Ecological diversification of a cyanobacterium through divergence of its novel chlorophyll d-based light-harvesting system

The evolution of novel traits can have important consequences for biological diversification. Novelties such as new structures are associated with changes in both genotype and phenotype that often lead to changes in ecological function.^1,2 New ecological opportunities provided by a novel trait can trigger subsequent trait modification or niche partitioning³; however, the underlying mechanisms of novel trait diversification are still poorly understood. Here, we report that the innovation of a new chlorophyll (Chl) pigment, Chl d, by the cyanobacterium Acaryochloris marina was followed by the functional divergence of its light-harvesting complex. We identified three major photosynthetic spectral types based on Chl fluorescence properties for a collection of A. marina laboratory strains for which genome sequence data are available,^4,5 with shorter- and longer-wavelength types more recently derived from an ancestral intermediate phenotype. Members of the different spectral types exhibited extensive variation in the Chl-binding proteins as well as the Chl energy levels of their photosynthetic complexes. This spectral-type divergence is associated with differences in the wavelength dependence of both growth rate and photosynthetic oxygen evolution. We conclude that the divergence of the light-harvesting apparatus has consequently impacted A. marina ecological diversification through specialization on different far-red photons for photosynthesis.