Isolation and Characterization of Purple Nonsulfur Bacteria (PNSB) from a Rice Paddy Soil in Bulacan, Philippines

Purple nonsulfur bacteria (PNSB) are phenotypically diverse group of microorganisms and are known for their biological nitrogen fixation in flooded rice paddies. Our research study, then, aimed to isolate and characterize PNSB and determine their population count. Rice paddy soil samples were collected from San Jose del Monte, Bulacan, and were inoculated into completely filled culture vials pre-filled with different enrichment media, e.g. Larsen’s medium, Van Niel’s medium, and Acetate Yeast Extract (AYE) medium. Following incubation for 4 weeks under an incandescent bulb, only the AYE medium resulted to bright “red bloom” indicating growth of PNSB. Morphological and cultural characterization of the isolated PNSB showed pinpoint, red colonies and Gram negative, non-spore forming, thin, elongated (0.5 x 2.0 μm) rods. In vivo absorption spectrum using spectrophotometer showed the presence of bacteriochlorophyll a and carotenoids. The isolated PNSB utilized pyruvate, malate, glucose, lactate, citrate and soluble starch as its carbon sources, and ammonium sulfate, peptone and yeast extract as nitrogen sources. Preliminary identification identified the PNSB isolates as Rhodopseudomonas sp. MPN estimated low number of PNSB (2 cells per ml) in the collected rice paddy soil.