Snow of the deep ocean

By Benedikt Heyerhoff, February 2022

The oceans are the heart of our planet, they cover more than 70 percent of its surface and represent the largest habitat on earth. Due to their size, they are the largest producers of oxygen, 50 – 70 percent of the oxygen comes from there. It is produced by phytoplankton.

Benedikt Heyerhoff

Due to their size, the oceans offer phytoplankton a huge habitat in which they occur in almost unimaginable numbers. Phytoplankton: these are floating plants, algae and bacteria that can carry out photosynthesis. Photosynthesis is what made the earth what it is today. While the first plants emerged 470 million years ago, cyanobacteria, often mistakenly called blue-green algae, were already producing oxygen 3.5 billion years ago. The genera Synechococcus and Prochlorococcus are the most important genera of cyanobacteria. They occur most frequently in the sunlit zones of the oceans. Bacteria of the genus Prochlorococcus play an important role because they are the smallest and most abundant photosynthetic organisms on this planet and produce up to 20 percent of the oxygen arising in the oceans.

Prokaryotes are single-celled microorganisms without a cell nucleus to which the domains bacteria and archaea are assigned. They occur in unfathomable numbers in every conceivable place on earth. In total, there are 4–6 × 1030 prokaryotes on earth, more than stars in the universe. An average of one million prokaryotes live in one millilitre of coastal seawater, and up to one billion per cubic centimetre of sediment in active ocean sediments. Depending on the habitat, a single grain of sand can be populated with up to hundreds of thousands of prokaryotes. Due to their number and their presence in all habitats on earth, prokaryotes are significantly involved in material cycles. Prokaryotes remineralise organic matter back into inorganic components such as carbon, nitrogen, phosphorus or oxygen. A surprisingly important role in material cycles is played by an often overlooked habitat – the sediments of the deep ocean.

In the deep ocean, biomass, i.e. the combined total mass of individuals per species, is relatively low compared to surface regions, but species diversity is unsurpassed. The deep ocean is a nutrient-poor habitat whose largest source of nutrients is “marine snow”. When organisms from the upper water layers die, they are broken down into small particles and sink. They are then called marine snow. Other particles such as sand, animal excrement and inorganic particles can also be present in marine snow. On its way down into the deep ocean, which can take several weeks, marine snow provides a habitat for prokaryotes which decompose the particles and dissolve them into their basic mineral building blocks. Only about three percent of the original material reaches the bottom of the deep ocean which is the most important source of nutrients for the prokaryotes living there in the sediment. Since not all particles are completely remineralised, the marine snow forms a layer which, due to the high pressure and over millions of years, turns into carbonate structures like the chalk cliffs on Rügen. The flow of organic material into the deep ocean forms an important carbon sink that binds significant amounts of carbon in the depths of the ocean sediments.

The sediments at depths of thousands of metres are a little explored, huge habitat of very high microbial diversity. Although deep-sea microorganisms are smaller than those living near the surface due to low nutrient availability, they are of global importance due to their high numbers. They form the interface between the biologically active surface world and the large geological reservoirs of biologically important substances. Thus microbial activity in the seafloor plays a fundamental role in the biogeochemical cycles of the Earth through a chain of reactions.

Microbial sulphate reduction in marine sediments produces pyrite which is a major source of ocean alkalinity. It is of particular importance because it influences the distribution of CO2 between the atmosphere and the ocean by determining in which form carbon occurs. The oceans are the world’s largest sink for CO2 and store it in the form of bicarbonate (HCO3-). When the alkalinity decreases, i.e. the oceans become more acidic due to more and more CO2 being absorbed, bound bicarbonate is converted into CO2 which outgases back into the atmosphere. This is only one example of many, but it shows that prokaryotes with partly unremarkable metabolic pathways form the basis of global material cycles that keep the earth as we know it in balance.

About the author: Benedikt Heyerhoff, doctoral student, benthic microbiology, Institute for Chemistry and Biology of the Marine Environment, University of Oldenburg, former Waldorf pupil.

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