How climate impacts microorganisms and why we should care

By: Dana Sackett

As Earth Day approaches, it seems timely to pause and examine some of the natural processes that enable life on Earth.  Remarkably, this big topic requires us to take a look at some of the smallest organisms on the planet.  Microorganisms (or organisms that cannot be seen with the naked eye such as bacteria, fungi, viruses, and phytoplankton) are integral to many of the Earth’s natural processes.   Understanding these microorganisms, how a changing climate may impact them, the processes they support, and ultimately life on Earth continues to be an area of science ripe for further study. 

Microorganisms are critical players in many of Earth’s nutrient cycles (the process that recycles elements such as carbon and nitrogen through our ecosystems). They are also involved in the production and consumption of greenhouse gases, like carbon dioxide, methane, and nitrous oxide.  For instance, in aquatic ecosystems, microscopic organisms such as phytoplankton or algae can remove carbon dioxide from the atmosphere during photosynthesis producing oxygen as a byproduct. Indeed, not only do these tiny critters often make-up the base of numerous food webs on Earth, but the continuous and countless rotations of nutrient cycles assisted by microorganisms through Earth’s history created the conditions upon which our ecosystems evolved.  Microorganisms may be very, very small, but taken as a whole, they have a very significant impact.

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A simplified depiction of the carbon cycle. Source: https://www.sciencelearn.org.nz/resources/1569-carbon-cycle

The nitrogen cycle is another example of the essential role that microorganisms play to all life on Earth.  This cycle involves nitrogen from the atmosphere, ground, and oceans being used to create the building blocks of all life (proteins and DNA) and requires bacteria, a microorganism, to work.  Bacteria are crucial to this cycle because they perform a process called nitrogen fixation, converting atmospheric nitrogen into a form that is usable by plants.  Plants then absorb the usable nitrogen and convert it into essential nitrogen-containing molecules such as amino acids and DNA.  Animals then acquire these essential nitrogen molecules by consuming plants or plant-consuming animals.

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A simplified depiction of the marine nitrogen cycle. Source: http://www.waterman.hku.hk/education/slide.aspx?code=6HXLG

In marine systems, the fixation of nitrogen by cyanobacteria (previously called blue-green algae) is a dominant source of usable nitrogen to all life in the ocean.  Studies have shown that microbial communities and their roles in our ecosystems are sensitive to climate change, but little is known about exactly how this sensitivity will impact ecosystems.  In a recent study, a group of cyanobacteria called Trichodesmium were exposed to increased carbon dioxide levels, mirroring what is expected over the next century.  This study exposed these bacteria to higher carbon dioxide for over 4 and a half years and found that the algae in the higher carbon dioxide environment evolved to fix nitrogen at rates 43% higher than the algae kept at present-day carbon dioxide levels. This evolution was irreversible.  After returning the bacteria to present-day levels, their rate of nitrogen fixation remained high even two years later.

Nitrogen in DNA

Nitrogen is the backbone of the bases that make-up the code in DNA and RNA. Source: http://logyofbio.blogspot.com/2015/09/nitrogens-purpose.html

While more usable nitrogen for the marine food web may sound like a good result, it may not work out that way.  The reason: nitrogen is not the only essential nutrient needed for these microorganisms to thrive; they also need elements such as phosphorus and iron to fix nitrogen.  Thus, with a higher rate of nitrogen fixation, these other elements are likely to run out more quickly, potentially resulting in less usable nitrogen being produced over time rather than more.  However, without knowing how rising carbon dioxide levels will impact these other elements cycles or other groups of cyanobacteria, it will be hard to say how all this will play out.

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Nitrogen fixation rates (a) and cell-specific growth rates (b) for long-term 750 ppm CO2 conditions (yellow), long-term 380 ppm CO2 conditions (red), and a switch from long-term 750 ppm to 380 ppm CO2 conditions. Source: Hutchins et al. 2015.

Beyond changes to the roles microbials play within nutrient cycles, a changing climate could also completely threaten the existence of some types of microorganisms.  One article recently pointed out that there are currently no published studies on how climate change may affect microbial extinctions, but anticipates the loss of many specialized microorganisms.  Given that microorganisms have such a major impact on the way our Earth cycles the nutrients and compounds needed for life, moving forward it will be vital to understand how our changing climate may impact microorganisms and the essential roles they play to life on Earth.

 

References and other reading material

Hutchins DA, Walworth NG, Webb EA, Saito MA, Moran D, McIlvin MR, Gale J, Fu F. 2015. Irreversibly increased nitrogen fixation in Trichodesmium experimentally adapted to elevated carbon dioxide. Nature Communications 6:8155. DOI: 10.1038/ncomms9155

Reese AT. 2016. How climate change endangers microbes—and why that’s not a good thing. Scientific American. https://blogs.scientificamerican.com/guest-blog/how-climate-change-endangers-microbes-and-why-that-s-not-a-good-thing/

Singh BK, Bardgett RD, Smith P, Reay DS. Microorganisms and climate change: terrestrial feedbacks and mitigation options. Microbiology 8:779-790.

https://www.sciencedaily.com/releases/2008/03/080311131851.htm

http://news.nationalgeographic.com/2015/09/15911-metals-extinction-ocean-oxygen-ordovician-silurian/

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