I always joke with friends that fungi can do anything the scientist puts their mind to, or the opposite if they haven’t pleased the fungal gods that day. They’re also quite fun because they ruin the party when you get into those “what is a species” and “what is a cell” debates with people. Fungi honestly blur, cross, destroy, and defecate on those lines. They’re everywhere and they do what they want. Although maybe I shouldn’t say that, it makes them sound selfish when they also have potential to be very cooperative, generous, and communicative. We wouldn’t have this lush world colonized with land plants if it weren’t for the fungi aiding with nutrient intake and clearing old debris. But today I want to share something I personally found unique– photosynthetic mycelium.

Arguably one of the factors that sets apart animals, plants, and fungi are methods of nutrient uptake. Animals typically ingest food for absorption, plants make it themselves, and fungi digest externally for intake. There are always exceptions with any of these and much more that goes into classification, but noting this generality can help you understand why the term “photosynthetic mycelium” for me feels just as alien as saying “photosynthetic skin” or something along those lines. Like no, humans can’t photosynthesize, what a sci-fi concept! The same goes for fungi when you recognize them as being a distinctly different type of eukaryote from plants.

This semi-recent paper called “Algal-fungal symbiosis leads to photosynthetic mycelium” looks at a how Nannochloropsis oceanica algal cells are internalized by Mortierella elongata fungi and create a long-term, persisting symbiotic relationship that gives us the photosynthetic mycelium. Previously, it is well-documented that algae and fungi interact mutualistically. Lichen are one of the easiest examples, and these relationships have formed multiple times independently over hundreds of millions of years, allowing organisms that often wouldn’t survive a niche alone to instead grow there when together. This occurs in non-lichen situations as well; scientists have noted that the chytrid (zoosporic fungi) Rhizidium phycophilum form a nutrient-sharing relationship with algae in the Bracteacoccus genus (here), for instance. Yet, never before has an internal relationship like this been noted prior to the study I want to talk about today.

First, the researchers tested how these species interact, although at this point they looked at external interactions as they did not yet see the internalization of the algae. When grow alone, N. oceanica has a smooth outer layer. When with M. elongata, though, fibrous extensions formed instead. They checked to see if it was potentially a particle released by M. elongata that caused this, but they saw that the N. oceanica had to be in physical contact with the fungi to show this morphology. They noted that these fibers acted as an anchor to keep the algae onto the fungus. An additional test with fungal enzymes (fungal hemicellulase and driselase) showed digestion of the smooth outer layer that revealed the fibers underneath. Whether the morphological change by touching the fungus is intentional or not, it allows improved adhesion.

Next, they used tracer experiments to see how carbon and nitrogen was transferred. Carbon was transferred in both directions, but contact was required for transfer to the fungus. There was concern that the exchange of carbon to the fungus was due to its saprotrophic nature, but when given heat-killed N. oceanica, hardly any carbon was transferred, showing it required live algae. To ensure the species were alive during these relationships, they did a Sytox Green stain which shows cell viability and saw there was primarily life. Overall, transfer to M. elongata requires physical contact and live cells, while it did not matter for transfer to N. oceanica. For nitrogen transfer, they saw that over twice as much nitrogen transferred from the fungus to N. oceanica than the reverse. In terms of physical contact, experimental conditions were different so the results could not definitively determine what caused the increase, just that there was transfer.

In situations of nutrient deficiency, N. oceanica had higher viability when co-cultured as well as increased chlorophyll. Both had higher biomass compared to when grown alone. The co-cultured media also had higher nutrient levels, suggesting why the N. oceanica could obtain them without contact to the fungi. Overall, growing together in nutrient-deficient media was beneficial to both species. This was determined to be specific to these species as well. They tried using other fungal species across three phyla, nine orders and thirteen families to see if they could co-culture it with N. oceanica, but they only saw neutral or negative results.

While co-incubated, the fungi continue to grow as the algae grew and divided within it, notably localized at hyphal tips. They hypothesize the hyphal tip is where the algae enter as it is constantly changing the cell wall and recycling plasma membrane. Sytox Green staining showed viability of the fungi in this state, while the green chlorophyll is only characteristic in living algae, confirming it is alive as well. Not to mention, a dividing cell is certainly alive if the chlorophyll and intact plastid doesn’t convince you. If you go to the paper, you’ll be able to also view videos of the cells under the microscope in Videos 2-5. It is expected these two species may interact in nature given they can share niches or do so with similarly functioning species. As for transfer, the authors say that it is unlikely the algae would transfer vertically, but they have not looked into it completely. In terms of size, vertical transfer is not impossible.

The last thing I want to add is that they mention Mortierella ecology is not well-understood, as they are found everywhere from the soil and plant roots, to Antarctica and desert crusts. I found this so fascinating as a marine fungi in my lab, Hortaea werneckii, has been discovered in similarly odd places such as the Atacama Desert. I’d be interested to know not only how they got there, but how they live there as well (assuming their samples were found alive).

Overall, this paper was a short and fun read, and while they didn’t go into as much detail as I’d have liked, it was surely worth it given the novelty of this work. I personally really wish they did more experiments on how the algae are internalized and nutrient transfer within the hyphae, or maybe checked photosynthesis rates, but I am sure it is something they or another group will tackle eventually with this knowledge in mind. Not to mention, it takes about a month before they’re even internalized! I expect it to take a long time to get a decent chunk of data. I will have to keep my eye out in case they publish more on the subject.

Happy reading,
-Beppa