The Sundarban mangrove forest (4,264 km 2 ) constitutes about 3% of the total area of the world mangrove. We measured diurnal and seasonal variations of air-water CO 2 exchange in relation to the occurrence of phytoplankton during January-December 2001. Diurnal variations of airflows showed that the minimum and maximum CO 2 flux of Ϫ16.2 mol m Ϫ2 h Ϫ1 and 49.9 mol m Ϫ2 h
Ϫ1, respectively, occurred during the higher sea breeze. The average ratio of dissolved inorganic nitrogen (DIN ϭ 13.85 Ϯ 7.19 mol L Ϫ1 ) to dissolved inorganic phosphorus (DIP ϭ 1.23 Ϯ 0.57 mol L Ϫ1 ) was 11 Ϯ 4 and the surface water was undersaturated with respect to dissolved oxygen. The mean value of 0.1 Ϯ 0.08 for the ratio of phytoplankton production (P) to community respiration (R) indicated that the ecosystem was heterotrophic. The saturation of dissolved carbon dioxide with respect to the atmosphere varied seasonally between 59% and 156%, with minimum levels in postmonsoon and maximum levels in premonsoon/early monsoon (June/July). Out of the 36 genera of diatoms, 1 blue green alga, and 3 dinoflagellates that occurred throughout the year, only 6 reached bloom proportions in postmonsoon, when mangrove water was a sink of atmospheric CO 2 . Although 59.3% of the emitted CO 2 was removed from the atmosphere by biological processes, on an annual basis, the Sundarban mangrove forest supplies 13.8 kg C ha Ϫ1 yr Ϫ1 of CO 2 from water surface to the atmosphere. Even though it is important to compare all in and out fluxes, there is no direct link between CO 2 emission and the later CO 2 removal by biological processes.
Inter-annual variations of phytoplankton abundance and community organization were observed over a two-decade period along with the ancillary parameters at the land-ocean boundary associated with the Sundarban mangrove forest (21°32′ and 22°40′ N and 88°05′ and 89°E), along the NE Coast of the Bay of Bengal. The number of definable Bacillariophyceae species exceeded Dinophyceae taxa, and the total number of bloom-forming species declined from a maximum of ten in 2000 and a minimum of two in 2007. Blooms of the diatom Coscinodiscus radiatus were common in 2000 and 2007. Tide cycles and the onset of the monsoon season played important roles in diurnal and seasonal variability of phytoplankton. Phytoplankton biovolume showed seasonality, with the highest levels during post-monsoon periods and lowest levels during the monsoon period. Phytoplankton abundance was correlated to rainfall patterns, which may be altered by long-term changes in climate.
The present study demonstrates the metal toxicity ameliorating and growth promoting abilities of three different bacterial isolates when applied to rice as host plant. The three bacterial strains included a cadmium resistant Ochrobactrum sp., a lead resistant Bacillus sp. and an arsenic resistant Bacillus sp. designated as CdSP9, PbSP6, and AsSP9, respectively. When these isolates were used as inocula applied to metal-treated rice plants of variety Satabdi, the germination percentage, relative root elongation (RRE), amylase and protease activities were increased. The toxic effect of metal was reduced in presence of these bacteria. The overall biomass and root/shoot ratio were also enhanced by bacterial inoculation. Hydroponic studies showed that the superoxide dismutase (SOD) activity and malondialdehyde (MDA) level, which had been increased in the presence of metal stress in rice roots, were lowered by the bacterial inoculation. In addition, all three strains were 1-aminocyclopropane-1-carboxylate (ACC) deaminase and catalase positive, whereas siderophore producing ability was lacking in PbSP6. However, both PbSP6 and AsSP9 were protease positive and could hydrolyse starch. The data indicate that these bacteria have promise for bioremediation as well as for plant growth promotion.
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