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LETTER: Microbes break down phosphates and nitrates

During the long winters, many of us long for the beauty of the ponds and lakes in the city to reveal themselves. And it happens, year after year, only to disappear again under a floating layer of algae and pond scum. The culprit? Organic matter, animal waste and fertilizer runoff from all of our green lawns, trails, et cetera.

Thanks to Mathias Baden for writing the interesting article on the floating islands in the Woodbury Bulletin of June 15, 2016. I hope this spurs creation of more of these structures that provide both an environmental and ecological function, as well as keep our city's water bodies beautiful.

However, as a chemist, I am compelled to comment on a statement made in the article: "The plant roots and poly material grow microbes to break down pollutants, like phosphorus and nitrogen ..." This statement is not quite correct. Phosphorus and nitrogen are both elements, and cannot be broken down any further. They are present as compounds in the fertilizers that people use — for example, as phosphates and nitrates. The microbes and algae use these as food to make proteins and other biological molecules such as DNA, et cetera, that are an integral part of the cell.

Therefore, the phosphates and nitrates are broken down, not the phosphorus and nitrogen.

To quote from the website of Midwest Floating Island: "BioHaven floating islands help manage Total Phosphorous (TP) and reduce total Nitrogen (TN), total suspended solids (TSS), dissolved organic carbon (DOC), copper and zinc in the water." Notice they don't say anything about breaking down of phosphorus and nitrogen — these elements are present as compounds — phosphates and nitrates. In fact, you cannot find phosphorus and nitrogen in the elemental form in the water in appreciable amounts.

Phosphorus is extremely reactive and forms an oxide on exposure to air, so is always in compound form in water.

Nitrogen is a gas that makes up 78 percent of the air we breathe — in water you can find only 15 milligrams of dissolved nitrogen in 1 liter of water — so most nitrogen in water comes from nitrates and nitrites.

Enjoy the summer, fertilize wisely, and pick up all the poop after your pet does the job!

Kannan Seshadri


Editor's note: Kannan Seshadri is a 3M research specialist.