Modern day vegetation may have evolved from water-based algae that colonised the land by ‘piggybacking’ on fungi, exchanging both nutrients and gases.
Algae are known to naturally exist in various symbiotic relationships alongside fungi.
Yet for the first time researchers have shown that algae can not only enter into mutually-beneficial relationships with fungi but can even end up living inside them.
Green algae, thought the ancestors of plants, colonised the land over 500 million years ago — but exactly how this difficult transition occurred had been unclear.
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Modern day vegetation may have evolved from water-based algae that colonised the land by ‘piggybacking’ on fungi, beneficially exchanging both nutrients and gases. Pictured: the algae Nannochloropsis oceanica attaches itself to the fungus Mortierella elongata
Biologist Zhi-Yan Du of the Michigan State University and colleagues studied a strain of marine algae and a soil fungus — Nannochloropsis oceanica and Mortierella elongata, respectively — that both come from old lineages.
The organisms form a very strong relationship when grown together, the team report.
‘Microscopy images show the algal cells aggregating around and attaching to fungal cells,’ Professor Du said.
‘The algal wall is slightly broken down, and its fibrous extensions appear to grab the surface of the fungus.’
Furthermore, the team found that if the two microorganisms are grown together for a longer period of time — around a month — some of the algal cells even enter the fungal cells.
‘Fungi are found all over the planet. They create symbiotic relationships with most land plants. That is one reason we think they were essential for evolution of life on land,’ said Professor Du.
‘But until now, we have not seen evidence of fungi internalising living algae.’
The researchers have dubbed this fusion a ‘photosynthetic mycelium’.
Not only do both microorganisms remain healthy in this relationship, but such appears to be mutually beneficial.
‘This is a win-win situation. Both organisms get additional benefits from being together,’ said Professor Du.
‘They exchange nutrients, with a likely net flow of carbon from alga to fungus, and a net flow of nitrogen in the other direction.’
Moreover, Professor Du explained, algal cells do not even need to be in physical contact with a fungus to benefit from their presence.
Fungal cell release nutrients into their surrounding environment, even when dead.
In contrast, the fungi need to be physically in contact with living algal cells in order to get nutrients and profit from the interaction.
‘Even better, when nutrients are scarce, algal and fungal cells grown together fend off starvation by feeding each other. They do better than when they are grown separately,’ said Professor Du.
The researchers suggest that this cooperative hardiness may explain how algae survived the transition onto land — by essentially piggy-backing their way along with fungi.
‘In nature, similar symbiotic events might be going on, more than we realise,’ Professor Du added.
‘We now have a system to study how a photosynthetic organism can live inside a non-photosynthetic one and how this symbiosis evolves and functions.’
Both organisms get additional benefits from being together,’ said Professor Du. ‘They exchange nutrients, with a likely net flow of carbon from alga to fungus, and a net flow of nitrogen in the other direction,’ he added
Alongside possibly shining light on the origins of modern vegetation, the findings may also have commercial applications, with both M. elongata and N. oceanica being strains that produce large amounts of oil and are used in the biotech industry.
Professor Du is exploring whether the two together could be used as an advanced platform to produce high-value compounds like biofuels or Omega 3 fatty acids.
‘Because the two organisms are more resilient together, they might better survive the stresses of bioproduction,’ he explained.
‘We could also lower the cost of harvesting algae, which is a large reason biofuel costs are still prohibitive.’
The full findings of the study were published in the journal eLife.