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Cyanobacteria as a primary producer provide energy and carbon sources for the marine food web, of which predation-interactions play central roles in regulating global element cycles and marine ecosystem stability. Here, we report the anti-predation activity of a typical marine cyanobacterium Prochlorococcus MED4 to defend the predation by Uronema marinum . MED4 synthesize formaldehyde as the anti-predation chemical, of which the synthesis was light-dependent and predator-inducible. Compared to other protists, both the higher concentration of accumulated formaldehyde in U. marinum and the lower formaldehyde tolerance of U. marinum resulted in the specific anti-predation of MED4 against U. marinum . This specific anti-predation could regulate the cyanobacterial growth and the U. marinum infection of marine fishes. Metadata analyses showed the mutually exclusion of Prochlorococcus and Uronema in global marine environments. These findings significantly advance our understanding of the marine food web and biogeochemical cycles.

Significance Statement

Predation-driven interactions in the ocean are critical regulators of global biogeochemical cycles, yet active defense mechanisms in marine picophytoplankton remain largely unknown. This study reveals that Prochlorococcus MED4 as the most abundant primary producer in the ocean synthesize light-driven and predator-inducible aldehydes to actively and specifically defend predation against the ciliate Uronema marinum . The anti-predation of Prochlorococcus against Uronema could have broad implications in biocontrol of Uronema infection in marine fishes and regulation of the marine food web.