A few weeks ago, I published a post that described a mechanism carried out by mycorrhizal fungi that actually reduces plant fitness. I had learned that networks of mycorrhizal fungi allocate more water to horizons of the soil that hold much of the seed bank. When the seed bank becomes water logged, seeds break down through the actions of other fungi and bacteria. The seeds used in the experiment I talked about were all from plants that form mutualistic relationships with fungi, so I concluded that present networks of mutualist fungi don’t recognize or value these seeds as future symbionts, and focus more on their own fitness rather than the fitness of potential future partners. Interactions like these are riddled throughout the forest floor. Species aren’t benevolent, sentient beings that look out for the well-being of other species. This isn’t the ‘70s. Organisms compete, tussling for space and nutrients, even if they have to knock out a potential propagule from a symbiotic species in which they co-evolved with. This past week, I found yet another negative interaction between plants and fungi in the scientific literature. This time, it is not a mycorrhizal fungus, but a saprobic one that’s interfering.
Over and over, when I start talking about saprotrophic fungi, I mention how their substrate is nutrient poor. Most of the carbon in wood is locked up in long polymer chains, which is never enzymatically efficient to access. These fungi are pretty limited to their substrate too, unless if they make a switch to a mycorrhizal lifestyle, but that takes hundreds of thousands of years. Advances in the field of mycology have however overturned the nutritional constraint of saprotrophic fungi. As it turns out, they are a bit opportunistic.
In 1972, Nellie Stark recorded the nutrient content of pollen from Pinus contorta, which turned out to be surprisingly rich. For every gram of pollen, there is 19.6 mg of nitrogen, 9 mg of potassium and 1.7 mg of phosphorus. A 1997 paper by Hutchison and Barron built their hypothesis around this informative bit. These authors wanted to looked closer at coniferous dominated ecosystems and the feeding ecology of saprobic fungi. Their results depict fungi that predominantly break down dead woody material to be more opportunistic than we previously thought.
156 species of filamentous fungi from conifer-dominated environments where used in Hutchison and Barron’s study. They grew each isolate on Petri dishes filled with nutrient agar dusted with pollen from Pinus nigra. Out of the 156 species, 41 of them where recorded penetrating and consuming the contents of pollen grains. In nitrogen limited environments, this study shows that pollen supplements the nutrient requirements of some saprobic fungi. To me, the most notable discovery was the utilization of pollen in Pleurotus ostreatus. Although P. ostreatus is an enzymatically productive decomposer, it is also known to trap nematodes to accrue more nitrogen. This study shows that this single species has not only evolved mechanisms to trap and consume nematodes, but over millennia has adapted to use another pool of nitrogen-coniferous pollen.
I titled this post, “Parasitism of Pollen” because Hutchison and Barron applied this term in the title of their 1997 paper 20 years ago. Technically, it is kind of a type of parasitism, with the fungi ruining any chance of pollination from those specific pollen grains. In reality though, the vast majority of pollen that makes it to the forest floor lose any viable chance of contributing to the coniferous gene pool. Pollen on the ground just isn’t going to come into contact with a fertile ovule. I predict that this type of “parasitism” actually benefits plant communities. Saprotrophic fungi get the nitrogen they need from pollen to synthesize the expensive enzymes required for lignin breakdown. The more pollen available, the more nitrogen some of these fungi obtain and the faster the carbon cycles in these ecosystems. Instead of carbon remaining in dead woody substrate for decades on end, pollen enhances ecosystem functioning, allowing fungi to mobilize the otherwise stagnant molecules locked up as lignin.