Biology is faced with the problem: the field in general does not understand much of what it studies. One method of solving this is to study the smallest organisms known, in an effort to “understand 100% of something small.” This is a typical engineering approach, to understand the smallest system first, then work up to attempting to understand larger systems. After understanding the organisms, of course, we have a better chance at modifying them (their DNA) into something useful (or, rather, something that actually lives at all).
Prochlorococcus is a cyanobacteria and noted as one of the most minimal genomes, at 1700 to 2400 genes, while containing no organic compounds (only CO2, N, P, S, Fe, Co, etc). This organism is common and found in both oceans and fresh water. It is highly efficient at photosynthesis.
In comparing all genomes of this bacteria, the number of genes in common (resulting in the core genome) is about 1,100 genes. This could represent the “minimal phototrope”. The Cyanosite; A Webserver for Cyanobacterial Research has a wealth of information about cyanobacteria in general.