Algae living inside fungi: How land plants first evolved

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Algae living inside fungi: How land plants first evolved

Scientists believe that green algae are ancestral plants living in the water, but we do not know exactly how the transition to terrestrial plants occurred.

New research from Michigan State University, published in the journal eLife, shows that algae may have grafted onto fungi to leave the water and colonize the soil over 500 million years ago.

"Mushrooms are found all over the world. They create symbiotic relationships with most terrestrial plants. This is one of the reasons why we believe they were essential to the evolution of life on earth. But so far, we have not seen any evidence of the internalization of living algae by fungi," said Zhi-Yan Du, co-author of the study and a member of Christoph Benning and Gregory Bonito's laboratories.

The researchers selected a strain of soil fungus and seaweed from old lines, respectively Mortierella elongata and Nannochloropsis oceanica.

When cultivated together, the two organizations form a strong relationship.

"Microscopic images show the algae cells aggregating around and attaching to fungal cells," says Du. "The wall of the algae is slightly decomposed, and its fibrous extensions seem to grasp the surface of the fungus."

Surprisingly, when they are grown together for a long period of time - about a month - some algae cells penetrate into fungal cells. Both organizations remain active and healthy in this relationship.

This is the first time that scientists have seen fungi internalize a eukaryotic photosynthetic organism. They call it a photosynthetic mycelium.

"It's a win-win situation. Both organizations derive additional benefits from being together," said Mr. Du. "They exchange nutrients, with a probable net carbon flow from algae to fungus, and a net nitrogen flow in the opposite direction. It is interesting to note that the fungus requires physical contact with living algae cells to obtain nutrients. Algal cells do not need physical contact or live fungi to benefit from the interaction. Fungal cells, dead or alive, release nutrients into their environment."

"Better still, when nutrients are scarce, algae and fungus cells feeding on each other feed on each other to avoid starvation. They do better than when they are grown separately," explains Du.

This increased hardiness may explain how algae survived the journey on land.

"In nature, similar symbiotic events can occur, more than we think," said Mr. Du. "We now have a system to study how a photosynthetic organism can live in a non-photosynthetic organism and how this symbiosis evolves and functions."

Both organisms are strains linked to biotechnology because they produce large quantities of oil. Du is testing them as a platform to produce high value-added compounds, such as biofuels or omega 3 fatty acids.

"Since the two organisms are more resistant together, they could better survive the stress of bioproduction," said Du. "We could also reduce the cost of harvesting algae, which is a major reason why the cost of biofuels is still prohibitive."