ABSTRACT
Microalgae represent promising sources of bioactive compounds for pharmaceutical and industrial applications. The emergence of antibiotic resistant bacteria leads to the need to explore new cost-effective, safe, and potent bioactive compounds from the microalgae. This study aimed to investigate the potential of local microalgae for their antimicrobial properties and bioactive compounds. Three local microalgae namely Chlorella sorokiniana (UKM2), Chlorella sp. UKM8, and Scenedesmus sp. UKM9 biomass methanol extracts (ME) were prepared and tested against Gram-positive and Gram-negative bacteria. Chlorella sp. UKM8-ME showed the highest antibacterial activity. UKM8-ME minimum inhibitory concentrations were in the range of 0.312 to 6.25 mg/mL. Cytotoxicity evaluation using MTT assay showed that the microalgae methanolic extracts did not exhibit cytotoxicity against Vero-cells. The UKM8-ME was mainly containing 28 compounds from the Gas Chromatography-Mass Spectrometry (GC-MS) analysis. Major compounds of UKM8-ME included phenol (18.5%), hexadecanoic acid (18.25%), phytol (14.43%), 9,12-octadecadienoic acid (13.69%), and bicyclo[3.1.1]heptane (7.23%), which have been previously described to possess antimicrobial activity. Hence, Chlorella sp. (UKM8) methanol extracts showed promising antibacterial activity. More comprehensive studies are required to purify these antimicrobial compounds and develop our understanding on their mechanism in UKM8-ME to unleash their specific potential.
ABSTRACT
Aims: Several harmful dinoflagellate species are widely found in Malaysian marine waters. These microalgae are capable of producing a variety of toxins that can intoxicate humans through consumption of contaminated seafood. Therefore, the present work aimed to investigate the potential of a marine bacterium sdPS-7 isolate as a biological control agent against Alexandrium minutum KB-5. Methodology and results: The isolate sdPS-7 was obtained from marine sediments in Malaysia and examined for its algicidal activity toward the toxic dinoflagellate A. minutum KB-5 in laboratory culture. Based on morphological and molecular characterization, this isolate was identified as Aeromonas hydrophila sdPS-7. Alexandrium minutum KB-5 cultures were exposed to cell free bacteria culture filtrate and the effect on dinoflagelate growth was evaluated based on direct cell counts in the treated samples compared to non-treated control cultures. The bacterial filtrate was treated at different temperatures, salinities and exposed to repeated freezing and thawing five times to study its algicidal activity stability thereafter. The results showed a potent inhibition of the growth of A. minutum KB-5. The strongest effect was observed when the bacterium culture filtrate was mixed with A. minutum KB-5 cultures that were in lag phase, resulting in 99% or higher mortality. The cell-free filtrate proved to be heat- stable when exposed to temperatures of 2 °C to 100 °C for one hour each. There was also no substantial salinity as well as (freezing – thawing) effect on the filtrate algicidal activity. Conclusion, significance and impact of study: This study illustrated the potential use of the marine bacterial Aeromonas hydrophila sdPS-7 filtrates in controlling the growth of the toxic dinoflagellate A. minutum KB-5.
