Chlorophylls are the key pigments in oxygenic photosynthesis. There are four variants: chlorophyll a, b, d, and f. To split water, almost universally, nature uses Chlorophyll a. However, there are a few very peculiar photosynthetic species that contain chlorophyll d and/or f. Of particular note is Chroococcidiopsis thermalis, which uses chlorophyll f and/or d to drive the chemistry of photosynthesis (see Science, 360 (2018) 1210-1213). Chlorophylls f and d are only a little different from chlorophylls a and b. But how does this affect their function? Why does nature strongly favor chlorophyll a? What are the consequences of the differences between the chlorophylls for photosynthetic function? Using our unique optical spectrometer, this project aims to address these key fundamental questions.