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.