In the present study, we report the nitrogen fixing potential of heterotrophic diazotrophs isolated from a tropical estuary and adjacent coastal sea. Results of the study revealed that most of the species that are capable of fixing nitrogen in the study area belongs to the genus Bacillus. The isolates from the estuary showed maximum homology with Bacillus megaterium, B. cereus, B. safencis, B. licheniformis, B. aerophilus, B. oceanisediminis, B. flexus, B. aquimaris, B. vietnamensis, and B. subterraneaus, whereas the diazotrophic isolates from coastal samples were closely related to B. subtilis, B. megaterium, B. circulans, B. aerophilus, B. flexus, and B. oceanisediminis. Experimental studies to determine the nitrogen fixation potential of isolates revealed considerable variation among different strains and the highest nitrogen fixing potential was recorded in B. megaterium (210.05 ± 7.0 nmol C H /mg protein/day) followed by B. flexus (108.76 ± 3.66 nmol C H /mg protein/day) and B. circulans (98.28 ± 4.32 nmol C H /mg protein/day). Molecular basis of nitrogen fixation by these heterotrophic Bacillus strains has been explored in terms of the presence of nifH gene in them. We observed that heterotrophic Bacillus sp. have potential ability to fix nitrogen.

Eight hundred and eighty-¢ve strains of bacterial isolates from various samples associated with the natural habitat of Macrobrachium rosenbergii were screened for their probiotic potential. Two putative probionts namely Bacillus NL110 and Vibrio NE17 isolated from the larvae and egg samples, respectively, were selected for experimental studies and were introduced to the juveniles of M. rosenbergii (0.080 AE 0.001g) through di¡erent modes such as through feed, water and both. The probiotic potential of the above bacteria in terms of improvements in water quality, growth, survival, speci¢c growth rate (SGR), feed conversion ratio and immune parameters was evaluated. The treatment groups showed a signi¢cant improvement in SGR and weight gain (Po0.001). Survival among di¡erent treatment groups was better than that in the control group. There were also signi¢cant improvements in the water quality parameters such as the concentration of nitrate and ammonia in the treatment groups (Po0.05). Improvements in immune parameters such as the total haemocyte count (Po0.05), phenoloxidase activity and respiratory burst were also signi¢cant (Po0.001). It is concluded that screening of the natural micro£ora of cultured ¢sh and shell¢sh for putative probionts might yield probiotic strains of bacteria that could be utilized for an environment-friendly and organic mode of aquaculture.

Microorganisms play a significant role in biogeochemical cycles, especially in the benthic and pelagic ecosystems. Role of environmental parameters in regulating the diversity, distribution and physiology of these microorganisms in tropical marine environment is not well understood. In this study, we have identified dinitrogen (N) fixing bacterial communities in the sediments by constructing clone libraries of nitrogenase (nifH) gene from four different stations in the Cochin estuary, along the southeastern Arabian Sea. N fixing bacterial clones revealed that over 20 putative diazotrophs belong to alpha-, beta-, gamma-, delta- and epsilon- proteobacteria and firmicutes. Predominant genera among these were Bradyrhizobium sp. (α-proteobacteria), Dechloromonas sp. (β-proteobacteria); Azotobactor sp., Teredinibacter sp., Methylobacter sp., Rheinheimera sp. and Marinobacterium sp. (γ-proteobacteria); Desulfobacter sp., Desulfobulbus sp. and Desulfovibrio sp. (δ -proteobacteria); Arcobacter sp. and Sulfurospirillum sp. (ε-proteobacteria). Nostoc sp. was solely identified among the cyanobacterial phylotype. Nitrogen fixing Sulfate reducing bacteria (SRBs) such as Desulfobulbus sp., Desulfovibrio sp., Desulfuromonas sp., Desulfosporosinus sp., Desulfobacter sp., were also observed in the study. Most of the bacterial nifH sequences revealed that the identities of N fixing bacteria were less than 95% similar to that available in the GenBank database, which suggested that the sequences were of novel N fixing microorganisms. Shannon-Weiner diversity index of nifH gene ranged from 2.95 to 3.61, indicating an inflated diversity of N fixing bacteria. Canonical correspondence analysis (CCA) implied positive correlation among nifH diversity, N fixation rate and other environmental variables.