Differential acclimation kinetics of the two forms of Type IV chromatic acclimaters occurring in marine Synechococcus cyanobacteria

Synechococcus is one of two most abundant phytoplanktonic organisms of the Ocean and also displays the widest variety of pigmentation of all marine oxyphotrophs, which makes it ideally suited to colonize the variety of spectral niches occurring in the upper-lit layer of oceans. Seven Synechococcus pigment types (PTs) have been described based on the composition and chromophorylation of their light-harvesting complexes, called phycobilisomes (PBS). The most sophisticated and abundant Synechococcus PT (3d) gathers cells capable of Type IV chromatic acclimation (CA4), i.e. to reversibly modify the ratio of the blue light-absorbing phycourobilin (PUB) to the green light-absorbing phycoerythrobilin (PEB) in PBS in order to match the ambient light color. Although two genetically distinct types of CA4-capable strains, so-called PTs 3dA and 3dB, have been evidenced and found to be equally abundant in the Ocean, reasons for their prevalence in natural Synechococcus populations remain obscure. Here, acclimation experiments in different blue to green ratios of representatives of these two PTs showed that in mixed blue-green light conditions, PT 3dB strains displayed significantly higher PUB:PEB ratios than their PT 3dA counterparts. Thus, PTs 3dA and 3dB seem to differ in the ratio of blue to green light required to trigger the CA4 process. Furthermore, shift experiments between 100% BL and 100% GL conditions, and conversely, also revealed discrepancies in the acclimation pace between the two types of chromatic acclimaters, which may explain their co-occurrence in some blue green light niches.