Nitrogen fixation in the dark ocean – a short tale of a journey through the water column

As a person from Denmark, born and raised on the coast of the Baltic Sea I cannot help but wonder how, out here on the great big blue, organisms manage to thrive. It seems like such a remote location with only scarce amounts of energy and nutrients being transported from the productive coastal regions. But nevertheless life has found a way to take advantage of the possibilities of this vast area of the earth’s exterior. Planktonic organisms that follow the paths laid out by the ocean currents and the wind are the foundation for all living creatures that wish to call this beautiful blue ocean their home.

The very fact that the ocean here at 20 degrees North; 26 degrees West has this brilliant blue colour is telling us that there is not much going on at the surface. But a journey down through the water column speaks a tale of evolutions amazing ability to produce lifeforms that can occupy areas that at a first glance appear to be a very unlikely place to find organisms that live, grow and reproduce, continuing the life cycle as they have done for millions, and for some, even billions of years. In these clear waters light can penetrate into the ocean to a place that has been hidden from view for most of humanities history. It is only within the last century or two that we have begun to understand the factors that govern this place in which many organism reside. It is a zone in the water column where nutrient rich water masses mix with the upper surface water. And everywhere in the ocean there is sunlight and nutrients photosynthetic algae can exist. They use the energy of the sunlight to turn carbon dioxide and water into sugar and oxygen. Such regions of high productivity can be found in all waters of the world, but out here this layer, which is of immense importance as input of energy and thereby a foundation for many of the larger life forms found here, can be located as deep as 100 – 150 meters.

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SnowCatcher provided by researchers from the MARUM institute at Bremen University used for collecting marine particles

But what happens below the point of last remaining light? No photosynthesis can contribute to the energy needed for life to exist. It is a dark and mysterious world that humans have only just begun to explore. But within the darkness, underwater cameras spot little flickers of light reflected by small particles. These are collectively referred to as Marine Snow, as they drift downward through the water in an endless stream resembling an eternal snowfall. They are produced by the aggregation of material from algal blooms and other small substances found in the marine environment such as fecal pellets from small animals and other kinds of debris drifting in sea. Most of the Marine snow not larger than 0,5 cm, and many smaller than the width of a human hair. Many people probably wouldn’t think more of these snow particles than a beautiful natural phenomenon. But down in the deep dark ocean they are a continuous supply of energy for the many bacteria that call this place their home. Marine Snow can be rich in carbon, but often has low amounts of the other elements that are needed for lifeforms to grow and reproduce. But compared to the nutrient depleted surrounding ocean they represent a habitat where microorganisms can form communities. Some of them release the nutrients bound in the Marine Snow and form what is known as a biofilm. In this thin layer of secreted substances many different bacteria can find a place to grow.

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Organisms and micro-structures of aggregates

An interesting theory has been proposed, that within this bacterial community we might find specialised organisms that, instead of using the Nitrogen bound in the Marine Snow, can take up the atmospheric nitrogen that is dissolved in the water and turn it into a form that can be used by organisms to grow and thrive. Such a finding would change our understanding of how Nitrogen can be supplied to the deepest layers of the dark ocean, and thereby further expand our knowledge of how the organisms that live in the deep oceans can get energy and nutrients in order to live, reproduce and flourish in this mysterious alien world.

Søren Hallstrøm

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