Objective: Researchers on the BioLINCS cruise will use a variety of drifting instruments to study how marine microbes take up and transform nitrogen compounds in the open ocean.
During the BioLINCS cruise, the research vessel Kilo Moana will spend 14 days near a patch of open ocean about 200 miles north of Oahu. Conditions in this area are similar to those at Station ALOHA, a mid-ocean research site that for almost 25 years has provided researchers with a wealth of background information about the chemistry, biology, and currents of the open Pacific.
Researchers on the Kilo Moana will conduct a number of experiments to study marine bacteria and archaea. (Archaea are single celled organisms that look similar to bacteria, but which are in an entirely separate biological domain.) The BioLINCS researchers are particularly interested in how these microbes take up nitrogen and convert it into different forms (nitrogen cycling).
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These experiments involve deploying a variety of research equipment in the ocean and allowing this equipment to drift with the currents for days at a time. Some of these drifting (“Lagrangian”) instruments are incubators, which allow researchers to run experiments on microbes in the environment from which the microbes were collected (in situ).
One of the largest of these drifting instruments is called the Environmental Sample Processor (ESP). The ESP will allow researchers to use the DNA of marine microbes to figure out what organisms are present. It will also be used to determine the abundances of genes necessary for taking up dissolved nitrogen gas from seawater—a process known as “nitrogen fixation.”
While the Kilo Moana follows these arrays of drifting instruments, researchers on the ship will collect water samples at various depths and acquire physical, chemical, and biological data throughout the water column. They will also conduct incubation experiments on board the ship using the collected seawater. The water-column data, shipboard measurements, and incubation experiments will allow researchers on the ship to understand the biological-chemical links (or “biogeochemical processes”) occurring in the water column. The water-column data will also become part of the long-term scientific record for Station ALOHA.
Conditions around Station ALOHA are typical of the mid-Pacific, with extremely clear water and low populations of microscopic photosynthetic organisms (primary producers), which form the basis for marine food webs. Primary producers are relatively sparse in the open ocean because the surface water contains very low concentrations of the chemicals (nutrients) that they need to grow. Oceanographers use the term “oligotrophic” to describe such low-nutrient waters, thus the acronym for Station ALOHA: ”A Long-term Oligotrophic Habitat Assessment.‘
One of the most important nutrients for primary producers is nitrogen, which can take several different chemical forms (nitrate, nitrite, ammonium, etc.). Different types of marine microbes use different forms of nitrogen as “fertilizer.“
In the open ocean, the “waste” from one group of microbes typically serves as an energy source or as a nutrient for another group of microbes. This biologically-controlled process of converting compounds from one form to another is called “biogeochemical cycling.” During the BioLINCS cruise, researchers will focus on learning about the biogeochemical cycling of nitrogen compounds.
Nitrogen gas is the only form of nitrogen that is available in high concentrations near the sea surface. However, only a few organisms exist that can take up nitrogen gas. These organisms “fix” nitrogen, converting nitrogen gas into energy-rich, “reduced” forms of nitrogen, such as ammonium, which can be utilized by other organisms. Thus nitrogen-fixing organisms can be thought of as providing fertilizer for other organisms. In fact, nitrogen fixation by microbes fuels most of the primary production in the surface waters of the open ocean.
In addition to being used by primary producers, nitrogen compounds are also nutrients for other marine microbes that do not necessarily rely on sunlight and photosynthesis for survival. These microbes get their energy not from light, but rather by absorbing reduced chemicals directly from seawater. In doing so, they convert these compounds from one chemical form to another. This is analogous to animals eating food (which contains reduced carbon) and converting it to carbon dioxide (an oxidized form of carbon), which is then released to the atmosphere
Although population densities of primary producers are relatively low in open-ocean areas, these areas cover much of the Earth’s surface. As the dominant organisms in this immense environment, marine microbes are critically important in maintaining the climate of the Earth. They also supply approximately a third of the oxygen in the our atmosphere.
In addition to providing oxygen, marine microbes have other important effects the atmosphere. Some of them release nitrous oxide (N2O), which is a greenhouse gas. Others release compounds such as dimethyl sulfide (DMS), which influence the formation of clouds.
Because of all these interactions between the open ocean and the atmosphere, studying the nitrogen cycle of the open ocean is more than an academic exercise. The results from the BioLINCS experiments could help improve computer models that predict how life in the oceans will respond to increasing carbon dioxide in the atmosphere, global warming, and ocean acidification.
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