Mississippi Dead Zone
By Kelly Poulos
The Gulf of Mexico dead zone is an area of oxygen-depleted waters at the outflow of the Mississippi River. In the case of the Gulf of Mexico, the dead zone is defined as being the portion of the water column with a dissolved oxygen concentration less than 2 milligrams per liter.
The record size of this hypoxic area is 8,776 square miles and it is caused largely by excess nutrient pollution into the Mississippi River.
The excess of nutrients, largely nitrogen and phosphorus from runoff fertilizers and treated sewage discharge from urban areas, encourages the growth of algae and development of algal blooms. When these algae die, they become a food source for bacteria, which consume dissolved oxygen as they decompose the algae, resulting in areas of the ocean with oxygen levels that are insufficient to support aquatic life, especially when there is strong stratification that prevents reoxygenation of the water column. As a result of this hypoxia, the microbial community in the water column shifts dramatically. This hypoxic zone can cover anywhere from 20%-50% of the Gulf of Mexico water column during the summer months and can persist through September.
Detailed Environmental Description
The dead zone in the northern part of the Gulf of Mexico was first recorded in the early 1970s. While the dead zone used to occur every couple of years, it is now a seasonal occurrence in the Gulf of Mexico. It is well researched as the dead zone is tied greatly to the fishing industry in the United States, however the microbial composition and metabolic potential of the microbes present in the dead zone is not well studied. Specifically, very little is known about the actual microbial ecology and physiology within the Gulf of Mexico dead zone. 
A connected issue to dead zones is that of agriculture and farming practices. While the Gulf of Mexico dead zone has been present since the 1970’s, it has continued to grow in size each year. Until nutrient cycling and the transformation of these nutrients is addressed, the Gulf of Mexico dead zone will continue to be a seasonal norm.
When the water column becomes anoxic, the microbial community present shifts greatly. As available oxygen decreases, anaerobic metabolisms, such as nitrate and sulfate reduction and anaerobic ammonia oxidation become more prevalent. In the absence of oxygen, there is still a substantial microbial community present in the water column along with increased nitrate and nitrate. The makeup of the water column shifts as a result of the formation of the dead zone.
Mississippi Dead Zone Microbial Ecology
(NEED TO DO THIS PART STILL)
Harmful Algal Blooms
Harmful algal blooms (HAB) are common in the Gulf of Mexico and are the result of the overgrowth of toxic phytoplankton. One such example of a HAB is red tide where the overgrowth of microorganisms leads to the red discoloration of the surface waters. A well researched microorganism that is a common culprit of the HAB is the dinoflagellate, Karenia brevi.
Typically, cyanobacteria, microflagellates, and dinoflagellates are the organisms responsible for the formation of red tides and they kill marine species by paralyzing their respiratory systems. Algae species associated with these blooms produce potent toxins, which are liberated when eaten by other organisms. Other algae are capable of killing other organisms without toxins. For instance, the serrated spines of certain nontoxic algae, like that of certain Chaetoceros diatom species, can lodge in fish gills and cause death.
Other algae species found in HAB can form cysts that remain in sediment until environmental conditions are conducive to the occurrence of a bloom. These cysts are toxic to filter feeders like oysters.  The toxins created by these different microbes can bioaccumulate and impact humans.
Dead Zone Restoration
While fully restoring dead zones requires reducing nutrient delivery, the Gulf of Mexico dead zone typically lasts until late August or September when it is broken up by hurricanes or tropical storms. For the dead zone to be fully addressed, nutrient loads into the Mississippi River need to be limited. This, however, requires the collaboration of dozens of states and thousands of non-point source polluters. For dead zones to be eliminated, nutrient loads must be shrunk. Such plans have been undertaken in the Chesapeake Bay and in the Black Sea, which suffer from seasonal dead zones as well.
Key Microbial Players
Alphaproteobacteria of the SAR11 clade of bacteria are abundant in oxygen minimum zones. They are aerobic heterotrophs that consume the dissolved organic carbon found in the ocean into carbon dioxide. A previous study found that SAR11 bacteria belonging to the family Pelagibacteraceae make up about twenty percent of all 16S rRNA genes and protein-coding metagenome sequences detected in an oxygen minimum zone.
In the hypoxic zone, ammonia oxidizing archaea (AOA) are more common than ammonia oxidizing bacteria (AOB) because they can outcompete bacteria in low energy conditions. The AOA have a higher affinity for ammonia and oxygen. Studies have found that there is often active ammonia oxidation and growth of AOA strains under low dissolved oxygen conditions. One such ammonia oxidizing archaea that studies have found increase in abundance in low DO samples are Thaumarchaeota. These nitrifying microbes dominate the hypoxic zone of the Gulf of Mexico and contribute to N2O production in oxygen limiting environments. 
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