Screening of microorganisms isolated from hypersaline environment for melanin production, optimization and determination of structural-biological properties

Melanin is a natural, dark pigment found in animals, plants and most microorganisms and is protective against UV radiation. In this thesis, bacterial and fungal isolates isolated from hypersaline environments were screened for melanin production and melanin was produced by batch culture method with the isolates identified and the melanin produced was extracted under acidic conditions. Box-Behnken experimental design model, was used for the optimisation of melanin production conditions. The presence of melanin was determined by spectrophotometric method, solubility, functional group (FT-IR), surface morphology and elemental content (SEM-EDS) were elucidated. Antimicrobial (microdilution), antioxidant (DPPH), cytotoxic activity (MTT), UV protective activity and melanin biosynthesis pathway were determined by pathway inhibition test. Bacterial isolates identified by molecular techniques, all three bacteria isolates are Bacillus atrophaeus, and microfungal isolates Aspergillus terreus, Alternaria alternata and Exophiala alcalophila were found to be halotolerant. When 11 melanin extracts were compared with synthetic melanin, it was revealed that their functional groups, microscopic and elemental contents were similar. The antimicrobial activity of melanin extracts MIC values varied between 156,25-1250,00 μg/mL and antioxidant activity 39,06-625,00 μg/mL for the test organisms used. In addition, melanin extracts were not cytotoxic up to a concentration of 250 μg/mL and showed protective activity in UV exposure up to 3 min. It was determined that melanin from B. atrophaeus and A. terreus was DOPA type, while A. alternata and E. alcalophila was DHN type. It was found that tyrosine and glucose increased the melanin production by B. atrophaeus, while tyrosine and copper sulphate, yeast extract and copper sulphate interactions played an important role in melanin production by A. terreus. In conclusion, the fact that halotolerant bacteria and microfungus-derived melanins are biologically active and have UV protective effects indicates that they are potential sources for biotechnological and industrial use.