Effects of different bacterial growth modes and of Bangladeshi aquaculture practices on environmental contamination by mercury and arsenic
Environmental contamination is an important public health concern as As in groundwater has led to the largest environmental poisoning in history. The work in this dissertation addresses the effects of differing growth modes on mercury methylation by SRB as well as the effects of Bangladeshi aquaculture practices on As mobilization.
Methylmercury is a known neurotoxin that bioaccumulates, particularly in the aquatic system. When people eat fish that have consumed contaminated lower species, people then take in the mercury that has bioaccumulated in the fish leading to skin problems, sensory impairment, and internal damage (Barringer et al 2005). Studies have investigated mercury methylation in planktonic cultures of SRB but bacteria exist primarily as biofilms in the environment. Thus, it is important and more applicable to study methylation by biofilm cultures. Our study uses inihibition and gene expression to compare mercury methylation by SRB strains ND132 and M8 and specifically addresses a less studied acetyl-CoA pathway. Results showed that biofilm cultures methylate up to four times more than planktonic cultures and through different and possibly more pathways. The acetyl-CoA pathyway was found to be important for both biofilm and planktonic cultures of ND132.
Arsenic in Bangladesh groundwater results in chronic exposure to the population, leading to millions of diagnosed cases of arsenicosis and other health concerns each year (Yu et al 2003). Using sequential extraction, solid-phase As host fractions were characterized for sediment collected from a new aquaculture pond in Bangladesh upon initial excavation and one year later. Sediment from the newly excavated pond was unfertilized and showed that As was found predominantly in the recalcitrant fraction with up to 35% of solid As found in phosphate- and hydrochloric acid-extracted fractions. However, extraction from one-year old sediment showed no As in the phosphate-extracted fraction and all in hydrochloric acid-extractable and recalcitrant fractions. This shift in host fractions in addition to an overall decrease in total arsenic from initial excavation to one year later suggests that treating the pond with cow manure altered host fractions and mobilized As. Dialysis experiments were performed with As, Ca2 , and NOM to investigate binding capabilities of NOM, as NOM is key to As mobilization. Cow dung and sediment treated with cow dung showed the greatest ability to complex As, which further suggests that fertilization practices are important and problematic. As speciation mostly complexed with NOM when NOM was present in geochemical models utilizing constants derived from experimental work with field samples. It should be noted that these studies imply that current practices of treating aquaculture ponds with cow manure is potentially exacerbating arsenic contamination in Bangladesh groundwater.