Bacteria help viruses end algal blooms sooner

Experiments and observations suggest Alteromonas bacteria intensify virus-driven deaths of bloom-forming algae.

Editorial illustration — not from the study.

Researchers studied the alga Gephyrocapsa huxleyi, a virus that infects it and bacteria collected from a 2018 bloom experiment near Bergen, Norway. In laboratory communities containing algae, viruses and bacteria, the bacteria increased virus-induced algal mortality; the effect also occurred in three other widespread microalgae.

Alteromonas showed the strongest combined effects on algal killing and bacterial growth. In observations from bloom communities, these bacteria were attracted to chemicals released by infected cells, accumulated in particles associated with bloom decline and increased during oceanic bloom collapse.

How bacteria hasten bloom decline

Bacteria significantly enhanced the ability of viruses to kill G. huxleyi algae. The researchers used a synergy index to measure the strength of the combined effect and a benefit index to assess how much the bacteria gained. Alteromonas bacteria consistently scored highly on both measures. They were attracted to extracts from virus-infected cells and became enriched in particles formed during bloom demise. The bacterial effect was also observed with three other globally widespread microalgae.

Why carbon pathways may change

Algal blooms can collapse together even though fewer than 30% of their cells show active viral infection. The bacterial contribution identified here may help explain that pattern by adding another route to algal mortality. Because the fate of algal cells affects whether carbon is recycled in surface waters or carried into sinking particles, this virus–bacteria interaction may alter the balance between those pathways and influence global carbon cycling.

Evidence and open questions

The study combined laboratory experiments using synthetic communities of algae, viruses and bacteria with observations from natural bloom communities and an experiment measuring bacterial attraction to infected-cell chemicals. The main model organisms and isolates came from a 2018 induced G. huxleyi bloom near Bergen, Norway, while the oceanic observations showed that Alteromonas increased during bloom collapse. These results identify a mechanism and an association in bloom communities, but the excerpts do not quantify how much this interaction changes carbon cycling globally or establish that it drives every algal bloom collapse.

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