# In Vitro Antibacterialential Activity of Marine Microalgae Extract against _Vibrio harveyi_
## 4 Discussion  
Many bioactive and pharmacologically active chemicals, particularly antibacterial compounds, are potentially produced by marine microalgae [6, 20, 21]. Furthermore, microalgae are gaining popularity due to their ability to create bioactive metabolites with anticancer, anti-inflammatory, antibacterial, and antioxidant effects [11, 22]. Previous studies have suggested that fatty acids [23], terpenoids, carbohydrates [24], peptides, polysaccharides, and alkaloids are responsible for antibacterial activity in microalgae [25]. Novel antibacterial substances were also discovered in microalgae, with _Cocomyxa onubensis_ fatty acid extracts inhibiting _E.coli_ and _P. mirabilis_[26]. Furthermore, microalgae have been increasingly treated with other compounds such as cyanovirin, oleic acid, linoleic acid, palmitoleic acid, -carotene, or phycocyanin, which have antioxidant or anti-inflammatory properties, as well as antimicrobial activity, such as against _Staphylococcus aureus_ and methicillin-resistant _Staphylococcus aureus_ (MRSA) [22, 23]. Table 1 shows variable degrees of inhibition on several species of microalgae extract against _V. harveyi_ from this investigation. As a result, the generation of antibiotics is heavily reliant on microalgal species [27]. Furthermore, the availability of antibiotic agents can vary greatly between various species within the same class or within a single species, depending on which ecotypes are adapted to certain conditions [27, 28]. The green microalga _Dunaliella_ sp. isolated from severely polluted waters, for example, was found to be more active against bacteria than ecotypes obtained from less polluted waters [29]. Recently, microalgae of the genus _Nannochloropsis_ have been shown to be rich in polyunsaturated fatty acid compounds (PUFAs), carotenoids, polyphenols, and vitamins, and have previously been used in aquaculture [30], while the microalgae strain of _Isoctrysis_ sp. was shown to be high in fucoxanthin and DHA [31]. _T. weissflogii_, on the other hand, has antibacterial properties due to its high presence of substances such as beta, lipids, phospolipid, polyunsaturated fatty acid, fucoxanthin (FX), and eicosapentaenoic acid (EPA) [32]. _C. gracilis_ showed a limited inhibited zone as compared to other microalgae extracts, particularly at higher concentrations of _V. harveyi_. This could be because the compound composition of _C. gracilis_ is minimal and contains only non-essential amino acids [33]. Apart from microalgae species, the presence of antibacterial chemicals in microalgae extracts is also strongly reliant on the extraction process and solvent utilised. According to the findings of this investigation, ethanol is the best extractant since it demonstrated the broadest inhibition against _V. harveyi_ in the disc diffusion method. Based on prior research, ethanol was also used to extract the thalli of _Gracilaria fisheri_ from India, the extract of which exhibited a high inhibitory effect against _V. harveyi_[34]. An ethanol extract of _Gracilaria corticata_ from India was found to be highly active against _V. cholerae_ and _V. parahaemolyticus_, but less active against _Pseudomonas aeruginosa_ and _Shigella flexneri_[35]. Another study on ethanolic extract of _Spiriluna_ also demonstrated more effective against two species of _Vibrio_[36]. These findings suggest that molecules with antibacterial action in microalgae are generally hydrophobic and might even be extracted more easily with organic solvents [27]. This could indicate that ethanol is a competent extraction method capable of extracting lipids with a specific set of chemicals present during extraction. The methanolic extract is a well-established and reported method to isolate active antimicrobial components from microalgae [37, 38]. In another study, methanol was found to be the best organic solvent to obtain bioactive compounds from _Dunaliella_ sp. [39]. The methanolic extract of freshwater microalgae exhibited antibacterial activity against important human bacterial pathogens [40].