Study reveals unexpected cooperation between ocean viruses and grazers in controlling phytoplankton
A new preprint study found that when a marine virus and a protistan grazer simultaneously attack the cyanobacterium Prochlorococcus, they produce more viruses, sustain grazer growth, and dramatically increase particle aggregation compared to acting alone. Prochlorococcus is one of the most abundant photosynthetic organisms on Earth and plays a major role in ocean carbon cycling. The findings suggest viruses and grazers form an unexpected trophic cooperation that could significantly influence how carbon is exported from the ocean's sunlit zone.
Researchers used a simplified laboratory ecosystem to study how the cyanopodovirus P-SSP7 and the protistan grazer Paraphysomonas bandaiensis individually and jointly affect Prochlorococcus MED4, a dominant marine cyanobacterium, along with co-occurring heterotrophic bacteria. While each agent alone caused substantial Prochlorococcus mortality, their combined effect on Prochlorococcus mortality was paradoxically lower than the sum of their individual impacts. Despite this reduced host mortality, virus production was enhanced and grazer populations were maintained, suggesting competition between the two was mitigated rather than intensified. The grazers appeared to sustain themselves by shifting to alternative food sources — including bacteria, damaged cells, and aggregates — that arose secondarily from viral lysis of Prochlorococcus. Most strikingly, the combined virus-grazer-phytoplankton interaction produced a dramatic increase in particle aggregation, which has direct implications for the biological carbon pump and the sinking of organic matter out of the photic zone. The study challenges the conventional assumption that viruses and grazers simply compete for the same host, revealing instead a form of trophic cooperation with broader ecosystem consequences.
What's missing
The study is a preprint posted on bioRxiv and has not yet undergone peer review. The experiments were conducted under simplified, controlled laboratory conditions using a single virus-host-grazer system, so generalizability to the full complexity of natural ocean ecosystems remains uncertain. The authors do not quantify the magnitude of particle aggregation's contribution to carbon export under in situ conditions, leaving the real-world biogeochemical significance of the observed aggregation effect unresolved.
What different sources said
- bioRxivCenter
Virus-grazer interplay enhances virus production, particle aggregation, and trophic efficiency during infection of Prochlorococcus
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