I want to know about the mechanism through which biological nitrogen fixation occurs in photosynthetic green microalgae apart from previously reported bacterial associations. Like in axenic lab conditions ?
Typical green microalgae (like Chlorella, Chlamydomonas, etc.) were not known to fix atmospheric nitrogen (N2) directly. They typically assimilate nitrogen from their environment in the form of nitrate (NO3-), nitrite (NO2-), or ammonium (NH4+). The idea of green microalgae fixing nitrogen in the same way cyanobacteria do is intriguing but is not well-established or understood in the literature up to that time.
Cyanobacteria are prokaryotic and are the only group of photosynthetic oxygen-evolving organisms that can fix atmospheric N2.
Among Microalgal organisms, Heterocystous Cyanobacteria only can fix nitrogen in auxenic medium. That's why we can keep the heterocystous marine cyanobacterium on nitrogen depleted medium (ASN-).
This kind of Cyanobacterium is having heterocyst on their morphology. In this heterocyst, there are some enzymes like hydrogenase and nitrogenase. Nitrogenase, which is responsible for reducing the atmospheric nitrogen to ammonium.
Examples for Heterocystous cyanobacteria: Nostoc and Anabaena.
Image reference: https://www.toppr.com/ask/question/certain-organisms-possess-specialized-structures-called-heterocysts-that-help-in-fixing-one-of-the-gases/
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