Respiration in Phototrophic Microorganisms

Phototrophic microorganisms include “anoxygenic phototrophs”, which are bacteria capable of growing photosynthetically with no oxygen generation, and Cyanobacteria which are “oxygenic phototrophs” because their photosynthetic apparatus generates oxygen. Several genera of anoxygenic phototrophs are capable of obtaining energy also from aerobic and anaerobic respiration in darkness; conversely, only a few filamentous cyanobacteria can grow in the dark on glucose or other sugars using the organic material as both carbon and energy source. The latter observation suggests that besides the bioenergetic aspect, respiration in cyanobacteria plays other roles such as to control the redox balance or to act as a scavenger for O2 during nitrogen fixation. Facultative phototrophs (capable of both respiration and photosynthesis) contain a photosynthetic apparatus whose synthesis is repressed by oxygen; an exception to this rule is the group of aerobic-anoxygenic phototrophs, mainly marine microorganisms, requiring the presence of oxygen to synthesize their photosynthetic apparatus.