Bacteria in sediments have previously been seen primarily as catalysts, decomposing organic matter as it sinks to the bottom. But now scientists have discovered that these bacteria are also a storehouse for phosphorus; it is in the bacteria that much of the active organically-bound phosphorus in sediments is found.

Emil Rydin, Stockholm University Baltic Sea Centre. Foto: Lisa Bergqvist

“This is a new way of looking at bacteria, which could have a major impact on our understanding of the dynamics of the Baltic Sea and what we can do to improve the eutrophication situation," says Emil Rydin, researcher at Stockholm University Baltic Sea Centre and lead author of the study.

Analysis of DNA and lipids

Using a new analytical method, the researchers examined sediment samples taken in the Baltic Proper, which is heavily affected by eutrophication and oxygen depletion, and compared them with samples taken in the northern Baltic Sea, where the sea floor is oxygenated and there are plenty of bottom-living animals moving around in the sediments. By measuring how much of the phosphorus in the sediments that is bound to DNA and phospholipids, which are found in cell membranes, respectively, they get a picture of how viable the bacterial community is and how much phosphorus that is fixed in bacteria.

“In the northern Baltic Sea, where the sea floor is oxygenated and bioturbating animals bring organic matter down into the sediments, there seems to be an active bacterial community that binds a lot of phosphorus. In areas where there is a lack of oxygen, however, the bacteria seem to starve and are only found thriving in the very top layer of sediment,” says Emil Rydin.

In oxic conditions benthic animals bioturbate the sediments and a thriving bacterial community bind phosphorus in its biomass.

In anoxic conditions the benthic animals cannot survive, the bacteria starve and more phosphorus is released to the water.

Lack of bacteria leads to phosphorus leaching

The lack of bacteria may result in that a larger proportion of the phosphorus that is mineralised at the sea floor is released back to water, where it can contribute to more eutrophication.

“We already know that iron cannot bind phosphorus in oxygen depleted sediments, which makes more phosphorus released to the water. This seems to be a parallel to that; eutrophication leads to a lack of oxygen, which reduces phosphorus-binding bacteria and releases more phosphorus into the water. It's a vicious circle that's hard to break," says Emil Rydin.

Eutrophication can lead to a vicious cycle where sediments bind less phosphorus, returning more to the water and contributing to further eutrophication.

Bottom trawling can affect phosphorus release

Understanding that bacteria play such a central role in phosphorus turnover can improve the ability to assess when phosphorus will be released from the sediments into the water, and affect estimates of how eutrophication will develop in the Baltic Sea. It also makes it important to protect bioturbating animals, which are a prerequisite for an active bacterial community, from other impacts, such as bottom trawling.

“It is important to preserve the benthic animals that bioturbate the sediments. They are killed off by lack of oxygen, but they are also damaged by bottom trawling, which can thus have a negative impact on phosphorus binding in the sediments and by extension on eutrophication,' says Emil Rydin.

Text and illustration: Lisa Bergqvist

Read the full study:

Contrasting distribution and speciation of sedimentary organic phosphorus among different basins of the Baltic Sea