What are the known energy capturing processes in biological systems?
Apart from phototrophy, there is only chemotrophy (obtaining energy from redox reactions of chemical substances) and radiotrophy (obtaining energy from ionizing radiation such as gamma rays). However, radiotrophy as of now is only known to be a supplemental mode of energy and not a primary one.
Diversity is a central theme of evolution. It would be interesting if there was a single common mechanism for all modes of energy-nutrition which is perhaps true for the case of the chemiosmotic energy generating mechanism (ATP-synthase like systems which generate energy from chemical gradients). Otherwise, diversity is expected as the organisms evolve to survive in different environments.