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When it comes to soil microbes, everything is NOT everywhere, but does this matter?

Written by Charlotte Alster, 2015-2016 Sustainability Leadership Fellow and PhD Candidate in the Department of biology

In the plant and animal realms of conservation biology, substantial biodiversity losses generally result in negative consequences for processes within an ecosystem, which can sometimes impact ecosystem services, or the benefits humans receive from an ecosystem (important things like food, fresh water, and air!). Only in the last decade has this concept really been examined for the plethora of microorganisms (microbes) residing in soil. In 1934, Dutch microbiologist Lourens Baas Becking coined the notion on microbial distribution, “everything is everywhere, but the environment selects.” What he meant is that all microbes should be all over the world because they can be transported so easily by wind, water, animals, etc., but that they are not necessarily found everywhere because of geographic and physiological constraints, which is known as environmental selection. Environmental selection is still a valid cornerstone of evolutionary biology, but the first part of the statement, which assumes microbes can be transported anywhere with no restrictions, is controversial. Despite this provocative statement, the Baas Becking hypothesis has endured as a reigning dogma in microbial ecology until about a decade ago. There is now increasing evidence that just like animals and plants, distributions of microbes reflect both historical and contemporary environmental conditions. Thus variation in microbial communities certainly exists, but do these differences necessarily mirror changes in microbial processes in ecosystems?

Understanding how variation in microbial community composition impacts ecosystem processes has potentially significant implications, especially in light of global climate and environmental change. Soil microbes perform scores of vital ecosystem processes such as making nutrients available for plants, for example, by converting organic nitrogen to inorganic nitrogen. Losing microbial diversity with ecosystem processes that influence plant productivity could have serious consequences for human nutrition and wellbeing. An often less considered point is that since there are so many microbes that live in the soil (the common number I hear quoted is that there are more microbes in a teaspoon of soil than there are humans on earth), they respire immense amounts of carbon dioxide (CO2). Small variations in microbial community composition and how they respond to environmental change may have enormous impacts for carbon budgets and subsequently global warming.

As microbial ecologists examine the variability of microbial communities in space and time, a follow-up question remains: how will this variability impact us as humans? Because there are so many different types of microbes residing in soils, coupled with the fact that most microbes can remain in a dormant state for up to thousands of years (yes, this really happens!) and can transfer genes to unrelated microbes (think of artificial genetic engineering except not artificial!), it is speculated that microbial communities exhibit a large degree of functional redundancy. Functional redundancy is an ecological concept whereby many different organisms, in our case microbes, perform the same function or process. As such, differences in microbial community composition may not necessarily result in changes in microbial community function. Under similar environmental conditions microbial function could remain the same. Consequently, the critical problem emerges in teasing out the relative influence that differences in microbial diversity has on changes in ecosystem processes.

Tackling this problem is actually pretty difficult to do, although new studies have recently come out providing evidence for both scenarios. There are two main hurdles: 1) it is difficult to isolate the effect of the microbial community from the soil environment, and 2) while methods are improving, there are still some major gaps that need to be breached in terms of connecting individual microbes to actual ecosystem processes. My own research is trying to get at these problems by isolating microbial communities from different soil types to see if it is the microbial community or the environment in which they live that determines rates of CO2 production and if that changes with increases in temperature. So at the very least, there is evidence finding that different microbial communities produce different amounts of CO2 regardless of soil type as well as many known fungal species that are fundamental to certain types of plant growth. While we are learning more about this dynamic, yet hugely important area of research, in my opinion it pays to be conservative in managing species diversity belowground, mainly preventing degradation of our valuable soils, while we’re still figuring it out.

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