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Phytoplankton Breakdown Causing Aquatic Hypoxia

Hypoxic zone on coast of Louisiana

Hypoxia is a term used to describe environments that lack oxygen, and in aquatic environments specifically this term applies to dissolved oxygen content. Bodies of water with a 0% dissolved oxygen content are considered anoxic or anaerobic environment, and those with 1 to 30% dissolved oxygen content are considered hypoxic or dysoxic. Most organisms, especially fish, cannot live below 30% dissolved oxygen saturation. Hypoxia is a product of anthropogenic causes like pollution of nutrients like phosphates and nitrates from fertilizers into waterways.

Scientists have determined that high concentrations of minerals dumped into bodies of water causes significant growth of phytoplankton blooms. As these blooms are broken down by bacteria, such as Phanerochaete chrysosprium, oxygen is depleted by the enzymes of these organisms.

Breakdown of Lignin

Tetrapyrrol ring, the active site of Ligninperoxidase enzyme

Phytoplankton are mostly made up of lignin and cellulose, which are broken down by enzymes present in organisms such as P. chrysosprium, known as white-rot. However, the breakdown of cellulose does not deplete oxygen concentration in water, whereas breakdown of lignin does. This breakdown of lignin includes an oxidative mechanism, and requires the presence of dissolved oxygen to take place by enzymes like ligninperoxidase. Other fungi such as brown-rot, soft-rot, and blue stain fungi also are necessary in lignin transformation. As this oxidation takes place, CO₂ is formed in its place.

            Ligninperoxidase (LiP) serves as the most import enzyme because it is best at breaking down lignin in these organisms. LiP disrupts C-C bonds and C-O bonds within Lignin’s three-dimensional structure, causing it to break down. LiP consists of ten alpha helices, two Ca²⁺ structural ions, as well as a heme group called a tetrapyrrol ring. Oxygen serves an important role in the catalytic cycle of LiP to form a double bond on the Fe²⁺ ion in the tetrapyrrol ring. Without the presence of diatomic oxygen in the water, this breakdown cannot take place because Ferrin-LiP will not be reduced into Oxyferroheme. Oxygen gas is used to reduce Ferrin-LiP into Oxyferroheme-LiP. Oxyferroheme and veratric alcohol combine to create oxygen radical and Ferri-LiP, which can now be used to degrade lignin. Oxygen radicals cannot be used in the environment, and are harmful in high presence in the environment .

Active site of tetrapyrrol ring binding oxygen

Once Ferri-LiP is present in the ligninperoxidase, it can be used to break down lignin molecules by removing one phenylpropane group at a time through either the LRET mechanism or the mediator mechanism. The LRET mechanism (long range electron transfer mechanism) transfers an electron from the tetrapyrrol ring onto a molecule of phenylpropane in a lignin. This electron moves onto a C-C or C-O bond to break one phenylpropane molecule from the lignin, breaking it down by removing one phenylpropane at a time.

Oxyferroheme is converted to Ferri-LiP with the addition of veratric alcohol, and gives off diatomic oxygen radical.

This is the breakdown of a confieryl alcohol by a hydrogen ion to make propanol and ortho-methoxyphenol.    

In the mediator mechanism, LiP enzyme is activated by the addition of hydrogen peroxide to make LiP radical, and a mediator such as veratric alcohol is added and activated creating veratric alcohol radical. Veratric alcohol radical transfers one electron to activate the phenylpropane on lignin, and the electron dismantles a C-C or C-O bond to release one phenylpropane from the lignin. It is to be noted that as the size of a lignin molecule increases, the more difficult it is to break these C-C or C-O bonds. Three types of phenyl propane rings include coniferyl alcohol, sinapyl alcohol, and-coumaryl alcohol .

Environmental Components that Drive Hypoxia

LiP has a very low MolDock score, meaning there is little energy required to form this enzyme and stabilize it to carry out reactions. LiP has a MolDock score of -156.03 kcal/mol. This is energetically favorable due to its negative free energy requirements, and therefore this reaction catalyzed by LiP is likely to take place spontaneously . Breakdown of propanol and phenols occur naturally in the environment because they are both water soluble.

Rate of phytoplankton breakdown

The breakdown of phytoplankton in the environment depends on the presence of oxygen, and once oxygen is no longer in the bodies of water, ligninperoxidases cannot continue to break down the lignin. When oxygen is not present in the water, the breakdown of phytoplankton changes from 10.7 days to a total of 160 days for this to take place. In this equation, G(t) is the amount of particulate organic carbon (POC) overall at a given time, t. G(0) is the concentration of POC before breakdown takes place. k is a rate constant in year-1, and t is time in years. For most POC of phytoplankton, the k is around 12.8 years-1, or about 28 days for nearly 96% of carbon to be broken down in these systems. Whereas for anoxic systems, POC breakdown takes 125 days, over four times longer. It takes approximately 1 mg of Oxygen to break down 1 mg of POC in the environment, and therefore, hypoxia takes place quickly as oxygen is used up quickly to digest POC. About 9% of POC in phytoplankton can be broken down in a single day at 18°C, therefore it takes about eleven days to completely break down a full phytoplankton .

After POC is broken down, this particulate matter can be turned into other dissolved organic carbon, such as carbon dioxide, bicarbonate ions, and carbonate. As much as 30% of phytoplankton can be broken down into dissolved organic carbon. When this particulate organic carbon interacts with 350 nm ultraviolet light, dissolved organic carbon is formed, removing even more oxygen from the environment in the forms of carbon dioxide, bicarbonate ions, and carbonate. Dissolved inorganic carbon is made at a rate of 2.3-6.5 mg/(m^3)day .

As phytoplankton breakdown, free phosphorus and nitrogen become available in the environment, which also fosters hypoxic conditions. As the breakdown of these phytoplankton takes place, the more phosphorus turns into phosphates, and nitrogens turn into nitrates. This depletes the oxygen even more so in the environment, further creating hypoxic zones in higher quantities. As more minerals such as phosphorus and nitrogen are displaced into these aquatic systems, the growth of phytoplankton greatly increases, and after their death, hypoxic zones are formed.