Posted by: Adam Kay | December 9, 2012

Report on a recent UST Biology publication: “Nitrate increases in Lake Superior are due to humans, and an usual group of microbes” by Chip Small

UST Biology’s Chip Small and colleagues recently published a study in Limnology & Oceanography, a leading ecosystem science journal. Below is a general audience description. The full article is available here.

Chip Small

Chip Small on Lake Superior

Gazing out across Lake Superior from a rocky beach on Minnesota’s north shore, it’s easy to imagine that this massive lake has been unaffected by humans.  The crystal-clear waters of this lake, which holds 10% of Earth’s liquid freshwater, hide the fact that, over the past century, nitrate levels have increased more than five-fold.  Scientists have assumed that this increase is simply due to external inputs such as nitrogen deposition from the atmosphere.  However, a new study published in the journal Limnology & Oceanography, my colleagues and I show that a group of microbes that was only recently discovered is playing an essential role in maintaining an imbalanced nitrogen cycle.

The process of nitrification is an essential link in the nitrogen cycle, in which a waste product (ammonia) from fish and zooplankton is converted into nitrate by a specialized group of microorganisms.  In order to understand how this process works in Lake Superior, our research team conducted experiments in a series of research cruises from 2009-2011. We added isotopically-labeled nitrogen to bottles of lake water from 10 different depths spanning the 150 m deep lake, and attached these bottles to cages on a long floating cable.  A day later, we retrieved the bottles, and measured how much of the labeled nitrogen had been transformed from ammonia to nitrate.

The actual rates we measured in and of themselves were not particularly exciting–similar to values measured in coastal ocean waters, and much lower than most other freshwater lakes.  But when we put our data together with the other pieces of this nitrogen puzzle, we found that nitrate is being produced by nitrification at a rate more than 50 times greater than external nitrogen inputs, and also at a rate much greater than it is being removed by other biological processes.

Until a few years ago, scientists believed that only a handful of bacterial species were responsible for transforming ammonia into nitrate.  Recently, a group of archaea (first described from samples collected at a wastewater treatment plant!) have been shown to be the dominant group of nitrifiers in the ocean, and our colleagues at Bowling Green State University showed that this same group of archaea are also responsible for nitrification in Lake Superior (the nitrifying bacteria that you would find in a textbook are not present).  We found that these nitrifying archae were absent from the shallow depths of the lake during the summer months, which temporarily shuts down the nitrogen cycle, until the water mixes again in the fall.  We know next-to-nothing about these organisms–what eats them, how effectively they compete with algae for limiting nutrients, how well they operate at different light and temperature levels.  We didn’t even know they existed until a few years ago, and yet it turns out they are playing a central role in this long-term nitrate buildup.  Humans are also playing an important role, too, and it’s likely that changes in forest composition and nitrogen deposition over the past century put this excess nitrogen in the lake in the first place.

So, the next time you have an opportunity to gaze across Lake Superior, consider how our actions, and the lifestyle of a group of tiny microbes, have combined to dramatically change the water chemistry in Earth’s largest lake.


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