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Latest Expedition Journal

October 25:  Day 23

Posted by Kristen Mitchell

The Potential for Plastic to Disrupt our Essential Element Intake

Plastic is crafted to be durable and long lasting, which means it persists in the ocean for long periods of time. We have seen that island ecosystems have emerged from the floating plastic pieces.  This new niche has the potential to change the natural ocean system including chemical cycling of elements and, specifically in my area of expertise, the element selenium. 

Selenium is named for the Greek selene, meaning moon.  Selenium is an essential micronutrient for nearly every form of life.  Selenium occurs in biological material mainly as selenocysteine, the 21st amino acid.  This amino acid is a main building block of over 20 selenoproteins which generally serve antioxidant functions. 

Selenium concentrations in the surface oceans are very low.  However, it exists in the surface ocean in several different chemical forms, which is an unusual chemical property.  This complexity makes it difficult to fully understand how it is cycled in the ocean.  Despite much interest and effort,  our understanding of the marine selenium cycle is far from complete. I fondly think of selenium as the teenager of the periodic table, who doesn't want to reveal its secrets to us quite yet.                    
    
Phytoplankton, along with other primary ocean producers, are key players in the selenium cycle.  These tiny organisms concentrate the minute amounts of selenium present in seawater through their normal metabolism.  As part of the food chain, the phytoplankton are eaten by zooplankton (marine invertebrates) and fish, and eventually, humans.  When phytoplankton die, they sink through the water column carrying organic selenium, which is buried in sediments.  Over millions of years and geological shifts, these sediments appear on land in the form of organic rich shale rocks.  These rocks provide relatively high concentrations of selenium in soils, allowing plants to absorb selenium and introduce it into the terrestrial food web. 

If the selenium cycle is changed by growth of selenium accumulators on plastic that are no longer eaten by zooplankton and fish, this could decrease the amount of selenium in higher food products, which could cause selenium deficiency in our food supply.  On geological timescales, if the plastic is less apt to sink it will keep selenium out of the marine sediments, and possibly decrease land sources of selenium.

Declining selenium could create potential problems for other nutrient elements like phosphorus and nitrogen, which will bioaccumulate on plastic.  Changes to the elemental cycling in the oceans, specifically nutrient elements, have the potential to alter multiple marine ecosystems.  The problems posed by marine plastic debris are complex and the scientific community has just begun to scratch the surface in their understanding.