Bacteria leave 4 times fewer algae than virus alone in lab test

Adding bacteria from an algal bloom to virus-infected algae accelerated their decline in a laboratory experiment, leaving approximately 4 times fewer algal cells after 7 days than infection with the virus alone.

The result addresses a puzzle: blooms can collapse across an entire population even though earlier measurements found active viral infection in fewer than 30% of algal cells during collapse. The findings suggest bacteria can add to the deaths caused by viruses.

Testing algae, viruses and bacteria together

For the study published in Nature, researchers built a laboratory community using Gephyrocapsa huxleyi, a single-celled alga that forms extensive blooms, its virus and bacteria collected during bloom collapse. They compared algae initially free of bacteria under four conditions: alone, with bacteria, with the virus, and with both. The experiment used five independent cultures.

The bacterial community did not significantly change viral production or the share of algal cells infected. The researchers said the bacteria appeared to increase algal deaths without increasing viral infection.

In separate tests, antibiotic treatment reduced deaths in virus-infected cultures of three other marine microalgae. The effect was not universal: antibiotics did not reduce mortality in Ostreococcus tauri infected with one specialist virus. The authors said the strength of the interaction may depend on the algae, virus and bacterial community involved.

From laboratory cultures to the ocean

The researchers identified Alteromonas bacteria as contributors to the effect. All three strains they tested increased the decline of virus-infected algae and gained a growth advantage during infection. The authors propose that viral infection creates conditions that help bacteria cause disease and grow.

Separate observations showed that Alteromonas were strongly attracted to material from infected cells, gathered in clumps formed as blooms collapsed and became more abundant during oceanic bloom collapse. Those observations are distinct from the laboratory tests of algal mortality.

The authors propose that the combined effect of viruses and bacteria could influence where carbon goes after a bloom. Viral activity can recycle nutrients into the surface-ocean food web, or help organic particles clump and sink, carrying carbon away from the surface. The laboratory results demonstrate an effect on algal death; the effect on those carbon pathways remains a proposed implication.