Abstract
Although methylation is regarded as one of the main detoxification pathways for arsenic (As), current knowledge about this process during manure composting remains limited. In this study, two pilot-scale compost piles were established to treat manure contaminated with As. An overall accumulation of methylated As occurred during 60 day-composting time. The concentration of monomethylarsonic acid (MMA) increased from 6 to 190 μg kg−1 within 15 days and decreased to 35 μg kg−1 at the end of the maturing phase; while the concentration of dimethylarsinic acid (DMA) continuously increased from 33 to 595 μg kg−1 over the composting time. The arsM gene copies increased gradually from 0.08 × 109 to 6.82 × 109 copies g−1 dry mass over time and correlated positively to the concentrations of methylated As. 16S rRNA gene sequencing and arsM clone library analysis confirmed the high abundance and diversity of arsM genes. Many of these genes were related to those from known As-methylating microbes, including Streptomyces sp., Amycolatopsis mediterranei and Sphaerobacter thermophiles. These results demonstrated that As methylation during manure composting is significant and, for the first time, established a linkage between As biomethylation and the abundance and diversity of the arsM functional genes in composting manure.
Highlights
The rapid expansion of the poultry and livestock industries in the past decades has generated vast quantities of farming waste, with attendant environmental impacts, notably in farming-intensive countries such as China, India and Brazil[1,2,3]
Based on the measured temperature profile, both compost piles progressed through mesophilic, thermophilic, and maturing phases as defined in other composting processes[28]
The rapid and sustained increase of temperature in the thermophilic stage in the two compost piles is an indicator of rapid establishment of microbial activity (Supplementary Figure S1), as biodegradation of organic matter resulted in a substantial heat production[20,29]
Summary
The rapid expansion of the poultry and livestock industries in the past decades has generated vast quantities of farming waste, with attendant environmental impacts, notably in farming-intensive countries such as China, India and Brazil[1,2,3]. As exists in inorganic and organic forms such as arsenate [As(V)], arsenite [As(III)], monomethylarsonic acid [MMA], dimethylarsinic acid [DMA], trimethylarsinic acid [TMA] and trimethylarsine oxide [TMAO] with varying biogeochemical behaviors and toxicity[11]. Methylated As species, both volatile (e.g. TMA and TMAO) and non-volatile (e.g. MMA and DMA), are less toxic than their inorganic forms[17]. Transgenic rice expressing an arsM gene from R. palustris was shown to methylate inorganic As into a variety of organic As species[27]. The mechanism of microbial As methylation is known and arsM genes have been detected in various environments, there remains a limited understanding of how the abundance and diversity of arsM genes correlate with the methylation process during manure composting
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