Soil protists as indicators of pollution


Breaking down the microbiology world one bite at a time

Soil protists as indicators of pollution

Let’s think about soil. What does 1 gram of soil contain? Besides organic and inorganic matter, even the smallest particle of soil contains A LOT of microorganisms! In 2018, Geisen and Bonkowski estimated that there are between 10,000 and 100,000 individual microorganisms per gram of soil. These are numerous bacteria, fungi, viruses, and protists. Protists, single-celled eukaryotic microbes, constitute a large portion of those soil microorganisms. While it is usually believed that they inhabit only aquatic systems, this is not entirely correct. Protists show remarkable morphological and physiological diversity, which allows them to live in different environments and adapt to various conditions. 

There is so much literature on bacteria and fungi in soil, but so little about soil protists. Scientists are claiming that these organisms have been unfairly overlooked for a long period of time. Their impact on the soil environment has gone unnoticed (or not noticed enough!). For example, photosynthetic protists are responsible for the carbon input into soil systems. Soil protists can also regulate and balance microbial populations because some of them feed on bacteria, fungi, and even other protists. Finally, some protists can significantly affect plant growth by stimulating bacterial activity and increasing nutrient input of carbon-based and nitrogen-based molecules (e.g. amino acids, carbohydrates etc.).

Rapid technological development and urbanization has led to many ecological problems and crises. Similar to air being contaminated with greenhouse gases, soil also gets contaminated with heavy metals, microplastics, and organic pollutants. Undoubtedly, such toxins affect soil organisms on an individual and on a species population level. Considering such an extensive list of benefits that protists have on soil in general, scientists are wondering how these environmental changes can influence protists themselves.

Protists can be an enormous help to plants that are struggling with growth. Source: Javier A. Ceja Navarro

Heavy metals are common soil pollutants. Back in 2012, it was already demonstrated that the increasing concentration of cadmium (Cd), zinc (Zn), lead (Pb), and mercury (Hg) significantly reduced the number of Acanthamoeba species, which are amoeba commonly recovered from soil (Note: amoeba are considered animal-like protists). Moreover, accumulated heavy metals damage the DNA of these organisms. This can lead to either extinction (since healthy genetic material cannot be passed down to the next generation) or mutations, which can have unpredictable consequences in the next generation. To investigate how well protists can tolerate heavy metals in their environment, many scientists use soil ciliates as model organisms. These organisms are important indicators of the soil condition because of their function. Not only do they release nitrogen that becomes available for plants, they also predate on bacteria present in soil. One of many experiments that focus on studying such interactions was conducted by Rico et al. He showed that the presence of heavy metals induced the production of reactive oxygen species (ROS), which negatively affected protists. This, in turn, disturbs normal functioning of cells, which can lead to unknown consequences for the stability of soil microbial populations.

But not all soil pollutants are necessarily metals or plastics. Organic pollutants, such as fertilizers, can also be harmful. Nowadays, when the demand for fresh fruits and vegetables is so high, it is hard to imagine a farm that isn’t using fertilizers to help plants to grow. However, there is some evidence that the soil protists community is more sensitive to this kind of treatment than even bacteria or fungi. A study from 2019 showed that nitrogen fertilizers indirectly change abiotic factors, like pH and salinity levels, of the soil environment, thus reducing protist diversity. Going forward, the main questions for concern are these: How severely do these changes affect the soil and the organisms in it? Will soil have the same microbial composition and properties without protists?

So how do protists adapt? One of the ways to co-exist with metal pollutants is to absorb and accumulate them. This is possible for protists because they have primitive zinc (Zn)- and copper (Cu)-binding proteins, like all eukaryotes do. This enables protists to take in and accumulate Zn or Cu until they are needed in biological reactions. This observation was made in the study by Hao et al. in 2015, and it is, indeed, a very unusual way of surviving. Keep your friends close and your enemies closer!

With everything that is already known about soil protists, more research on the relationship between the microbial world and pollutants is still needed. To better understand this connection, scientists intend to use soil protists as indicators of pollution. The logic here is simple: Since protists are so abundant in soil, any disturbances of the soil environment will affect protists. If industrial pollutants accumulate in soil and negatively impact soil health, the activity and “biological” duties of soil protists will also be interrupted. This, in turn, will give scientists enough information to estimate the risks associated with increased accumulation of pollutants. Let’s hope that the secret of how to fight pollution will soon be revealed!

Link to the original post: Chenyuan Wu, Yuanqing Chao, Longfei Shu, Rongliang Qiu, Interactions between soil protists and pollutants: An unsolved puzzle, Journal of Hazardous Materials,2022

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