From the equator to the arctic, life forms have adapted to their particular climate and regional conditions. In steamy sub-tropical estuaries, mangrove forests dominate the landscape. They bridge the salt- and fresh-water worlds. In northern Canada and Russia, the evergreen trees of the taiga forest endure incredibly cold winters and long periods of almost complete darkness. These differences are visible to us living on the earth’s surface.
But what about the tiny life within the soil? Can the millions of microbes in a single teaspoonful of soil be as specialized as the trees they live beneath?
When we focus on the communities within the soil, we find that there are indeed big differences among microbes. These differences depend on whether the communities come from different latitudes or even different parts of a single state!
The case of rhizobia bacteria
One group of soil bacteria found to be regionally adaptable are rhizobia. Rhizobia are very important in natural and agricultural systems. This is because they form symbiotic relationships with specific plants called legumes, helping these plants to get the nutrients they need to survive. And what do the rhizobia get in return? A free home! Legumes house the microbes within their roots, forming lumpy structures known as nodules.
There’s more to this exchange than just housing, though. In the nodules, rhizobia take nitrogen from the surrounding air, and through their natural metabolism create compounds that the plants can use. This is called nitrogen fixation. In exchange for these nitrogen compounds, the plant “feeds” the rhizobia carbohydrates it makes during photosynthesis. Nitrogen, as you may know from other Soils Matter blogs, is a limiting and vital nutrient in most soil ecosystems, especially agricultural systems. Rhizobia bacteria help legume plants capture nitrogen they could otherwise never use! This symbiotic relationship can enrich natural landscapes and can also help farmers improve fertility in their fields without using fertilizer.
So now you know why we care about rhizobia, but what about the part where they adapt?
Rhizobia across latitudes
A genetically similar group of rhizobia is called a strain. Within species, strains are like the different “breeds” of rhizobia. Studies of rhizobia strains from arctic regions have shown that these bacteria can continue to grow even at 0°C, which is freezing! At this temperature, rhizobia strains from temperate or tropical regions are dormant, do not grow, and possibly might not survive. Research showed that hardy arctic rhizobia strains were better able to form nodules and improve growth of legume plants at cold temperatures1. These studies showed this happened both in the lab and in the field when compared to strains from a warmer region.
In other studies of rhizobia, scientists found that strains from within the same country differed in their adaptation to climate. Strains from the northernmost part of Finland had more nitrogen-fixing enzyme activity than strains from southern Finland when grown at cold temperatures2.
In these studies and others, the reverse was true as well. At warmer temperatures, the rhizobia from more temperate regions grew faster and formed stronger symbiotic relationships with plants than the arctic or northern-latitude rhizobia. These bacteria have adapted to survive best under the conditions of their native regions, just like plants, animals, and other life forms!
In our lab in Minnesota, we study the rhizobia that live on the roots of legumes like clover, field peas, and hairy vetch. As we continue with our research, we anticipate that there will be differences in rhizobia strains even within our own state! Rhizobia from Grand Rapids, MN, in the northern region of the state, may have different characteristics or be differentially adapted to certain conditions than those from Lamberton, MN, in the southwest region. We expect to see even bigger differences when these strains are compared to those from other parts of the country with different climate conditions, like North Carolina or New York.
The scale of rhizobia diversity is not only across continents or countries, however. Rhizobia communities can be vastly different even between fields on the same farm.
In our research, we frequently sample soils from farms. We then use that soil (with its microbial community) to “inoculate” legume plants we grow in the lab. Depending on what rhizobia are present in the soil from that field, legumes may grow really well or pretty poorly.
In image 4 are four red clover plants that were part of one of these rhizobia experiments. These clover are hosting rhizobia communities from two different fields on the same farm. You can see there is a big difference in growth among them! Possibly one of the fields never had clover growing in it before, so the clover rhizobia were not very abundant there. Or possibly the conditions of the fields were very different; for example, maybe one was well-drained and one was wet. These local climate variations can lead to differences in local microbes as well.
So, can soil microbes adapt to different climates and different regions?
The answer, at least for rhizobia bacteria, is yes! Microbes from certain climates and regions appear to be better adapted to the stresses of that climate. This allows them to survive and thrive where others would not.
Rhizobia (and other soil microbes) are also locally diverse, with their “demographics” shifting even within the bounds of a single farm.
Soil microbes and their diversity are still mostly a mystery even to soil scientists and microbiologists! Stay tuned for further research in this area in the years to come, as we are bound to discover much more about the microbial communities of soil and their regional adaptations.
Answered by Charlotte Thurston, University of Minnesota
To learn about soil microbes in Antarctica, click here.
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- Lipsanen, P. & Lindstrom, K., 1986. Adaptation of red clover rhizobia to low temperatures. Plant and Soil, 92, pp.55–62.
- Prévost, D. et al., 2003. Cold-adapted rhizobia for nitrogen fixation in temperate regions. Canadian Journal of Botany, 81(12), pp.1153–1161.