Abstract
Background:
Marine microalgae extracts are a promising alternative to commercial drugs because of their antimicrobial properties.

Aim:
This study aimed to investigate the antibacterial properties of crude extracts containing bioactive compounds derived from Chlorella sp., Isochrysis galbana, and Chaetoceros calcitrans.

Methods:
Crude extracts from microalgae were obtained via maceration using ethanol and hexane as solvents. Antibacterial activities were initially screened using the agar well diffusion method, whereas antimycobacterial activity was assessed using the microplate 7H11 agar proportion method. Subsequently, the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of the crude extracts were determined. Gas chromatography-mass spectrometry (GC-MS) was used for phytochemical composition analysis to identify significant antimicrobial compounds in the extracts.

Results:
The ethanolic extract of I. galbana demonstrated detectable antibacterial activity, with an inhibition zone of 12.40 ± 0.06 mm against Bacillus subtilis, 12.22 ± 0.28 mm against Staphylococcus aureus, 11.10 ± 0.11 mm against Pseudomonas aeruginosa, and 10.99 ± 0.33 mm against Aeromonas veronii at 100 mg/ml. Extracts from I. galbana and C. calcitrans inhibited Mycobacterium marinum, suggesting potential benefits for betta fish health. MIC values were 25 mg/ml for A. veronii in the ethanolic extracts of I. galbana, C. calcitrans, Chlorella sp., and C. calcitrans hexane extract. The GC-MS chromatogram of the ethanolic extract of I. galbana revealed the presence of several bioactive compounds. The major components identified include Phthalate, Tetradecanoic acid, Hexadecanoic acid, and Phytol.

Conclusion:
This study demonstrated the promising antimicrobial potential of crude extracts from marine microalgae, particularly the ethanolic extract of I. galbana, which showed strong activity against both terrestrial and aquatic pathogens. The findings suggest that bioactive compounds in these extracts, especially those from I. galbana and C. calcitrans, can be effective in managing bacterial infections such as A. veronii and M. marinum in betta fish. These results highlight the potential of marine microalgae as natural, sustainable alternatives for disease control in aquaculture.

Key words: Marine algae, Mycobacterium marinum, Antimycobacteriosis, Antibacterial activity, Siamese fighting fish