Arctic seafloor bacteria are active all year round, researchers say

Despite the pronounced seasonality of their habitat, the bacterial community of Arctic sediments is very stable taxonomically and functionally.

Researchers in Bremen, Germany, studied the composition and function of bacteria in the seabed off Svalbard during alternating periods of polar night and midnight sun. To do this, they specially developed a sample collection device, the Ellrott grab. Unlike bacterial communities in overlying waters, sediment bacteria hardly change with the seasons. This is probably due to the fact that on the seabed, difficult-to-digest foods are available all year round.

The Arctic is cold and hostile to life, but it is nevertheless home to a large number of microorganisms whose activity has a significant impact on life on our planet. For example, bacteria present in the seabed play a central role in processing the biomass of dead organisms, thus transforming the contained carbon into substances that are difficult to degrade and can remain stored for a long time.

Besides the cold, unusual seasonality is a striking feature of polar habitats: day and night do not alternate every twelve hours, but the whole year oscillates between the midnight sun and polar night. This has a considerable impact on local primary production, which depends on sunlight. In summer, tiny algae thrive in seawater, as does land life. In winter, primary production largely stops. Little research has been done on the extent to which the resulting large fluctuations in organic matter input influence seafloor bacteria.

To fill this knowledge gap, a team of researchers from the Max Planck Institute for Marine Microbiology in Bremen, Germany, visited the Svalbard archipelago at different times of the year to study the local bacterial community in the sediment. They now present their findings in The ISME magazine.

Enzymes change more than bacteria

Surprisingly, the seafloor bacterial community does not behave as expected given environmental conditions. “Although the supply of organic matter and its renewal rate fluctuate considerably during the year, the composition of the bacterial community hardly changes at first glance,” reports lead researcher Katrin Knittel.

Seabed bacteria thus behave very differently from those in water, where many of them exhibit pronounced seasonality. “Benthic bacterial communities, that is to say those of the seabed, are very complex,” adds Knittel. “That’s what makes them so stable and robust, and it makes it very difficult for us to study their dynamics.”

To better understand this unexpected behavior, Knittel and his team at the Max Planck Institute in Bremen studied the functional diversity of the bacteria. How does the activity of seafloor bacteria change between the midnight sun and the polar night?

To find out, they analyzed which genes bacteria have to break down the sugars in algae and to what extent they use them. “Although the composition of the bacterial community hardly differs between seasons, we found that the gene expression of carbohydrate-degrading enzymes changes between winter and spring,” explains first author Sebastian Miksch , who participated in the project as part of his doctoral thesis.

In winter, enzymes that break down a-glucans (e.g. glycogen) predominate. A-glucans are intracellular storage compounds of heterotrophic bacteria, animals and fungi. They are also available the rest of the year, but are less important then. In spring, however, there are more enzymes that break down b-glucans, such as the algal component laminarin. Then there are so many b-glucans that some of them can be put aside as storage for later in the year.

“These enzymes reflect the algae components, particularly the algae sugars, available to bacteria during different seasons,” says Knittel. “It’s not that different from going to the farmers’ market here: although there are lots of different fresh fruits and vegetables available during the sunny season, at some point during the winter, only the stored potatoes.”

Seabed bacteria can thus use fresh materials that flow from the water column, especially in spring and summer, such as the aforementioned laminarin. However, they can also consume materials already present in the seabed or produced there. This includes tasty treats like mucin, but also hard bits like chitin. On these, bacteria nibble on all year round. This food source is particularly important in winter, when other inputs are scarce. Their long-term availability Their long-term availability stabilizes the bacterial community of the seabed.

“These findings occur on a very small scale, but they are important in a broader context: when bacteria consume the sugars in algae, they release carbon dioxide. And carbon dioxide is a very important greenhouse gas “, notes Knittel. Thus, the tiny inhabitants of the oceans can have an influence on global processes.

Small, light, practical: the Ellrott grapple

Overall, the bacterial community on the Arctic seafloor is therefore surprisingly uniform across the seasons. Despite the strong seasonality, the community is present and active in both seasons. However, it wasn’t just internal dynamics that made it difficult for Knittel’s team to study the bacteria found in Spitsbergen’s seabed. It is also a methodological challenge.

“It is very difficult to obtain intact samples of the seabed and the pore water contained between the sand grains,” explains doctoral student Chyrène Moncada, who is also working on the project. “That’s why we developed our own device: the Ellrott bucket.” This sampling device, presented in an article in the journal Limnology and oceanography: methods and named after its developer and co-author Andreas Ellrott, allows samples of sandy sediments to be taken without disturbing them.

The grab is so small and light that it is perfect for use on small research vessels. “Andreas is a brilliant engineer and designed and built the dumpster from scratch, manufacturing many of the components in a 3D printer himself,” says Moncada. “To date, we have already collected more than 100 sediment samples from the Wadden Sea and Svalbard fjords with the Ellrott grab, and we plan to collect many more.”