The toxicity of Hg on PNSB has been studied but only by an in vitro test i e. Hg at different concentrations (k 100 HM) inhibited the growth of Rhodobacter sphaeroides 2.4.1 by affecting the photosynthetic apparatus and the bind- ing with C so, C-N, C-S, and C-SH groups of the amino acids (Asztalos et al., 2012).In addition, Hg2+ at 0.03 mM gave a negative effect on R. sphaeroides R26.1 as the growth rate decreased while the lag-phase increased following the increase of the Hg concentrations from 0.001 to 0.050 mM (Giotta et al., 2006). However, no work has been done on the effect of Hig on PNSB in shrimp ponds. The Hgr concen tration in the sediment was significantly higher than in the water column (Table This was because most of the Hg in 3). he water was attached to particles of suspended sediment such as organic matter including metal oxides (Domagalski 2001) and these precipitated onto the sediment. In addition the Hig in the sediments was strongly bound to s2 to form a highly insoluble mercury sulfide (Hgs)