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A microbial system with unexpectedly low complexity?

Algae blooms regularly make for pretty, swirly satellite photos of lakes and oceans. They also make the news occasionally for poisoning fish, people and other animals. What's less frequently discussed outside narrow scientific circles is the outsize role they play in global carbon cycling. Everyone in our field's favourite stat is that 50% of the oxygen in every breath originated in the sea as a product of algal photosynthesis. And a significant proportion of that marine photosynthesis occurs during bloom events.

Regardless of what precipitates the bloom (be it agricultural runoff, seasonal changes, upwelling from the deep ocean), the exponential growth of algae leads to a massive transfer of CO2 into algal biomass. But these are of course fundamentally transient phenomena. Algae die either of starvation, having consumed all of the available nutrients, or they are consumed by grazers eating them from the outside and viruses eating them from the inside. And when they die a large fraction of their biomass is consumed by bacteria.

Over the course of the last decade, we've been studying North Sea spring blooms at the island of Helgoland - Germany’s only true offshore island, famous for its seabirds, seals and duty-free shopping rather than for microscopic algae. But what we were interested in was the fate of the organic matter once the algae die, and by what processes it is remineralised and ultimately returned to the atmosphere.

Near the Helgoland Harbour, photo credit: Hanno Teeling

A substantial fraction of algal biomass is composed of different types of polysaccharides, and hence we wanted to look at specifically the this part of it. We went for a targeted metagenomic analysis of the Bacteroidetes phylum of bacteria, since these are most associated with polysaccharide consumption (they do this in your gut too!). Bacteroidetes genomes frequently carry characteristic operon or regulon-like gene clusters called polysaccharide utilisation loci (PULs), that comprise a SusC-like TonB-dependent transporter, a SusD-like accessory binding protein, and then variable numbers of degradative carbohydrate-active enzymes or CAZymes (typically glycoside hydrolases and polysaccharide lyases). PULs have been found to be specific to a particular polysaccharide substrate.

In our study we reconstructed over a thousand metagenome assembled genomes (MAGs) belonging to more than a hundred Bacteroidetes species, and found that, contrary to what we might have anticipated, the diversity of PULs was in fact limited. We found only five major polysaccharide classes were being regularly targeted by multiple species, namely beta-glucans (such as laminarin - the main diatom storage compound), alpha-glucans (such as starch and glycogen - also algal and bacterial storage compounds), mannans and xylans (typically algal cell wall components), and alginates (mostly known as slimy stuff produced by brown macroalgae). PULs targeting other substrates weren't typically associated with multiple bacterial clades, implying that the most widely available and therefore most important polysaccharide substrates released by dying algae are made up of a fairly small set of basic components.
Krüger et al. (2019)

Given what we know of algal and bacterial species diversity, and the enormous potential complexity of glycans and polysaccharides, it came as no small surprise to see such a limited spectrum of PULs, and in only a relatively small number annually recurrent bacterial clades.

Ben Francis
Doctoral Candidate, Max Planck Institute for Marine Microbiology and University Greifswald

„Grün ist das Land, rot ist die Kant, weiß ist der Sand. Das sind die Farben von Helgoland.“

Living and working on Germany’s only open sea island

The cliffs with view on the lighthouse and the southern harbour

This year's spring bloom campaign on Helgoland was carried out from February 14th to May 19th. During this time, sea water samples of the “Kabeltonne”, specific coordinates between the dune and the main island, were taken daily (OMICS) or on Tuesdays and Thursdays (Glycobio). The water samples were pulled onto different pore-sized filters before shipping them to Bremen or Greifswald. However, how exactly can one imagine life and work on the rough North Sea island?

First of all, you need to know that everything on Helgoland is about the sea and the weather. Is it stormy? What is the sea state? What it the visibility? The island depends on its shipping like a fish on water. Science is no exception.

View on BAH (Biological Institute of Helgoland), taken from the research vessel Aade

A regular day for the OMICS and Glycobio sampling starts at approximately 8:30 am. There is no need to start earlier because of the before mentioned weather dependency. AWI’s (Alfred-Wegener Institute) small science vessel, Aade, won’t arrive at a pinpointed schedule. During good weather she can be fast to collect all samples and return to the harbor by 9:00 am. But let the weather be bad, it can take until 10:00 am or later. So after we’ve prepared everything for the day, e.g. labeling all sampling tubes, filtration set-up, have have to practice patience until the ship appears on the horizon with the wanted cargo. Now, physical labor is demanded. The water cans (on Tuesdays and Thursdays approximately 120 L) are brought back to the lab with nothing but carts and muscles. A short “Moin” to the sailors followed by “Danke” and “Bis Morgen”.

Unloading the water canisters off the Aade

Back in the laboratory, todays work can finally start. Seeing that the flow-rate is quite low, there might be an algal bloom. It’s going to be a long day. A visual check of the 3 μm filter is also a good indicator to determine whether there are many algae in the water or not. The greener the filter is, the more material there is on it. After the OMICS filters are prepared and shock frosted, we continue with the filters for subsequent cell sorting. Downstairs in the “Glycobio” laboratory, 100 L of raw seawater have to pass the different filters. This also, is going to take a while. Meanwhile, it is noon and it’s time to take a break. Since the sun is shining, we enjoy our free time at the water. It’s fairly easy to find a nice lunch because of all the local snack bars. Now all that is left to do is find a nice spot to spend the lunch break. Soon work calls again. The break passes quickly here at the ocean’s edge.


Filtration set-up “OMICS”		Filtration set-up “FISH”

The “Glycobio” Laboratory		Filtration set-up “Glycobio”

Next up for the OMICS procedure are the FISH filters, which are prepared with a formaldehyde-fixed seawater sample. But the day isn’t over yet. After all the daily samples are taken and processed, the staining of a filter section by means of DAPI and the subsequent counting of bacterial cells, still follows. As we leave the darkroom where the microscopes are located, the sun already sits deep on the horizon. Down in the basement laboratory the filtration is still ongoing, but no worries there are always the nice colleagues to talk to.

The colleagues in the basement laboratory always fancy a talk

Today’s work is done and we quickly prepare everything for the next sampling day. The now empty water canisters have to go back to the ship for a new water sample and all material needs to be rinsed. This is necessary, so we receive a clean sample from tomorrow's water and prevent algae growth in the bottles. Finally, after this we’re done - it’s time to call it a day.

Shopping district around the Siemensplaza, Underland

To be able to buy groceries at the local Edeka, the only supermarket chain on Helgoland, you have to be quick now. At 6:00 pm the shops are closed. Only restaurants and bars are still open and only during the season. It’s much nicer to spend the evening on the cliffs of the “Oberland” even if the nesting birds are quite noisy. The Lange Anna (“Tall Anna”), Helgoland’s famous sandstone stack, is dipped in the orange light of the setting sun and the wind is picking up. And so the day on Helgoland ends.

View of the Lange Anna in the evening sun

Text: Lilly Franzmeyer and Doreen Schultz

Photography: Lilly Franzmeyer