A Secret Nitrogen Reserve in Microalgae

This guest post was adapted by Jana Pilátová (@JanaPilatova), co-author of the cited paper, from a Czech article. Guest posts do not necessarily represent the views and policies of ISEP.


Joint research of physicists and biologists brought a surprising result of large amounts of crystalline guanine in photosynthetic microscopic algae. Massive guanine reserves are used a source of nitrogen for numerous cell divisions of tested microalgae. Guanine is one of the nucleic acid bases of DNA and RNA, it also occurs in organisms in the form of various guanosine phosphates as a signaling molecule or source of chemical energy. However, crystalline guanine has so far only been known from the animal kingdom, e.g. on fish opalescent scales or as a reflexive layer at the back of dark-living animal eyes, as well as in chameleons that can change their skin color thanks to guanine nanocrystals.


Zooxanthellae in Aiptasia sp. contain large amounts of guanine crystals that are nicely visible under polarized light as twinkling stars (Jana Pilátová)

The powerful spectroscopical approach of Raman microscopy used in the study published in PNAS (online Dec 2020) is still not vastly used among biologist despite its undoubtful advantageous assets and unforeseen applications. Thanks to the microspectroscopic method, the researchers found that many species of microalgae accumulate crystalline guanine in large quantities, including toxic algal bloom forming dinoflagellates as well as zooxanthellae living in symbiosis with corals.


"It is a paradox that the existence of crystalline inclusions inside zooxanthellae has been known for about 80 years, but biologists have long speculated about their chemical composition. Now we have shown with Raman microscopy that it is mainly crystalline guanine, and in some cases also other purines, " explains Associate Professor Peter Mojzeš from the Institute of Physics, Charles University, who is the main author of the study.

. Rapid uptake of guanine by N-starved A. carterae led to the accumulation of intracellular guanine inclusions (Mojzeš et al., 2020)

In algae, crystalline guanine acts as a kind of nitrogen reservoir that the cells need to grow. "Guanine crystals are a very dynamic structure that quickly form while there are sufficient external nitrogen sources, and then they are consumed, if such sources are not available anymore. Thus, guanine crystals are a massive nitrogen storage that our fellow biologists had no idea of its existence," says dr. Mojzeš.


Researchers believe that their research will contribute to a better understanding of how microalgae deal with a lack or excess of nitrogen in the environment. The existence of corals, which live in symbiosis with each other, also depends on these microorganisms. At present, corals are strongly endangered not only by the warming of the seas, but also by the leaching of artificial fertilizers, which also disturbs the fragile balance in nitrogen recycling and the coexistence of coral with its symbionts.


Coral reef (Jana Pilátová)

According to the authors of the study, crystalline guanine in algae probably has other functions, i.e. photonic manipulation of light, which scientists want to test in future research. According to them, crystalline guanine is an evolutionarily ancient multifunctional structure that biologists have long overlooked, mainly because they lacked a simple method for its identification and study directly in cells. As dr. Mojzeš says, this method definitely is Raman microscopy.


Paper: Guanine, a high-capacity and rapid-turnover nitrogen reserve in microalgal cells. Peter Mojzeš, Lu Gao, Tatiana Ismagulova, Jana Pilátová, Šárka Moudříková, Olga Gorelova, Alexei Solovchenko, Ladislav Nedbal, and Anya Salih. PNAS, 2020.


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Background image: Nannochloropsis sp., CSIRO

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