Molecular clues reveal that bacteria devour fungi in the ocean

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.”