Photoperiods and light differentially influence growth and potential niches of phycocyanin‐ and phycoerythrin‐rich picocyanobacteria

Abstract Strains from the picocyanobacteria genus Synechococcus are currently found across a wide range of photoperiods and photosynthetically active radiation. Future scenarios now forecast range expansions of marine Synechococcus into new photic regimes. We found that strains of temperate, coastal phycocyanin‐rich and phycoerythrin‐rich Synechococcus grew fastest under moderate photosynthetically active radiation, and a 24‐h photoperiod, despite a cumulative diel photon dose equivalent to conditions where growth was slower, under higher light and shorter photoperiods. Under optimal conditions, a phycoerythrin‐rich Synechococcus strain achieved a highest recorded cyanobacterial chlorophyll‐specific exponential growth rate ( μ ) of 4.5 d −1 . Two phycoerythrin‐rich strains demonstrated wider ability to modulate light capture capacity, whereas two phycocyanin‐rich strains showed less change in light capture across increasing cumulative diel photon dose. All four coastal strains showed a decrease of effective absorption cross‐section for photosystem II photochemistry, vs. increasing cumulative diel photosynthetically active radiation doses. Within each strain, μ showed consistent, saturating responses to increasing cumulative diel photosystem II electron flux, with more variations in responses of μ to cumulative photosynthetically usable radiation. As photoperiod opportunists, coastal picocyanobacteria show potential to expand into longer photic regimes as higher latitudes warm.