Stimulation of Microbially Mediated Arsenic Release in Bangladesh Aquifers by Young Carbon Indicated by Radiocarbon Analysis of Sedimentary Bacterial Lipids

Abstract

The sources of reduced carbon driving the microbially mediated release of arsenic to shallow groundwater in Bangladesh remain poorly understood. Using radiocarbon analysis of phospholipid fatty acids (PLFAs) and potential carbon pools, the abundance and carbon sources of the active, sediment-associated, in situ bacterial communities inhabiting shallow aquifers (<30 m) at two sites in Araihazar, Bangladesh, were investigated. At both sites, sedimentary organic carbon (SOC) Δ14C signatures of −631 ± 54‰ (n = 12) were significantly depleted relative to dissolved inorganic carbon (DIC) of +24 ± 30‰ and dissolved organic carbon (DOC) of −230 ± 100‰. Sediment-associated PLFA Δ14C signatures (n = 10) at Site F (−167‰ to +20‰) and Site B (−163‰ to +21‰) were highly consistent and indicated utilization of carbon sources younger than the SOC, likely from the DOC pool. Sediment-associated PLFA Δ14C signatures were consistent with previously determined Δ14C signatures of microbial DNA sampled from groundwater at Site F indicating that the carbon source for these two components of the subsurface microbial community is consistent and is temporally stable over the two years between studies. These results demonstrate that the utilization of relatively young carbon sources by the subsurface microbial community occurs at sites with varying hydrology. Further they indicate that these young carbon sources drive the metabolism of the more abundant sediment-associated microbial communities that are presumably more capable of Fe reduction and associated release of As. This implies that an introduction of younger carbon to as of yet unaffected sediments (such as those comprising the deeper Pleistocene aquifer) could stimulate microbial communities and result in arsenic release.

Human and livestock waste as a reduced carbon source contributing to the release of arsenic to shallow Bangladesh groundwater

“…At Site B, a 7-m thick clay/silt layer caps the sandy aquifer whereas the sandy formation extends essentially to the surface at Site F (Dhar et al, 2008). As described in detail in Whaley-Martin et al (2016), sediments were collected in January 2013 by gravity coring, sectioned directly into whirlpacks, immediately frozen, shipped on ice and frozen until future analyses. Data for groundwater samples collected between 2002 and 2015 from pre-existing well nests at the two sites (Dhar et al, 2008) using a battery operated downhole pump (Groundwater Essentials©) is also presented.…”

“…Prior to extractions, all sediments were freeze-dried for 48–72 h. A detailed description of the organic compound extraction and separation is provided in Whaley-Martin et al (2016). Sediment samples (~300 g to 1 kg) were extracted twice overnight using a modified Bligh and Dyer extraction and the organic phase was removed after phase separation in separatory funnels.…”

“…A three-fraction (Fraction 1 = dichloromethane (DCM), Fraction 2 = acetone, Fraction 3 = methanol) silica gel chromatographic elution was carried out. Methanol fractions were utilized for phospholipid fatty acid analyses reported in a previous study (Whaley-Martin et al, 2016). Preliminary tests revealed elution of sterols occurred in both the Fraction 1&2 in agreement with a recent study (Birk et al, 2012) showing high recoveries using elution with DCM and acetone.…”

“…Recent evidence based on Δ 14 C analysis of microbial lipids, DNA and biogenic methane from a few sites in Bangladesh has indicated that reductive dissolution of iron (Fe) oxides at depths <30 m may be driven primarily by relatively young surface-derived carbon sources as opposed to older sedimentary carbon (Harvey et al, 2002; Mailloux et al, 2013; Whaley-Martin et al, 2016). Determining which of the potential sources of surface derived reactive carbon, e.g.…”

“…The sterol distributions are compared with other indicators of unsewered wastes (Cl/Br mass ratios), as well as As and Fe concentrations. The organic extracts used for the present study are the same that were previously used for Δ 14 C analysis of bacterial lipids (Whaley-Martin et al, 2016) and showed that young carbon was driving microbial metabolisms.…”