4. Discussion
Although this research was initially focused on the PNSB the results showed that other groups of APB, PsB and AAPB were also detected based on the amplification of the pufM genes (Tables 1 and 2 and Fig. 2). Our results are in agreement with the previous work that focused on the diversity of purple bacteria in a permanently frozen Antarctic Lake; however, in addition the bacteria, AAPB were also detected due use of the pufM ene probes (Karr et al., 2003). The AAPB that belong to the a Proteobacteria (Fig. 2) were the group that showed varied species found in shrimp ponds not surprising as the both sets of samples. This is a-Proteobacteria are the most abundant bacterial group in seawater and seawater is normally used for shrimp cultivation. In addition, aeration is always used in intensive shrimp ponds to improve water quality and increase shrimp yields; and all the 16 shrimp ponds studied were provided with mechanical continuous aeration, particularly at the end of culture. This is the reason why the AAPB such as R. denitrificans and R. elongatum were detected from all water samples; and R. denitrificans was detected from most sediment samples. The AAPB are aerobic and carry out anoxygenic photosynthesis by capturing energy from light using simple organic compounds as a carbon source for their growth. In contrast, a greater diversity of the PNSB was found in the sediment samples than in the water samples with a higher detection rate of the pufM gene; this is because the sediment conditions in shrimp ponds are enriched with organic matter with only a limited amount oxygen and far from the sunlight, but the PNSB also grow in dark conditions using fermentation. Hence, PNSB could compete better with the PSB in the bottom of the ponds because light is limiting for photosynthesis. The high percentage of pufM amplified uncultured' bacteria in both sample types indicated that there could be many species of purple bacteria and AAPB in the shrimp pond environment that were uncultured species. In addition, it was possible that the extracted DNA template of these spe- cies were at low levels, thus resulting in a reduced-specificity for amplification of the pufM gene, so any incomplete sequence was classified as an uncultured' bacterium.