Microbial community composition of a household sand filter used for arsenic, iron, and manganese removal from groundwater in Vietnam. Sand filter microbial community

Household sand filters are used in rural areas of Vietnam to remove Fe, Mn, and As from groundwater for drinking water purposes. Currently, it is unknown what role microbial processes play in mineral oxide formation and As removal during water filtration. We performed most probable number counts to quantify the abundance of physiological groups of microorganisms capable of catalyzing Fe- and Mn-redox transformation processes in a household sand filter. We found up to 104 cells g-1 dry sand of nitrate-reducing Fe(II)-oxidizing bacteria and Fe(III)-reducing bacteria, and no microaerophilic Fe(II)-oxidizing bacteria, but up to 106 cells g-1 dry sand Mn-oxidizing bacteria. 16S rRNA gene amplicon sequencing confirmed MPN counts insofar as only low abundances of known taxa capable of performing Fe- and Mn-redox transformations were detected. Instead the microbial community on the sand filter was dominated by nitrifying microorganisms, e.g. Nitrospira, Nitrosomonadales, and an archaeal OTU affiliated to Candidatus Nitrososphaera. Quantitative PCR for Nitrospira and ammonia monooxygenase genes agreed with DNA sequencing results underlining the numerical importance of nitrifiers in the sand filter. Based on our analysis of the microbial community composition and previous studies on the solid phase chemistry of sand filters we conclude that abiotic Fe(II) oxidation processes prevail over biotic Fe(II) oxidation on the filter. Yet, Mn-oxidizing bacteria play an important role for Mn(II) oxidation and Mn(III/IV) oxide precipitation in a distinct layer of the sand filter. The formation of Mn(III/IV) oxides contributes to abiotic As(III) oxidation and immobilization of As(V) by sorption to Fe(III) (oxyhydr)oxides.

越南农村地区广泛应用家用砂滤器，以去除地下水中的铁（Fe）、锰（Mn）与砷（As），满足饮用水需求。目前，净水过滤过程中微生物过程在矿物氧化物形成与砷去除环节所扮演的具体角色仍不明确。本研究采用最大可能数（most probable number, MPN）计数法，对某家用砂滤器中可催化铁、锰氧化还原反应的生理类群微生物丰度进行定量分析。结果显示，反硝化亚铁氧化细菌与三价铁还原细菌的丰度最高可达每克干砂10⁴个细胞，未检出微好氧亚铁氧化细菌，而锰氧化细菌的丰度最高可达每克干砂10⁶个细胞。16S rRNA基因扩增子测序结果验证了MPN计数结果：仅检测到丰度较低的已知铁、锰氧化还原功能类群微生物。与之相反，砂滤器表面的微生物群落以硝化微生物为优势类群，例如硝化螺旋菌属（Nitrospira）、亚硝化单胞菌目（Nitrosomonadales），以及一株隶属于候选属氨氧化古菌（Candidatus Nitrososphaera）的古菌操作分类单元（operational taxonomic unit, OTU）。针对硝化螺旋菌属与氨单加氧酶基因的定量聚合酶链反应（quantitative PCR, qPCR）结果与DNA测序结果一致，进一步凸显了硝化微生物在砂滤器中的数量重要性。基于本研究对微生物群落组成的分析，并结合此前关于砂滤器固相化学特性的相关研究，我们得出结论：该滤器中，非生物亚铁氧化过程相较于生物亚铁氧化过程占据主导地位。然而，锰氧化细菌在砂滤器特定层段的二价锰氧化与三/四价锰氧化物沉淀过程中发挥着关键作用。三/四价锰氧化物的形成可通过吸附至三价铁（氧）氢氧化物表面，促进非生物途径的三价砷氧化以及五价砷的固定。