Salty sediments? What bacteria have to say about chloride pollution

Lexi taking water quality measurements at Cedar Creek in Cedarburg, WI.

The activities are as follows:

In snowy climates, salt is applied to roads to help keep them safe during the winter. Over time, salt – in the form of chloride – accumulates in snowbanks. Once temperatures begin to warm in the spring, the snow melts and carries chloride to freshwater lakes, streams, and rivers. This runoff can quickly increase the salt concentration in a body of water. 

In large amounts, salt in the water is harmful to aquatic organisms like fish, frogs, and invertebrates. However, there are some species that thrive with lots of salt. Salt-loving bacteria, also known as halophiles, grow in extreme salty environments, like the ocean. Unlike other bacteria and organisms that cannot tolerate high salinity, halophiles use the salt in the environment for their day-to-day cellular activities. 

Lexi is a freshwater scientist who is interested in learning more about how ecosystems respond to this seasonal surge of chloride in road salts. She thought that there may be enough chloride from the road salt after snowmelt to change the bacteria community living in the sediment. More salt would support halophiles and likely harm the species that cannot tolerate a lot of salt. 

By taking a water sample and measuring the chloride concentration, we can see a snapshot in time of how toxic the levels are to organisms. However, the types of bacteria in sediments take a while to change. Halophiles may be able to tell us a long-term story of how aquatic organisms respond to chloride pollution. Lexi’s main goal is to use the presence of halophiles as a measure of how much chloride has impacted the health and water quality of river or stream ecosystems. This biological tool could also help cities identify areas that may be getting salted beyond what is necessary to keep roads safe.

Lexi expected that there would be few, or maybe no, halophiles in rural areas where there are not many roads. She also thought halophiles would be widespread in urban environments where there are many roads. Because salt impacts the streams year after year, she expected that halophiles would become permanent members of the microbial community and increase in winter and spring. Therefore, she also wanted to track whether halophiles remain in the sediment throughout the year, increasing in numbers when salt levels become high. 

She began to sample sediments from two different rivers in Southeastern Wisconsin. The urban Kinnickinnic River site is in Milwaukee, WI, and the Menomonee River site is in a rural environment outside of the city. She selected these sites because they offer a good comparison. Because there are more roads, and thus saltier snowmelt, the Kinnickinnic site in the city should have higher chloride concentrations than the Menomonee site. 

When visiting her sites throughout the year, Lexi collected multiple water and sediment samples. Every time she visited, she also recorded important water quality characteristics such as pH, conductivity, and temperature of the water. She then brought the samples to the laboratory and analyzed each for its chloride concentration. To measure the quantity of halophiles in the sediment, Lexi used a process where the sediment is shaken in water to separate the bacteria from the sediment and suspend them in the water. Samples from the water were then plated on a growth medium that contained a very high salt concentration. Because the growth medium was so salty, Lexi knew that if bacteria colonies grew on the plate, they would most likely be halophiles because most bacteria do not thrive in salty environments. Lexi counted the number of bacteria colonies that grew on the plates for each sample she had collected.

Featured scientist: Lexi Passante from the University of Wisconsin-Milwaukee

Flesch–Kincaid Reading Grade Level = 12.0

Some videos about Lexi and her research:

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Fertilizer and fire change microbes in prairie soil

Christine collecting samples from the experimental plots to measure root growth.
Christine collecting samples from the experimental plots to measure root growth.

The activities are as follows:

Stepping out into a prairie feels like looking at a sea of grass, with the horizon evoking a sense of eternity. Grasses and other prairie plants provide important benefits, such as creating habitat for many unique plants, mammals, insects, and microbes. They also help keep our water clean by using nutrients from the soil to grow. When plants take up these nutrients, they prevent them from going into streams. High levels of plant growth also keeps carbon bound up in the bodies of plants instead of in the atmosphere.  

Prairies grow where three environmental conditions come together – a variable climate, frequent fires, and large herbivores roaming the landscape. However, prairies are experiencing many changes. For example, people now work to prevent fires, which allows forest species to establish and eventually take over the prairie. In addition, a lot of land previously covered in prairie is now being used for agriculture. When land is used for agriculture, farmers add nutrients through fertilizer. With all these changes, prairie ecosystems have been declining globally. Scientists are concerned that as they disappear so will the benefits they provide. 

Lydia and Christine are two scientists contributing to the effort to learn more about how to preserve prairies. They both became interested in studying soil because of their appreciation for prairies at a young age. For Lydia, she lived in an area that was covered by trees and farmland, but knew at one time it used to be prairie. This made her want to learn more about prairie environments and how places like where she grew up have changed through history. For Christine, she grew up surrounded by prairies where she developed a passion and curiosity for the natural world. Especially for the organisms living in the soil that you cannot see, called microbes. 

These are two different experimental plots within the large field experiment at Konza Prairie Biological Station. The one with lots of trees is an unburned plot, the one with lots of grass is a burned plot.
These are two different experimental plots within the large field experiment at Konza Prairie Biological Station. The one with lots of trees is an unburned plot, the one with lots of grass is a burned plot.

Lydia and Christine read about how grassland scientists have been doing research to learn more about what happens when fire is stopped and excess nutrients are added. These changes reduce biodiversity and affect which species of plants can grow in the prairie. However, Lydia and Christine noticed that the research had been mostly focused on what happens aboveground.  Lydia and Christine had a hunch that the aboveground communities were not the only things changing. They thought that belowground components would be changed by fire and fertilizer too. They turned their focus to microbes in the soil, because they also use nutrients. In addition, they thought these microorganism would be affected by the changes in aboveground plant biodiversity. 

To see if this was true, they used data that they and other scientists collected at Konza Prairie Biological Station from a large field experiment. The experiment was set up in 1986 and the treatments were applied at the field site every year until 2017! Lydia and Christine focused on the fertilizer (nitrogen) addition and prescribed burning treatments to answer their questions. The nitrogen treatment had eight plots where nitrogen had been added and eight with no nitrogen as a control. Similarly, the prescribed burn treatment was applied to eight plots, while eight plots had no burning as a control. These two treatments were also crossed with each other, meaning that some plots were burned and nitrogen was added.

Lydia and Christine expected the types of microbes in the soil to change in response to the nitrogen and burning treatments because of the different aboveground plant communities and difference in soil nutrients. Soil microbial communities can change in multiple ways. First, the number of unique species can increase or decrease, measured as richness. The other way is how many individuals of each species there are in the community, measured as evenness. Taken together, richness and evenness give a measure of diversity, which can be summarized using the Shannon-Wiener index. The value will get bigger if either richness or evenness increases because it incorporates both. For example, a community with five species that has equal abundance of each will have a larger Shannon-Wiener index than a community with five species where one species has a lot more individuals than the other four.  

Featured Scientists: Lydia Zeglin and Christine Carson from the Konza Prairie Biological Station. Written By: Jaide Allenbrand

Flesch–Kincaid Reading Grade Level = 10.4

Alien life on Mars – caught in crystals?

Magnesium sulfate crystals trapping liquid water.

The activities are as follows:

Is there life on other planets besides Earth? This question is not just for science fiction. Scientists are actively exploring the possibility of life beyond Earth. The field of astrobiology seeks to understand how life in the universe began and evolved, and whether life exists elsewhere. Our own solar system contains a variety of planets and moons. In recent years scientists have also discovered thousands of planets around stars other than our Sun. So far, none of these places are exactly like Earth. Many planets have environments that would be very difficult for life as we know it to survive. However, there are life forms that exist in extreme environments that we can learn from. On Earth there are extremely hot or acidic environments like volcanic hot springs. Organisms also live in extremely cold places like Antarctic glacier ice. Environments with extremely high pressure, like hydrothermal vents on the ocean floor, also support life. If life can inhabit these extreme environments here on Earth, might extreme life forms exist elsewhere in the universe as well?

A view of the astrobiology lab.

Charles is an astrobiologist from Great Britain who is interested in finding life on other planets. The list of places that we might look for life grows longer every day. Charles thinks that a good place to start is right next door, on our neighboring planet, Mars. We know that Mars currently is cold, dry, and has a very thin atmosphere. Charles is curious to know whether there might still be places on Mars where life could exist, despite its extreme conditions.While there is no liquid water on the surface of Mars anymore, Mars once had a saltwater ocean covering much of its surface. The conditions on Mars used to be much more like Earth. Liquid water is essential for life as we know it. If there are places on Mars that still hold water, these could be great places to look for evidence of life. Charles thought that perhaps salt crystals, formed when these Martian oceans were evaporating, could trap pockets of liquid water.

Charles and his fellow researcher Nikki knew that there are a number of kinds of salts found in Martian soils, including chlorides, sulfates, perchlorates and others. They wanted to test their idea that water could get trapped when saltwater with these salts evaporate. They decided to compare the rate of evaporation for solutions with magnesium sulfate (MgSO4) with another commonsalt solution: sodium chloride, or table salt (NaCl). They chose to investigate these two salts because they are less toxic to life as we know it than many of the other chloride, perchlorate, or sulfate salts. Also, from reading the work of other scientists, Charles knows the Martian surface is particularly rich in magnesium sulfate.

Charles and Nikki measured precise quantities of saturated solutions of magnesium sulfate and sodium chloride and placed them into small containers. Plain water was used as a control. There were three replicate containers for each treatment – nine containers in total. They left the containers open to evaporate and recorded their mass daily. They kept collecting data until the mass stopped changing. At this point all of the liquid had evaporated or a salt crust had formed that was impermeable to evaporation. They then compared the final mass of the control containers to the other solutions. They also checked the resulting crusts for the presence or absence of permanent water-containing pockets. Charles and Nikki used these data to determine if either saltmakes crystals that can trap water in pockets when it evaporates.

Featured scientists: Charles Cockell, UK Centre for Astrobiology, University of Edinburgh, & Nikki Chambers, Astrobiology Teacher, West High School, Torrance, CA

Flesch–Kincaid Reading Grade Level = 8.7

Additional teacher resource related to this Data Nugget: