Comparative proteomic and physiological analysis of diurnal changes in Nostoc flagelliforme

Nostoc flagelliforme is a terrestrial cyanobacterium species whose metabolism follows an obvious diurnal pattern. Diurnal changes at physiological and proteomic levels of N. flagelliforme were obtained. In the morning (7:00 H), net photosynthesis, dark respiration, as well as the activities of total Rubisco, nitrogenase, glutamine synthetase, SOD and CAT were comparatively high. All these physiological activities significantly decreased in the afternoon (13:00 H), and then slowly increased in the evening (19:00 H). Thirty-one differentially expressed proteins with a variety of important functions were reproducibly detected and identified over a diurnal cycle. These proteins were categorized according to their predicted functions into secretion and regulation (15.79 %), antioxidative processes (21.05 %), nitrogen metabolism (10.53 %), carbohydrate and energy metabolism (10.53 %), as well as cell division (2.63 %). The remaining proteins had unclassified/unknown functions (21.05 %) or were unidentified (18.42 %). The results suggested a metabolic shift from active (7:00 H) to quiescent (13:00 H) and then to active (19:00 H) over the diurnal cycle. The differential expression level of ferritin, Mn-CAT, SOD and Fe-SOD may serve as molecular markers for the diurnal metabolism in N. flagelliforme.