Mycoloop

The mycoloop is a trophic pathway in aquatic food webs where parasitic fungi, particularly chytrids, facilitate the transfer of nutrients and energy from large, inedible phytoplankton (algae) to zooplankton. This process enhances nutrient cycling and supports higher trophic levels in aquatic ecosystems. Chytrids infect large, inedible phytoplankton, such as diatoms or cyanobacteria, and produce zoospores (free-living, motile spores, 2–5 μm in diameter). These zoospores are rich in nutrients like polyunsaturated fatty acids (PUFAs) and cholesterol, making them an excellent food source for zooplankton, such as Daphnia and rotifers. By consuming the zoospores or fragmented phytoplankton, zooplankton gain access to nutrients that would otherwise be unavailable from inedible phytoplankton, creating the trophic link called the mycoloop. In this manner, the mycoloop channels carbon, phosphorus, and other nutrients from phytoplankton to zooplankton, bypassing the limitations of inedible phytoplankton. The mycoloop can influence phytoplankton blooms by reducing host populations (via parasitism) and supporting zooplankton growth, potentially stabilizing aquatic food webs. It can also influence the carbon cycle by altering carbon fluxes, reducing the sinking of large phytoplankton and redirecting carbon to higher trophic levels. The concept of the mycoloop was developed by Maiko Kagami et al. in 2007. The term "mycoloop" combines myco (referring to fungi, specifically chytrids) with loop (referring to the cycle of nutrient transfer). The discovery of the mycoloop, and its potential impact on nutrient cycling indicates the importance of fungal-algal interactions in natural systems. Chytrids have also been reported to stabilize food webs, while also reducing the amount of organic material that reaches benthic environments.

Background Most food web studies do not incorporate what is perhaps the most common trophic interaction - parasitism. Despite their ubiquity, parasites are often overlooked because of their cryptic nature, the difficulties in quantifying their effects, and their assumed low biomass. However, they can account for greater biomass than predators and participate in the majority of trophic links. Parasites can modulate trophic flows in a number of ways. They can drive reductions in host biomass, not only by increasing host mortality rates, but also by influencing growth, fecundity, nutritional status, susceptibility to predation, or behaviour. While their role as consumers is better known, parasites can also be prey for other organisms. They can be consumed together with their host (i.e. concomitant predation) or as free living life stages. Given the enormous reproductive output of parasites, free living infecting stages potentially constitute a significant nutrient source and can account for a substantial transfer of material and energy to higher trophic levels. Chytrids are a type of microscopic fungi belonging to the phylum Chytridiomycota. These fungi are primarily aquatic or found in moist environments. Chytrids can be saprophytic (decomposing organic matter), parasitic (infecting plants, algae, or animals), or mutualistic, and play key ecological roles in breaking down organic material and nutrient cycling. Notably, the chytrid Batrachochytrium dendrobatidis causes chytridiomycosis, a deadly disease in amphibians, contributing to global population declines. They are unusual among fungi in that they reproduce with motile spores, driven by flagella, called zoospores. Most chytrids do not sexually reproduce. Asexual reproduction occurs through the release of zoospores.

Saprotrophic chytrids decompose inedible organic matter releasing zoospores that zooplankton consume, further contributing to nutrient cycling. Zooplankton grazing on zoospores may suppress chytrid outbreaks, regulating parasite populations. The mycoloop can stabilise ecosystem by alleviating competition among phytoplankton and supporting zooplankton production, especially in nutrient-rich environments. Studies show chytrid zoospores, which can have densities up to a billion spores per litre, are a high-quality food source, doubling zooplankton feeding rates compared to uninfected phytoplankton. The mycoloop is significant both in freshwater lakes and marine environments, with chytrids like Zygorhizidium facilitating nutrient transfer from algae like Asterionella to zooplankton like Daphnia.

Parasitic fungi derive nutrients from living hosts, often causing harm. However, fungi have many other ecological roles they can play apart from being parasitic. For example, they can be mycorrhizal (forming mutualistic relationships with plants), endophytic (living inside plants without causing harm), lichenized (forming symbiotic relationships with algae or cyanobacteria), or saprotrophic (breaking down dead organic matter). Some fungi even switch roles depending on environmental conditions or host availability.

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