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Probiotics play vital roles in improving growth, survival, and immune responses and inhibit the growth of pathogenic bacteria in freshwater fish. This study was conducted to isolate potential probiotics from Channa punctatus and Channa striatus and to evaluate their effect on Labeo rohita fingerlings. Among the isolates, Bacillus paramycoides PBG9D and BCS10 (1) exhibited antimicrobial activity against the fish pathogen Aeromonas hydrophila. Both strains showed tolerance to acidic and alkaline pH (2, 3, 4, 7, and 9) and bile salts (0.3%) and exhibited strong adhesion capacity. After in-vitro assessment, these strains were evaluated on the growth performances of rohu fingerlings challenged by Aeromonas hydrophila for 4 weeks. The study consisted of six groups, each containing 6 fish. Group (I) was the control, fed a basal diet; group (II) contained a pathogen and was also fed a basal diet; group (III & IV) was given a probiotic supplemented experimental diet; Fourth group (V & VI) contained a pathogen and was given a probiotic supplemented experimental diet. After the 12th day of experiment, rohu fingerlings of pathogen (II) and probiotic + pathogen (V & VI) groups were intraperitoneally injected with 0.1 mL of Aeromonas hydrophila. After 4 weeks, no significant differences in weight gain, weight gain %, and feed conversion ratio were observed in probiotic (III & IV)- fed groups compared to control. However, the specific growth rate was significantly improved in probiotic fed groups compared to other groups. Survival rate and condition factor were significantly similar in all groups. After injection, abnormal swimming, loss of appetite and weight loss were observed in the pathogen (II) group, while no such symptoms were found in the probiotic + pathogen (V & VI)- groups, confirming the effects of probiotics. The overall results of the study revealed that dietary supplementation with Bacillus paramycoides strains could improve the specific growth rate and disease resistance against Aeromonas hydrophila in Labeo rohita.

Nutrient cycling is an important environmental process and has been the focus of ecological research. Nutrient cycling refers to the sufficient supply of key elements provided through the ecological processes within and between various biotic or abiotic components of a cell, community, or ecosystem. Nutrient cycling also includes the recovery and reuse of industrial, agricultural, and municipal organic debris that are considered wastes. Nutrient cycles include biotic and abiotic components involved in biological, geological, and chemical processes known as biogeochemical cycles. Changes occurring in such cycles may indicate or even alter the functioning of the ecosystem. Plants take up soil nutrients in terrestrial ecosystems for healthy growth and development, wherein soil acts as a nutrient reservoir. Nutrients are lost from such sites due to soil erosion, denitrification, and food production, which cause reduced availability of nutrients. Therefore, analyzing nutrients’ assimilation, transport through biota, and their release for subsequent re-assimilation is mandatory. Nutrients to be recycled essentially for the survival of organisms include macronutrients (C, O, H, N, K, P, Ca, Mg, S, and Cl) and micronutrients (Fe, Mn, Mo, Cu, Zn, Bo, Ni, Co, Na, Se, and I). This chapter presents the role of nutrients and nutrient cycling for environmental sustainability