Abstract Photosynthesis in the world’s oceans is primarily conducted by phytoplankton, microorganisms that use many different pigments for light capture. Synechococcus is a unicellular cyanobacterium estimated to be the second most abundant marine phototroph, with a global population of 7 × 1026 cells. This group’s success is partly due to the pigment diversity in their photosynthetic light harvesting antennae, which maximize photon capture for photosynthesis. Many Synechococcus isolates adjust their antennae composition in response to shifts in the blue:green ratio of ambient light. This response was named type 4 chromatic acclimation (CA4). Research has made significant progress in understanding CA4 across scales, from its global ecological importance to its molecular mechanisms. Two forms of CA4 exist, each correlated with the occurrence of one of two distinct but related genomic islands. Several genes in these islands are differentially transcribed by the ambient blue:green light ratio. The encoded proteins control the addition of different pigments to the antennae proteins in blue versus green light, altering their absorption characteristics to maximize photon capture. These genes are regulated by several putative transcription factors also encoded in the genomic islands. Ecologically, CA4 is the most abundant of marine Synechococcus pigment types, occurring in over 40% of the population oceanwide. It predominates at higher latitudes and at depth, suggesting that CA4 is most beneficial under sub-saturating photosynthetic light irradiances. Future CA4 research will further clarify the ecological role of CA4 and the molecular mechanisms controlling this globally important form of phenotypic plasticity.
Significance Of all cyanobacteria on Earth, marine Synechococcus are those displaying the greatest pigment diversity. The most sophisticated pigment type is cells able to reversibly modify their color by a phenomenon called type IV chromatic acclimation or CA4. Two genetically distinct CA4 types (CA4-A and CA4-B) have evolved in different lineages. Together, they represent almost half of all Synechococcus cells in oceanic areas and are equally abundant but occupy complementary ecological niches. While the molecular mechanism of CA4-A has recently started to be deciphered, the CA4-B mechanism was so far uncharacterized. Here, by unveiling this mechanism and demonstrating its singularity relative to CA4-A, we provide highlights on the evolutionary history of Synechococcus acclimation to light color in the oceans.
