| Summary: | 碩士 === 國立臺灣大學 === 海洋研究所 === 105 === The Dongsha Atoll National Park is the seventh national park in Taiwan. So far, the atoll remains close for tourism, and thus the environment is still in relatively pristine state. Two sand ridges extend to the west of the island embracing a small lagoon which connect to the main lagoon of the atoll. This lagoon is an important habitat for sea grasses and sedimentary bacteria closely related to the nitrogen cycle. In this study, we isolated the nitrate-reducing or denitrifying bacteria sampled from Dongsha small lagoon. Preliminary biochemical and physiological characterizations of the isolates were performed. We used most-probable-number (MPN) to estimate the abundance of denitrifying and nitrate reducing bacteria. After serial dilutions, samples were inoculated in polypeptone-yeast extract-nitrate (PYN) broth medium. After incubation, the MPN value of nitrate-reducing bacteria was between 1.1 × 107 to 2.4 × 109 cells/g wet wt. and the value of denitrifying bacteria was between 3.0 × 101 to 4.3 × 107 cells/g wet wt. According to the phylogenetic analysis of the partial 16S rRNA gene, sixty-four isolates belong to nine genera, including Pseudoalteromonas, Vibrio, Marinobacter, Marinobacterium, Shewanella, Neptunomonas, Idiomarina, Alcanivorax and Halomonas. We also used specific primer pairs in conserved regions of nitrite reductase gene (nirS and nirK) to determine the nir gene type of denitrifying or nitrate reducing isolates with PCR. The result showed that Marinobacter and Vibrio were the major genera of bacteria which participate the denitrification in the lagoon of Dongsha Island. Compared with previous studies, whether there were contenetal nitrogen source or not, bacteria of Marinobacter played key role in denitrification. This study provides important information about bacteria which involve in nitrogen cycle in Dongsha lagoon and can be an useful basic knowledge for microbial research in future.
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