Microbial infection by bacteria has caused severe health problems worldwide. Treatment with antibiotics as the current solution has several drawbacks and triggers the phenomenon of bacterial resistance. Therefore, there is an urgency to look for a natural antimicrobial that is safer and has fewer side effects. One of the most promising antibacterial agents is Spirulina platensis. This research was conducted to evaluate the antibacterial activity of microalgae S. platensis against Propionibacterium acne, Staphylococcus epidermidis, and Enterobacter aerogenes and identify compounds from the active fraction of microalgae. Biomass was extracted with ethanol 96% using the reflux method then partitioned with immiscible solvents such as hexane, ethyl acetate, and water. Partial purification was carried out by chromatography techniques such as thin-layer chromatography and column chromatography. The compounds of active fractions were identified by GC-MS analysis. The result showed that ethyl acetate extract had vigorous antibacterial activity against all tested bacteria. The highest activity (14.4 ± 0.63 mm and 16.9 ± 1.48 mm) was achieved against P. acne; followed by S. epidermidis (13.05± 0.14 mm and 13.15 ± 0.0 mm), and E. aerogenes (11.7 ± 2.05 mm and 12.6 ± 1.90 mm), at concentrations 20,000 ppm and 30,000 ppm, respectively. The results indicated that the extract is more sensitive to Gram-positive bacteria (P. acne and S. epidermidis) than Gram-negative bacteria (E. aerogenes). Purification of the extract resulted in fraction 2 and fraction 6 as the most potential fractions for further analysis and identification. Based on the antibacterial activity, inhibition zones of fractions are wider than extracts. It could be assumed that the purification process enhances the activity of a sample. GC-MS analysis revealed that the dominant compounds of fractions 2 and 6 were bis (2-ethylhexyl) phthalate (67.76%) and 1,2-Benzendicarboxilic acid, bis (2-ethylhexyl) ester (50,88%), respectively. This result indicated that the ethyl acetate fraction of the microalgae S. platensis has the potential as a natural antibacterial.

The demand for natural antioxidants has increased due to the harmful effects of synthetic antioxidants, such astoxicity and carcinogenic properties. Microalgae face this requirement as they can produce numerous substances that havepotential as an antioxidant. This study aimed to evaluate the antioxidant activity of Cosmarium extracts using ABTS radicalcation assay and identify compounds probably responsible for it. Dried biomass was extracted using hexane, ethyl acetate,and ethanol solvents using the maceration method. Extracts were partially purified with TLC and column chromatography.Compounds identification were conducted using GC-MS analysis. The result showed that ethanol extract has the best activitywith the IC 50 at 55.95 ppm, followed by hexane and ethyl acetate extracts with IC 50 as much as 104.339 ppm and 180.07 ppm,respectively. Two active fractions were the selected fraction after partial purification with chromatographic analysis. Fraction1 gave IC 50 at 99.6 ppm while fraction 2 gave better IC 50 at 53.562 ppm, both categorized as strong antioxidants. Compoundsidentification by GC-MS revealed that both fractions contain fatty acids compounds with 9.12-octadecadienoic acid (linoleicacid) and hexadecanoic acid (palmitic acid) as the dominant compound in fractions 1 and 2, respectively. This study gaveinsight into the potential of ethanol fraction from Cosmarium sp. as a natural antioxidant.

Chroococcus turgidus belongs to cyanobacteria, a photosynthetic prokaryote, which produces many valuable bioactive compounds and shows interesting biological activities. In this study, we investigated the antimicrobial activity of the ethanol fraction from C. turgidus and identified compounds that responsible for the activity. C. turgidus biomass was extracted using the reflux method and then fractionated by column chromatography. The antimicrobial activity was evaluated against Staphylococcus aureus, Escherichia coli, Bacillus subtilis, and Candida albicans using agar disc diffusion method. The result showed significant activity against all tested microbes. The largest zone of inhibition was 18.1 ±0.14 mm, achieved by the fraction against S. aureus. Characterization using Gas Chromatography-Mass Spectrometry revealed the volatile organic compounds with 1, 2-Benzenedicarboxylic acid, bis(2-ethylhexyl)ester as the dominant compound. This study revealed that the bioactive compounds produced by C. turgidus could be a potential source of natural antimicrobials.