Microalgae are the source of various compounds with high potentials for being used in different
industries. The production of such compounds can be raised under extreme conditions. In the
present study, four cyanobacteria and one coccoid green alga were examined which were isolated from
hot springs in high background radiation areas in Ramsar, a city in the north of Iran.
Methods: Cadmium adsorption from aqueous solution, response towards cadmium stress, antioxidant
activity, total phenolic compound and drought tolerance were investigated in these microalgae.
Results: The results showed that these extremophile microalgae contain valuable biological compounds
which can be useful in remediation of heavy metals from contaminated water and soils and pharmaceutical
applications. The unicellular cyanobacterium, Chroococidiopsis thermalis IBRC-M50002, was the
best strain with the highest biological activity in various testes such as cadmium adsorption (225 mg
g-1), cadmium tolerance stress (100 mg ml-1), antioxidant activity (IC50= 18 μg mg-1) and total phenol
content (100 μg ml-1). The coccoid green algae Grasiella emersonii IBRC-M50001, also exhibited significant
antioxidant activity (IC50=10 μg mg-1) and total phenol compound (116 μg ml-1), but its cadmium
adsorption, tolerance at cadmium stress and desiccation were lower than Chroococidiopsis thermalis.
Conclusion: HBRAs microalgae, isolated from extreme conditions, are useful microorganisms for the
production of bioactive substances and natural antioxidants. In other words, they exhibited high capacity
to be used in pharmaceutical, industrial and commercial applications.