What are Protists?

In every corner of the globe, you can find microbes. From boiling ponds to glaciers on the Tibetan Plateau, microbes thrive in some of the harshest places on our planet. Protists are fascinating and important members of microbial communities. Protists are eukaryotes, meaning their cells are organized into tiny organs known as organelles. Protists can perform a variety of essential jobs in their ecosystems. In many ways, they are microscopic versions of the plants and animals we see around us every day.

Protists Play Critical Roles in Aquatic Environments

Protists are found in all ecosystems, but they are especially important in lakes, oceans, and other aquatic environments. There, protists are part of a food web known as the microbial loop. Like other food webs, the microbial loop has its producers, consumers, and decomposers. Instead of trees and bushes, microscopic plants (algae) harness the sun to grow and provide sugars for the rest of the food web. Much like deer and rabbits, herbivorous (plant-eating) protists graze on algae, while bacteria munch on sugars released by the algae. Predatory protists are the wolves of these ecosystems, devouring anything smaller than them and leaving the scraps for decomposers.

The members of the microbial loop work hard to convert essential nutrients, like carbon and nitrogen, into forms that fish and other animals can use. In fact, all the life we find in the oceans exists because of microbes. Because of protists’ importance, scientists called microbial ecologists aim to understand their role in cycling these nutrients.

Protists Function as a Community

No one protist can do every job needed to support the microbial loop. Instead, protists and other microbes work together (and against each other) in complex communities. These microbial interactions can occur in positive, negative, or neutral combinations. When both organisms benefit, the interaction is known as mutualism. An interaction where both organisms suffer is known as competition, and it occurs when organisms compete for food, space, or other resources.

Interactions are always changing. Some organisms are friendly when the water is warm and nutrients are plentiful, but when things get colder, they begin competing for nutrients to survive. Changes in their environment can have big impacts on the microbial loop, which in turn impacts the rest of the ecosystem. Microbial ecologists are working to unravel how microbial interactions shift with natural environmental changes and how they may be altered by the effects of climate change. However, observing interactions of these tiny plants and animals has proven to be a big challenge.

Imagine you had a high-tech pair of binoculars that could see everything when you pointed them at a pond or lake. As you sit by the shore, you can see fish and beavers; you see reeds and lily pads bobbing in the waves, and mosquitos buzzing too close to hungry frogs. If you keep looking, you can see the protists too: little green algae and even smaller cells zipping around, just small whispers compared to the croaking of frogs and splashing of fish. In most aquatic ecosystems, animals and plants are very loud or busy, making it nearly impossible to make detailed observations of the microscopic protists. Researchers have created some ways to focus in on protists, such as DNA sequencing or strong microscopes. By using these methods, they can figure out which protists are around and learn a bit about how they interact. However, a lot of protist activity is still too quiet to hear. To silence the noise further, researchers can observe quieter lakes.

Studying Protists in McMurdo Dry Valley Lakes

What better lakes to explore than those in the coldest, driest places imaginable: the McMurdo Dry Valleys. These valleys are found in Antarctica, nestled in the Transantarctic Mountain range. The McMurdo Dry Valleys are one of the largest ice-free regions in Antarctica because of the low humidity and the mountains protecting the area from surrounding ice sheets. This region has many habitats, from sandy dunes to wind-carved rocks, and from giant glaciers to ice-covered lakes. Because of the harsh conditions and limited nutrients, no plants or animals can survive. The environment has been compared to that of Mars! In such an inhospitable place, protists reign supreme [1].

This region of Antarctica, and the extreme microbes living there, have been well-studied for over 30 years by a group of scientists from the McMurdo Long Term Ecological Research Project (McM LTER). The lakes are areas of intense investigation. These lakes are covered in thick ice year-round, which keeps their only inhabitants, microbes, protected from the harsher conditions outside. The lack of plants and animals, along with the protective ice cover, make these lakes a nice, quiet environment to study microbial interactions (Figure 1).

The Antarctic Lake Microbial Loop

Even though they contain only microbes, the McMurdo Dry Valley lakes are diverse. However, a few groups of critical protists steal the show, including Chlorophyta, Ciliophora, and Thraustochytrid (Figure 2) [2]. These three groups do not represent the full diversity, but they fulfill critical roles in the microbial loop within these lakes.

Chlorophyta, also known as Chlorophytes or green algae, are the ancestors of plants. Like plants, chlorophytes are primary producers, using light to turn carbon dioxide into sugars. A well-studied Chlorophyte from Antarctica is Chlamydomonas priscui. This Chlorophyte can be found in deep parts of the lake, where it has evolved to thrive in very low light and high salinity (saltiness). Despite relying on light as their energy source, Chlorophytes like C. priscui survive polar night, a period of 24-h darkness in the winter.

Ciliophora, also known as ciliates, are considered to be the smallest animal. Ciliates are covered in fine hairs called cilia, which they use to move and to bring food to their cytostome (mouth). They have very simple digestive systems (basically a bag of enzymes) and they even poop! Ciliates are some of the largest organisms in these lakes, making them predators of bacteria, algae, and other protists. As top predators, ciliates help control prey populations, redistributing limited nutrients. Thraustochytrids, some of the tiniest protists, occupy lower parts of the food web, but this does not make their contributions any less important. Thraustochytrid cells are covered in stiff hairs. Unlike ciliates or Chlorophytes, Thraustochytrids are saprotrophs—organisms that decompose fallen leaves, dead animals, etc. In the McMurdo Dry Valley lakes, saprotrophs decompose dead algae and cell debris, which moves through the permanent ice like a conveyor belt or floats down the water column. Thraustochytrids decompose algae by attaching to them, producing an ectoplasmic net (an extension of their cell membrane), and excreting powerful enzymes that can break down tough materials.

How do Protist Interact?

Chlorophytes, Ciliophora, and Thraustochytrids each fulfill major roles in the microbial loop. These organisms impact their environment and can influence each other’s lives. They can do so indirectly, by releasing nutrients back into the water for other organisms to use, or by direct interactions (Figure 3). How protists work together or against one another can majorly impact their jobs in the food web. Chlorophytes pull CO₂ from the water and turn it into sugars, which can help the cells grow or are released into the water. These sugars can attract bacteria, which trade resources (such as vitamins) that the Chlorophytes cannot get themselves. These relationships are positive and help both of organisms grow, with bacteria essentially farming the algae.

Heterotrophic nanoflagellates, which have whip-like flagella, act as secondary consumers that directly feed on bacteria. By hunting bacteria, nanoflagellates hinder the positive relationship between Chlorophytes and bacteria. At the same time, they release nutrients that other heterotrophs can use. These protists are necessary to maintain a balance in prey populations.

At the top of the food web are the Ciliates. Due to their size, ciliates voraciously devour algae, bacteria, and even heterotrophic nanoflagellates. Ciliates control the populations of bacteria and algae, which is important because too much of one group of algae can block out light needed by deeper-dwelling algae. Being at the top of the food web means that ciliates are often most impacted by sudden changes.

Ultimately, many interactions are driven by the need for nutrients, such as carbon, nitrogen, phosphorous, and sulfur. Interactions between chlorophytes, ciliates, and nanoflagellates are all examples of direct interactions. Saprotophs, such as Thraustochytrids, indirectly interact with other protists. Saprotrophs release many nutrients via decomposition, which other predators cannot consume on their own. Dissolved nutrients can then be taken up by algae, bacteria, and other protists, thus keeping the microbial loop in balance. However, environmental changes can disrupt interactions and throw food webs out of balance and as our climate continues to change, this balance is expected to be further disrupted.

Applying Mcmurdo Lake Protist Interactions to Other Environments

While the protist species in the McMurdo Dry Valley lakes are unique, the critical roles they play as producers, consumers, and decomposers are common across all aquatic ecosystems [3]. This means that what is learned in these lakes can be applied to other environments. Scientists hope that studies of protist interactions in these lakes can help predict how climate change might affect the microbial loop in other ecosystems, thus allowing them to predict how the services microbes provide to the rest of the food web may change. These predictions will hopefully inform ways to protect these tiny but important organisms! Finally, if you would like to learn more, you can check out McM LTER Outreach or Polar TREC.

Glossary

Protists: ↑ Eukaryotic, single-celled organisms with diverse shapes, niches, and structures.

Food Web: ↑ A simple explanation describing how energy and matter flow in an ecosystem.

Microbial Loop: ↑ The microbial portion of the aquatic food web that produces and recycles carbon and energy generally used by larger plants and animals.

Microbial Ecologist: ↑ A scientist that studies how microbes interact with their environments, each other, and larger plants and animals.

Enzyme: ↑ Proteins that help specific chemical reactions happen, such as building or breaking apart compounds.

Saprotroph: ↑ An organism that feeds on dead or rotting organic material by releasing enzymes and absorbing the nutrients that are released.

Heterotroph: ↑ An organism that feeds on other organisms to gain energy and carbon.

Conflict of Interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

This work was supported by the NSF office of polar programs (OPP-1637708, OPP-1937546). Written informed consent was obtained from the individual(s) for the publication of any identifiable images or data included in this article.

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