BIOENERGETIC PROCESSES ARE MODIFIED DURING NITROGEN STARVATION AND RECOVERY IN PHORMIDIUM LAMINOSUM (CYANOPHYCEAE)1

In the non-N₂-fixing cyanobacterium Phormidium laminosum (Agardh) Gomont (strain OH-I-pCl₁), N starvation induced an increase in the rate of respiration and a decrease in the rate of O₂ evolution. When NO₃^? was added to illuminated N-starved cells, O₂ evolution immediately increased to levels shown by NO₃^? grown cells, even though N-starved cells had lost most of their in vitro photosynthetic activities. Stimulation of noncyclic electron flow was maximal under light-saturating conditions and after 2–3 days of N starvation. The respiratory rate of N-starved cells was stimulated by the addition of NO₃^? or NH₄⁺ and partially inhibited at very low irradiances, even in the presence of DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea). Results indicate that N-starved cells obtain the energy supply for N assimilation through a process different from that used by N-sufficient cells. N-starved cells were able to take up NO₃^? in the dark and when illuminated in the presence of DCMU under anaerobiosis. Following NO₃^? addition, the photosynthetic yield of the in vivo noncyclic electron transport slightly increased, whereas it decreased after NH₄⁺ addition. Addition of NO₃^? or NH₄⁺ favored photoinhibition of photosystem II, the effect being faster after NH₄⁺ addition.