When growing the Dunaliella microalga under stress conditions in the laboratory such as nitrogen deficiency and high salinity in order to extract beta-carotene. It does not change the color from green to orange.
I have not direct experience with Dunaliella, therefore please verify my suggestion with others.
However, the high production of carotenoids and other pigmented anti-oxidant in microalgae are generally stimulated by intense light -stress. This is the typical case of Haematococcus with synthesis of astaxanthine. Indeed, carotenoids perform a double activity: protection from free-radicals and shielding from solar radiation.
Other environmental stress, that you mentioned in your question, can induce a different metabolic response of the cell, in which carotenoids are only a component of a more complex combination of actives
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