By: Selina Våge, UiB
Seawater seen under the microscope with 1000 times magnification – bright blobs are bacteria and the tiny dots are viruses. There are roughly one billion bacteria and ten times more viruses in 1 liter of seawater. Epifluorescence image: Gunnar Bratbak
In the central Arctic Ocean, where we are right now, microbes definitely steal the show of life – only few other organisms thrive in this harsh environment. Studying the microbes living here is thus an important part of our mission. In the course of our cruise we have found them free-floating in the water column or attached to sea ice, living either inside brine channels or growing as mats underneath the ice. As we moved northward, larger waterborne microbes became less abundant, but the sea ice community is still hanging in there!
Some microbes (most of which have cells that look similar to our own) can convert carbon gas into biomass, producing oxygen as a by-product, just like trees do on land (they are called photosynthetic), whereas other microbes need to eat (parts of) other organisms to get organic carbon building blocks for their bodies. We have everything to thank for the photosynthetic microorganisms in the ocean!
Phytoplankton (top row) are beautiful photosynthetic microorganisms that produce oxygen and organic carbon from CO2 and water. They are strong competitors of bacteria for limiting nutrients. So-called heterotrophic flagellates and ciliates (bottom row) are ferocious microbial predators of bacteria. Photos taken on board by Wencke Eikrem (NIVA) using a light microscope.
They were the ones turning Earth’s atmosphere into an oxygen rich medium that we can breathe some 2.8 to 2.45 billion years ago! Marine microorganisms still produce roughly half of the oxygen we all breathe today.
Most bacteria in the Arctic Ocean cannot produce oxygen. Their sheer abundance is mind-blowing however; imagine you were brave enough to take a bath in the Arctic ocean (we are currently measuring freezing temperature of -1.8 °Celsius in the surface water) – then a single droplet of sea water running down your cheek would harbor a million bacteria! Even more impressive, there are about 10 million viruses in that drop of water! Fortunately, most marine viruses mean no harm to us – it’s the bacteria that have to worry!
Bacterial and viral abundances are mind-blowing – in each of these small tubes there are about two million bacteria and 20 million viruses. We study their activity by giving them radio-actively labeled food (aminoacids) that we can trace when the bacteria have taken it up. Photo: Selina Våge
When bacteria get infected by viruses they explode and turn into food for other microorganisms. In an environment where vertical mixing of the water column is limited due to strong density gradients, just like here in the central Arctic Ocean, this “recycling” of organisms near the surface is critical for life to persist since supply of new nutrients from deeper waters is very limited. Bacteria also disintegrate other dead microbes, slowing down their sinking and thus further helping to retain essential nutrients near the surface.
Bacteria have a tough life – not only are they under constant viral attack, they are also hunted by ferocious and extremely efficient predatory microbes. Besides, they have to share the little nutrients we find in the Arctic Ocean with their beautiful “cousins”, the phytoplankton (photosynthetic microorganisms). Under optimal growth conditions, these phytoplankton can monopolize limiting nutrients, creating a short-cut through the microbial food web with reduced recycling that bacteria and viruses stand for, resulting in an efficient transfer of energy from the bottom of the food web to animals like shrimp and fish. Events like that are the reason why Norway could rise as a nation from fisheries. Although regularly occurring in sub-polar regions like the Barents Sea or Norwegian Sea, such optimal growth events are rarely found in the central Arctic Ocean. Understanding how this might change in the future is one of our research goals.
Scientists on board studying abundance, activity and elemental composition of microorganisms from seawater samples. If you want to become an observational marine microbiologist, get ready for some filtration fun. Photos: Bodil Bluhm & Selina Våge
As you see, microbes show many different faces, linking ocean chemistry to harvestable resources, and we have not even started to talk about the fascinating evolutionary games they play! Rapid co-evolution between bacteria and viruses can tell us a whole lot about the development of infectious diseases like the Covid-19 for instance… I would be happy to talk more about this on our next cruise.