10.22376/ijpbs.2019.10.1.p1-12 Volume 4 Issue 2 2013 (April - June) The antimicrobial compounds were extracted from two marine microalgae namely lessThan i greaterThan Nanochloropsis oculata lessThan /i greaterThan and lessThan i greaterThan Chaetoceros calcitrans lessThan /i greaterThan using different organic solvents. The efficiency of those extracts on aquatic pathogens such as lessThan i greaterThan Staphylococcus aureus lessThan /i greaterThan , lessThan i greaterThan Pseudomonas aeruginosa, Vibrio harveyi lessThan /i greaterThan , lessThan i greaterThan V. parahaemolyticus and Aeromonas hydrophila. lessThan /i greaterThan Among the extracts obtained from microalgae, chloroform + methanol (2:1) extract of lessThan i greaterThan N lessThan /i greaterThan . lessThan i greaterThan oculata lessThan /i greaterThan and butanolic extract of lessThan i greaterThan C lessThan /i greaterThan . lessThan i greaterThan calcitrans lessThan /i greaterThan had the maximum inhibiting activity against all tested aquatic pathogens. Most effective crude extracts were again purified by column chromatography and inhibitory activity of the eluted fractions tested against same pathogens. Active fraction was analyzed through FTIR for knowing its functional group. Column purified extracts of antimicrobials obtained from lessThan i greaterThan N. oculata lessThan /i greaterThan was found to be light sensitive compound which was activated at an intensity of 200 lux for 18 h. Analysis on mode of activity showed inhibition of cell wall formation in the pathogen. Identified active functional groups were carboxyl and nitro group in lessThan i greaterThan N lessThan /i greaterThan . lessThan i greaterThan oculata lessThan /i greaterThan and nucleoside compound in lessThan i greaterThan C lessThan /i greaterThan . lessThan i greaterThan calcitrans lessThan /i greaterThan . SELVENDRAN.M. Active compounds, functional group, Nannochloropsis oculata, Chaetoceros calcitrans. mode of activity. 876-888