results further showed that utilization of the ocean's largest reduced carbon reservoir, 14 C-depleted, dissolved organic carbon, was negligible in this environment. This isotopic portrait of carbon assimilation by the in situ, free-living microbial community, integrated over >50,000 L of seawater, implies that recent, photosynthetic carbon is not always the major carbon source supporting microbial community production in the mesopelagic realm.microbial metabolism ͉ particle flux ͉ particulate organic carbon (POC) ͉ chemoautotrophy

Pacific carbon cycling constrained by organic matter size, age and composition relationships

“…Multiple alternate deep DOC sources have been proposed, which might alter DOC Δ 14 C values at depth, including hydrothermal DOC, chemosynthesis, and particle solubilization [Hansman et al, 2009;McCarthy et al, 2011;Smith et al, 1992]. If any of these processes were in fact significant, this would result in source heterogeneity consistent with the mismatch between our model results in surface versus deep ocean.…”

Molecular transformation and degradation of refractory dissolved organic matter in the Atlantic and Southern Ocean

“…As a result, radiocarbon depleted degraded DOM mixes with the surface DOM. Hansman et al (2009) found that chemoautotrophic fixation of 14 C depleted carbon in the deep sea may be a significant in situ source of fresh-but-old DOM (i.e., inducing an apparent higher DOM reservoir age at depth). Based on chlorofluorocarbon data the renewal time of the WDW is long (240 a; Klatt et al, 2002) compared to the residence time of Weddell Gyre surface water (2.5-3 a; Gordon and Huber, 1990;Hoppema et al, 1999).…”

“…Most of the DOC is derived from primary producers using dissolved inorganic carbon (DIC) as their carbon source. It is directly released into the oceans from photosynthetic plankton or through heterotrophic transformation processes (Carlson, 2002) and also transported from land by rivers (Hedges et al, 1997) and through the atmosphere (Willey et al, 2000) with minor allochthonous and autochthonous sources in the deep ocean (e.g., Hansman et al, 2009;Pohlman et al, 2011). On the shelves and in the pelagic ocean the majority of new DOC is remineralized to CO 2 mainly by heterotrophs (respiration from prokaryotes or higher trophic level organisms; del Giorgio and Duarte, 2002) with a small contribution from photodegradation (Mopper et al, 1991), or it is transformed to a fraction of degraded dissolved organic matter (DOM) that is highly resistant to microbial decomposition (Jiao et al, 2010).…”

Advection of surface-derived organic carbon fuels microbial reduction in Bangladesh groundwater

“…Compound-specific radiocarbon analysis of cellular biomass can be compared with the radiocarbon content of different OC sources that have a unique radiocarbon signature to provide a direct measure of carbon substrate utilization in the environment (15)(16)(17)(18)(19)(20)(21). For example, this method has been used to document the preference of microbes for degradation of petroleum hydrocarbons and utilization of organic matter derived from shale weathering.…”

“…Radiocarbon analysis of lipids is usually performed on sediment samples but drilling methods to obtain large amounts of pristine sediments are not available for aquifers in Southeast Asia. Radiocarbon analysis has been performed on DNA extracted from human cells (22) and surface water samples (19,21) but not groundwater, in part because it is difficult to collect sufficient DNA and to remove humic substances when purifying the DNA. The radiocarbon signature of DNA is a direct measure of the carbon used during microbial respiration and growth.…”