With the autumn mushroom season in full swing, many of the larger, choice edibles seem to get the spotlight. There is nothing wrong with this and I encourage you to learn the edible species that grow in your own neck of the woods, so you too can enjoy the fruits of the forest floor. However, smaller non-edible species are also occurring in great abundance and they should not be overlooked.
On my last hike, a lush patch of moss caught my eye. From a few meters away, the patch seemed to have huge sporophytes. Bryophytes hold a special place in my ecological heart, because they represent an ancient Earth when plants were just getting a foothold in terrestrial habitats. I crouched down for a closer look. There, I realized I was wrong. These structures were not large moss sporophytes, but the fruiting bodies of moss loving Rickenella fibula.
Key identifying features of this mushroom include its small size, orange color and pale creamy to light orange gills. Also, its gills are decurrent, slightly traveling down the stipe. One feature that really helped me distinguish it from other dainty orange mushrooms are its fine bristles that you can see if you examine the cap closely. These bristles are sterile structures which are also known as ‘cystidia,’ and occur on its cap, gills, and upper stem. These aforementioned characteristics are extremely useful, but its habitat in moss is the most important feature attributed with this species.
Most species that occur with moss are saprobes that share similar niche requirements with the moss. That is, many of these organisms can only exist in a certain range of temperature, moisture, pH, and nutrient content of the substrate. Much of the time, they don’t directly interact. Moss loving fungi break down dead plant material that may leach and be absorbed into the plant, but these interactions are not considered mycorrhizal. Recent studies indicate that Rickenella fibula doesn’t just coexist with the moss it is found growing with. There is actually a direct interaction going on here.
Moss utilize arbuscular mycorrhizae (AMF) and as we know, AMF are from the ancient fungal division Glomeromycota. Members of the Glomeromycota represent ‘lower’ fungi, and don’t form fruiting bodies we can see with the naked eye. So, what exactly is going on here? Well, in 1981, Scott A. Redhead described R. fibula as a moss parasite, stealing plant sugars from primitive root structures called rhizoids. He closely analyzed the mycelia of the fungus and found that hyphae actually penetrated moss rhizoids.
The moss that I was looking at was lush, and supported a plethora of sporophytes indicating its fit ecological stature. A parasitic fungus would decrease moss fitness, something I did not see. This fungus is not a parasite. It then dawned on me; this union between fungus and moss offers a glimpse into Earths past. This species interaction represents how higher fungi gained a mycorrhizal ecology, setting an ectomycorrhizal dominated forest floor.
Ectomycorrhizae did not evolve in a single ‘big bang’ moment. It occurred in a series of steps. Small fungal species with ancestors that coexisted with moss, that then penetrated moss rhizoids indicate the origin of ectomycorrhizae. As bryophytes evolved new structures and strategies, so too did their fungal symbiote. Ectomycorrhizae has come a long way from its ancestral condition. Today we see ectomycorrhizae in relation with higher plants that surround plant roots in a cocoon like structure called the Hartig net-a structure that maximized resource transfer.
More research needs to be done to confidently categorize this interaction between R. fibula and moss mycorrhizal. It could be an indirect positive interaction, but hyphae that actually penetrate moss rhizoids reveal a more direct interaction. This mushrooming season, remember, even if you’re looking for delicious species like Laetiporus sulphureus, or Grifola frondosa, make sure to keep an eye out for more discrete fungi. Who knows, you might find Rickenella fibula symbiotically living with some moss.