The plant organic matter in the
peat layer deposited around 6650+/-110 and 4800+/-80 years BP, during the transgressive and regressive phases of sea level changes, respectively in the dried part (playa) of the present Pulicat lagoon in Palar Basin (southeast coast of India) was studied to elucidate the biogenic
pyrite generation and associated
trace elements. The scanning electron microscopic (SEM) observations show strongly curved unique C-shaped bacteria of uniform coccoidal shape and size (1 microm) freely scattered on the plant epidermal microfragments. These form spheroidal microcolonies 8-15 microm in diameter attached to the epidermis in a linear fashion or haphazardly enclosed in the translucent sheath as observed in surface view. These colonies are dense with a number of toxic
trace elements as analyzed under an SEM-EDX spectrometer indicating its affinity toward
metal binding. On maturity the sheath
ruptures disseminating tetrad form of bacteria along with some C-shaped bacteria and few rhombohedral crystals of framboidal
pyrite. The
pyrite crystals intact in spheroidal shape in cavities on the epidermal fragments indicate its biogenic precipitation mediated by the bacteria. The
pyrite crystals are rich in
sulfur,
iron, and traces of
arsenic when analyzed by the SEM-EDX spectrometer. It is suggested that cubic and pyritohedran forms of crystals are perhaps
arsenopyrite precipitated biogenically in saturated coastal wetland ecosystem in the past mediated by tetrad strain of the observed coccoid bacteria. Thus, the strata-bound
peat layer could be the potent residing place for
arsenopyrite or other toxic
trace elements that may contaminate the groundwater aquifer.