Parasites: Maybe They’re Good?

Infection of filamentous phytoplankton by fungal parasites enhances herbivory in pelagic food webs (2020) Frenken et al., Limnology and Oceanography. https://doi.org/10.1002/lno.11474

Image Credit: MarekMiś, CC BY 4.0, Image Cropped

The Crux

Pelagic ecosystems (see Did You Know) make up more than 70% of the Earth’s surface, and the base of the food web is composed of primary producers like phytoplankton. Primary producers produce their own energy and provide an important service to the rest of the food web (and planet!). Not only do they provide a resource for the upper levels of the food web, but they also contribute to the global climate by making carbon available to other organisms. Because of these large-scale ramifications for any changes in phytoplankton primary production, many studies have investigated how things like nutrients, light, and temperature are able to affect phytoplankton.

A key aspect of certain phytoplankton is that they have morphological characteristics that make them more resistant to consumption by grazers further up the food web, like zooplankton. However, chytrid parasites (the same fungus that is ravaging amphibian populations the world over) are able to get around these defenses and reconnect phytoplankton to their zooplankton consumers. Chytrid infects phytoplankton, it then releases a free-living infectious stage, the zoospore, which is eaten by zooplankton. This indirect connection between inedible phytoplankton (like cyanobacteria) and zooplankton is called the mycoloop, and it can provide zooplankton with up to 40% of their food. Interestingly, studies have shown that zooplankton populations do better when their food, the inedible cyanobacteria, is infected by chytrid. Today’s study investigated how exactly chytrid is able to reduce the cyanobacteria defenses and provide zooplankton with more food.

What They Did

The authors used Planktothrix rubescens as their inedible cyanobacteria primary producer, Rhizophydium megarrhizum as the chystrid parasite, and Daphnia longispina clones as the zooplankton consumer. First, two cultures of cyanobacteria were set up, one infected by chytrid and one uninfected. These cultures were sampled every three days over the course of 18 days, and on each of these sampling days the authors set up grazing assays (a term referring to a trial). For each grazing assay, four treatments were used to determine how chytrid affects phytoplankton and zooplankton grazing. The first treatment had cyanobacteria alone, the second had cyanobacteria infected with chytrid, the third had uninfected cyanobacteria with Daphnia grazers, and the fourth had infected cyanobacteria with Daphnia grazers. 

After each of these grazing tests, the authors measured the clearance rate, how much cyanobacteria had been consumed by Daphnia. They also measured both the infection prevalence (how much of the cyanobacteria was infected) and the filament length of the cyanobacteria. The filaments are a defensive structure, and longer filaments makes it harder for Daphnia to graze on them.

Did You Know: The Pelagic Zone

Scientists love naming things, and parts of the ocean are no exception. The pelagic zone is the part of the ocean that is in the water column, aka the “open sea” part of the ocean and not the ocean floor. The pelagic zone is divided into smaller sections named (from bottom to top) hadalpelagic, abyssopelagic, bathypelagic, mesopelagic, and epipelagic. Because the pelagic zone makes up such a significant part of the Earth’s surface, anything the affects the pelagic zone has the potential to affect a LOT of other organisms.

The pelagic zone, aka the water column of the ocean. While the pelagic zone is a single, continuous water column it is divided into different regions by depth (Image credit: Amy Apprill, CC BY-SA 4.0).

What They Found

Uninfected cyanobacteria populations grew over the course of the 18-day experiment, but they shrank when chytrid infections and Daphnia were present because the parasite killed them as it spread through the population. Daphnia consumed more cyanobacteria in the chytrid-infected treatments, while they consumed considerably less when the cyanobacteria were not parasitized. The filament length of the cyanobacteria were reduced in the Daphnia treatments, but it was even more reduced when cyanobacteria was infected with chytrid.

Problems?

I really did not have an issue with any aspects of this study, it was an elegantly-designed and thorough exploration of the questions that the authors set out to answer. The only caveat would be that this is a relatively new field of study, so it will be exciting to see what further research will discover.

So What?

Today’s study has uncovered a further level of complexity in the mycoloop, showing that parasitic infection facilitates further transfer of energy upwards through the food web. Due to the important nature of phytoplankton (like the cyanobacteria in this study) in the pelagic zone, the results of this study imply that parasites have a key role in food web dynamics and ecosystem functioning.

Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions and is currently working like a madman to finish his PhD. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.

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