A discovery in the depths of the sea sheds new light on the life forms that inhabit it

The deep, light-free zone of the ocean, located between 200 and 1,000 meters below the surface, is surprisingly poor in iron, to the point of limiting the growth of bacteria, which, however, they compensate by producing molecules that facilitate their absorption from the surrounding water:this is what emerges from new research led by scientists from the University of South Florida, subjected to peer review and published in the prestigious scientific journal Nature.The authors explained that the experiment could revolutionize our understanding of microbial processes in the deep oceans and, above all, provide new estimates and parameters regarding carbon uptake by the oceans, which strongly depends on bacterial activity and is essential for mitigating climate change.

The analyzed region, called “twilight zone”, is characterized by the fact that sunlight cannot penetrate, thus creating a dark and cold environment.The lack of sunlight therefore limits photosynthesis and, consequently, primary production (i.e. the production of organic matter by autotrophic organisms such as plants) is almost absent in this area.However, as explained by the authors of the study, it is one ecologically significant area, as it acts as a transit for organic matter that descends from the epipelagic (the surface area) towards the ocean depths.

To lead the research, scientists collected samples from the upper 1,000 meters of the water column during an expedition across the eastern Pacific Ocean, from Alaska to Tahiti.What they found in the samples "surprised" them:in the twilight zone the levels of iron – an essential nutrient for the growth of marine bacteria and many other organisms – are very low but, nevertheless, a high presence of siderophores was found, or the molecules that bind to iron and they make it more easily absorbable, both in surface waters and between 200 and 400 meters of depth, i.e. in an area where nutrient concentrations were thought to have a reduced impact on bacterial growth.

«Unlike surface waters, we did not expect to find siderophores in the ocean's twilight zone.Our study demonstrates that iron deficiency is high for bacteria living in this region across much of the eastern Pacific Ocean and that the bacteria use siderophores to increase iron uptake.That is has a knock-on effect on the biological carbon pump, because these bacteria are responsible for breaking down organic matter as it sinks through the twilight zone,” said Tim Conway, associate professor of chemical oceanography at the USF College of Marine Science and co-author of the research.Just as anticipated by the professor, in fact, bacteria play a fundamental role in the decomposition of organic matter that sinks towards the ocean depths.After breaking down the carbon, these they release it as a gas or store it in ocean sediments, thus facilitating the process described by Conway.

In conclusion, as explained by scientists, discovering the quantity of siderophores present in the deep ocean is essential for prediction how and to what extent the oceans can contribute to mitigating climate change.Thanks to the study of these molecules and the fact that therefore the ability of bacteria to recover iron in the twilight zone could be greater than expected, further studies could find that the oceans could actually collect more carbon than previously thought, and that this factor will certainly have to be taken into consideration in understanding oceanic processes linked to carbon storage.“For a complete picture of how nutrients shape marine biogeochemical cycles, future studies will need to take these findings into account.In other words, near-surface experiments need to expand to include the twilight zone,” he concluded Daniel Repeta, scientist at Woods Hole Oceanographic Institution and co-author of the article.

[by Roberto Demaio]