Oxygenated water can give you more energy

Oceanography: The ocean is running out of air

Yet they are anything but inanimate. Some organisms have adapted to the extremely low oxygen content and seek refuge there from predators. As Euphausia mucronata, the most common krill species in the Humboldt upwelling area off South America. The little crabs spend the day in the depths and only hike up at night under cover of darkness to eat algae and breathe. Various fish and copepods can also survive in the "death zone". To do this, however, they have to shut down their metabolism - which makes them sluggish and easy prey. Deep diving marine mammals such as elephant seals therefore prefer to hunt here. Likewise, the vampire squid, which occurs at a depth of 600 to 900 meters. It can even cope with an oxygen saturation of just three percent - thanks to large gills and the blue blood pigment hemocyanin, which binds oxygen much more efficiently than our hemoglobin at low concentrations. In the long term, all of these animals still need oxygen to survive.

This does not apply to numerous microorganisms: They become active in the first place without the gas or, when they run out, simply switch to anaerobic breathing. Some meet their energy and carbon needs by oxidizing organic compounds, such as sugars or amino acids. In doing so, they reduce nitrate (NO3) to nitrite (NO2), Nitrous oxide (N.2O) and finally to molecular nitrogen (N2). Others use iron, manganese or sulfate ions (SO42−) instead of oxygen. And some unicellular inhabitants of the oxygen minimum zone, like algae, are able to use photosynthesis to remove CO dissolved in the water2 to fix. Instead of sunlight, they use chemically bound energy, which is contained in reduced inorganic compounds such as ammonium (NH4+) and hydrogen sulfide (H2S) is stuck. When they are oxidized, energy is released that allows the organisms to convert sugar and other molecules from CO that are required for growth2 to synthesize.

The microbial processes in the oxygen-poor regions of the oceans play a central role in the global cycle of some biologically relevant elements. This is particularly true of the nitrogen cycle: nitrogen is an important nutrient and makes up 78 percent of the earth's atmosphere. In the air he is mainly as N2Molecule in which the two atoms are linked by a strong triple bond. In this form it is worthless to most living things. Only a few single-cell organisms can break the bond and convert atmospheric nitrogen into ammonia in order to synthesize amino acids, for example. Humans imitate this natural process in the Haber-Bosch process and thus produce ammonium nitrate and urea as fertilizers for agriculture.

The "death zones" regulate the productivity of large parts of the seas in the long term

Animals and plants take up the biologically available nitrogen. When they die, microorganisms recycle their biomass and release the nitrogen again, mainly in the form of ammonium and nitrate. Two groups of bacteria that researchers have found in large numbers in minimum oxygen zones convert the compounds back into atmospheric nitrogen. On the one hand, there are so-called denitrifiers, which oxidize organic carbon with the help of nitrate, and, on the other hand, anammox bacteria only discovered in the mid-1990s that breathe ammonium with nitrite to generate energy for the CO2-Gain fixation. The high availability of nutrients and the shortage of oxygen in the upwelling areas create optimal conditions for these processes: Scientists estimate that 20 to 40 percent of the loss of usable nitrogen in the ocean is at the expense of the oxygen-free zones, although they make up less than one percent of the total volume . In this way, the "death zones" regulate the productivity of large parts of the seas in the long term. Because the water comes to the surface again at some point, and the amount of biologically available nitrogen often limits the growth of algae.

The upwelling areas are also important in the global carbon cycle because large amounts of CO2 fixed during photosynthesis and transported into the deep sea as marine snow. On average, however, barely one percent of the carbon reaches the sea floor. Microorganisms put the rest back as CO2 free as it crosses hundreds or even thousands of meters of water. In the absence of oxygen, however, this happens more slowly, so that more organically bound carbon reaches the ground in upwelling areas and is withdrawn from the cycle than in other marine regions.

Humans make large amounts of nitrogen biologically available and overfertilize the oceans

The emissions of greenhouse gases - especially CO2, but also methane and nitrous oxide - raise the temperature of the atmosphere and thus that of the ocean. Because the solubility of oxygen in seawater is falling at the same time, oceanographers assume that the oxygen-poor zones will expand in the coming decades. In fact, their volume has quadrupled worldwide since the middle of the 20th century, as long-term studies show. The reduced solubility explains less than half of the observed decrease in oxygen. The majority can be traced back to a weaker ocean circulation - another consequence of ocean warming: As the surface water heats up and loses its density, the stratification of the water column becomes more stable. So it takes more energy to transport oxygen-rich water down into the depths. The ventilation of the ocean is therefore decreasing in the course of climate change.