Abstract: Mutant photosynthetic microorganisms having reduced chlorophyll and increased photosynthetic efficiency are provided. The mutants have a locked in high light-acclimated phenotype, in which many of the photosynthetic parameters characteristic of high light acclimated wild type cells are found in the LIHLA mutants when acclimated to low light, such as reduced chlorophyll, reduced NPQ, higher qP, higher Ek, higher Pmax per unit chlorophyll with little to no reduction in Pmax per cell, and higher rates of electron transport through photosystem II over a wide range of light intensities. Provided herein are constructs for attenuating or disrupting genes are provided for generating mutants having the LIHLA phenotype. Also provided are methods of culturing LIHLA mutants for the production of biomass or other products.