In the productive zones of the sea, organic material, the “marine snow”, is constantly sinking. In particular, the smallest of these particles are crucial for regulating the nitrogen balance in the oceans, as researchers have now discovered. According to this, this fine “sea snow” in particular contributes to transporting the nutrients bound in the particles to the degrading bacteria in the water and sediment.
Especially near the coast and at the bottom of the oceans, oxygen-poor “death zones” are increasingly forming – areas in which the oxygen content of the water is so low that marine animals can no longer survive. These minimum oxygen zones are a natural phenomenon, but they are intensified by the warming of the sea water and man-made overfertilization.
Due to the increased input of nutrients, an excess of nitrate and ammonium gets into the water and promotes the growth of plant plankton. If this then dies, it is decomposed by bacteria with the consumption of oxygen and oxygen becomes scarcer. As soon as there is no more oxygen available, nitrate and ammonium are converted into nitrogen by anaerobic processes, which then leaves the ocean as a gas. As a result, the minimum oxygen zones are responsible for up to 40 percent of the global nitrogen loss in the ocean.
What role does marine snow play?
Scientists working with Clarissa Karthäuser from the Max Planck Institute for Marine Microbiology have examined in more detail which factors regulate the degradation processes of nitrogen compounds in an oxygen minimum zone in the eastern South Pacific off Peru. Above all, they wanted to get to the bottom of the so-called anammox process, in which ammonium is converted into molecular nitrogen with nitrite under anaerobic conditions. The impetus was given by the observation that this anammox process is particularly intense where there is a lot of organic material in the form of so-called marine snow. These particles sink towards the deep sea, especially after algal blooms.
The researchers suspected that the nitrogen-rich “snowflakes” could serve as a source of ammonium for the anammox reaction. However, this was contradicted by the fact that no anammox bacteria were found directly on the small organic particles, which normally convert the ammonium to atmospheric nitrogen. This raised the question of how the bacteria living freely in the water column find this source of nutrients. With the help of underwater cameras, the researchers investigated this on particle particles of four different size classes at different sea depths.
Small flakes supply bacteria
The evaluations confirmed that the marine snow has a decisive influence on the breakdown of ammonium in the oxygen minimum zone. Obviously the smaller flakes are especially important. “We have observed that the anammox process mainly takes place where there are many small particles,” reports Karthäuser. “For the anammox process, small particles are more important than large ones – whereby small means that they are about the size of a hair’s breadth and are barely visible.” These small particles sink only slowly and therefore have longer residence times than larger flakes. At the same time, the organic material is more closely bonded to the smaller particles. As a result, they transport about as much material per particle as the larger lumps.
“We found that the ammonium concentration in the boundary layer, that is, around the particle, is significantly higher,” explains Karthauser ‘s colleague Soeren Ahmerkamp. Overall, the smaller particles released around 75 percent more remineralized nitrogen than larger particles, according to the researchers. “First of all, due to the high number and long residence times of small particles in the water column, it is very likely that bacteria will encounter them by chance,” concludes Ahmerkamp. “Second, the high ammonium concentration in the boundary layer quickly supplies a lot of bacteria.” In fact, a model simulation by the scientists showed that anammox bacteria were, on average, around twice as likely to encounter ammonium from smaller particles.
Expanded nitrogen cycle
“With the study, we have clarified an important aspect of the anammox process and thus made a significant contribution to a better understanding of the nutrient balance of the oceans”, summarizes Karthäuser’s colleague Marcel Kuypers. “With this data we can now expand biogeochemical Earth system models in order to better assess the effects of the human-influenced oxygen deficiency and the increased plankton growth on the nitrogen cycle.”
Source: Max Planck Institute for Marine Microbiology, Article: Nature Communications, doi: 10.1038 / s41467-021-23340-4