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New study highlights the role of fungal organic matter in the marine carbon cycle

How can we know what bacteria eat? In contrast to animals, they have no trunks, specialized teeth or beaks that provide clues whether the preferred food consists of leaves, meat, small insects or grain. However, bacteria have specific enzymes, which can tell us a lot about their favorite dish. Analyzing these enzymes enables to uncover the otherwise hidden interconnections in the complex bacterial food web. Now scientists from the Max Planck Institute for Marine Microbiology in Bremen reveal that bacteria sometimes spice their meal with fungal sugars.

Marine bacteria play a decisive role in the decomposition of biomass from higher marine organisms, such as animals, algae and fungi. In case of marine fungi, not much is known about the underlying mechanisms. Some marine fungi are parasitic and can infect marine microalgae, in particular during algal blooms. When researchers at the Max Planck Institute for Marine Microbiology in Bremen, Germany, studied a North Sea bacterium known to thrive on algae during blooms, they were surprised to find an enzyme targeting a specific polysaccharide (a kind of sugar) known to occur only in fungi. The researchers could show how this novel enzyme digests a fungal polysaccharide. Similar polysaccharides occur in the yeasts that humans use for baking bread or brewing beer, and the mechanism is surprisingly similar albeit not identical to a mechanism employed by human gut bacteria. The scientists have reported their findings recently in The ISME Journal.

Underestimated players in the marine food web

Bacteria in the oceans are known to feed on algal sugars, so-called polysaccharides. These make for a nutrient-rich meal especially after the demise of algal blooms, when the seawater is full of algal biomass. A team of scientists around Vipul Solanki from the Max Planck Institute for Marine Microbiology has now found that a bacterium that thrives on algal polysaccharides is more versatile than expected: It can also decompose an exclusively fungal polysaccharide. The bacterium itself was isolated from seawater collected off Helgoland Island in the German Bight. "This is the first time we find how a bacterium degrades a polysaccharide from a marine fungus", says Solanki. "We believe that the role of fungi in the marine food web, in particular during algae blooms, has so far been underestimated."

Substrates from marine fungi are food for bacteria

The fungal polysaccharide detected is an alpha-1,6-mannan. Other mannans have been found in algae, but this particular type of mannan is specific to fungi. The scientists got on its trail, because they found the genes coding for enzymes that target alpha-1,6-mannans, including the key enzyme, termed ShGH76. "Such enzymes are known from bacteria of the human gut. There they target mannans from the yeasts that humans use to bake bread or brew beer", Solanki explains. "However, we did not expect this in a marine bacterium, which is why we studied this enzyme in detail". An interdisciplinary team of researchers, uniting expertise in glycobiology, protein crystallography, biochemistry and genomics was necessary to describe this enzyme and identify its unexpected substrate. Solanki points out that "in particular X-ray diffraction of crystallized ShGH76 together with synthetic oligomannan substrates did provide information about the mechanism of this enzyme".

A missing piece in the marine carbon cycle

What exactly is the marine fungus of origin doing? Most probably, it infects algae. The scientists suspect so, as ShGH76-like genes are particularly abundant in microbial communities during algae blooms. Therefore, the fungi might play two important roles in the marine carbon cycle: First, their parasitism on algae makes algal biomass accessible for marine bacteria, and second, they themselves represent a food source for bacteria. "The turnover of such fungal organic matter does play a role in the marine carbon cycle, yet the extent and detailed mechanisms are currently not well understood", Solanki stresses. "Our study provides a first stepping stone towards a deeper understanding of this neglected part of the marine carbon cycle."

X-ray crystal structures of ShGH76 with active site highlighted as black box © Vipul Solanki/Max Planck Institute for Marine Microbiology

Dr. Vipul Solanki fishing for crystallized enzymes in the laboratory. © Katrin Matthes/Max Planck Institute for Marine Microbiology

Original publication

Vipul Solanki, Karen Krüger, Conor J. Crawford, Alonso Pardo-Vargas, José Danglad-Flores, Kim Le Mai Hoang, Leeann Klassen, D. Wade Abbott, Peter H. Seeberger, Rudolf I. Amann, Hanno Teeling and Jan-Hendrik Hehemann (2022): Glycoside hydrolase from the GH76 family indicates that marine Salegentibacter sp. Hel_I_6 consumes alpha-mannan from fungi. The ISME Journal (published 12 April 2022)
DOI: 10.1038/s41396-022-01223-w

Max Planck Institute for Marine Microbiology, 28359 Bremen, Germany

Max Planck Institute of Colloids and Interfaces, 14476 Potsdam, Germany

Lethbridge Research and Development Centre, Agriculture and Agri-Food Canada, Lethbridge, AB T1J 4B1, Canada

University of Bremen, Center for Marine Environmental Sciences, MARUM, 28359 Bremen, Germany

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Molecular clues reveal that bacteria devour fungi in the ocean

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New study highlights the role of fungal organic matter in the marine carbon cycle How can we know what bacteria eat? In contrast to animals, they have no trunks, specialized teeth or beaks that provide clues whether the preferred food consists of leaves, meat, small insects or grain.

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Yeasts, rusts, smuts, mildews, molds, and mushrooms.

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- salt efflux pumps in cell membranes which regulate their internal environment by removing excess salt
- create osmolytes to regulate osmosis
- have proteins that work at high pressures

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Certain animals such as sea slugs feed on marine fungi and certain types of marine bacteria feed on substrates from marine fungi.

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Inorganic carbon produced by phytoplankton is converted into organic matter. The organic matter is transported to the deeper ocean interior by the BCP and decomposed by the fungi.

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Eukaryotes have a nucleus wrapped in a nuclear membrane.
Eukaryotes contain many internal specialized compartments and complex cytoskeleton
Some examples of eukaryotic organisms are fungi, plants and animals

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Yes, some marine fungi can be bioluminescent. However, in our case study, the bioluminescence in our ocean waters is usually caused by dinoflagellates and phytoplankton. The most recent algal bloom was in November of 2022.

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As parasites of phytoplankton, macrophytes, algae, and animals’ marine fungi can cause leaching of dissolved organic matter and destruction of their host populations.

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As endobionts, fungi can provide energy to the host or even be commensals benefiting from host protection.

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Fungi can be associated to organisms such as sponges or protozoans, where both individuals benefit

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As saprobes, fungi can colonize and transform the organic matter originated from plants, algae, and animals into valuable nutrients.

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A marine fungi is any fungus that is able to grow and/or sporulate in marine environments, form symbiotic relationships with other marine organisms, adapt and evolve or be metabolically active in marine environments.

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Fungi (land) and Marine Fungi.

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by means of spores (budding or binary fission)

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What roles do they perform in the marine environment?

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What roles do they perform in the marine environment?

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What are all these tiny organisms that live in the oceans? 

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What are all these tiny organisms that live in the oceans? 

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Fungi are eukaryotic organisms that include microorganisms such as yeasts, moulds and mushrooms.

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Are Fungi single-celled or multicelullar?

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Are Fungi single-celled or multicelullar?

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Fungi can be single celled or very complex multicellular organisms.