Effects of nitrogen limitation and starvation on Chroococcidiopsis sp. (Chroococcales)

Chroococcidiopsis sp. cells from hot desert rock (Timna National Park, Israel) were grown in modified BG-11 medium which was either N-free or which contained different concentrations of nitrogen, provided as nitrate. The life cycle, ultrastructure and physiological activities of the cyanobacterium were investigated over a period of 4 months. No alterations were detected in cells grown in medium containing 250 or 125 μg N ml-1. In contrast, cells grown in medium with 62 μg N ml-1 or lacking nitrogen occurred only as isolated viable forms, after 3 months and 1 month of growth, respectively. In these cells a decrease in the content of cellular chlorophyll and phycocyanin paralleled undetectable O2 evolution and depressed O2 uptake. In addition, such isolated cells were characterized by a multilayered envelope, whereas, in the cytoplasm, vesiculated thylakoids and glycogen granules were observed. N-limited and N-starved cells recovered their cellular organization and physiological activities upon N repletion. These 'survival' cells had a spore-like form, and were functionally comparable to akinetes

Differential electron flow around photosystem I by two C(4)-photosynthetic-cell-specific ferredoxins

In the C(4) plant maize (Zea mays L.), two ferredoxin isoproteins, Fd I and Fd II, are expressed specifically in mesophyll and bundle-sheath cells, respectively. cDNAs for these ferredoxins were introduced separately into the cyanobacterium Plectonema boryanum with a disrupted endogenous ferredoxin gene, yielding TM202 and KM2-9 strains expressing Fd I and Fd II. The growth of TM202 was retarded under high light (130 µmol/m(2)/s), whereas KM2-9 grew at a normal rate but exhibited a nitrogen-deficient phenotype. Measurement of photosynthetic O(2) evolution revealed that the reducing power was not efficiently partitioned into nitrogen assimilation in KM2-9. After starvation of the cells in darkness, the P700 oxidation level under far-red illumination increased significantly in TM202. However, it remained low in KM2-9, indicating an active cyclic electron flow. In accordance with this, the cellular ratio of ATP/ADP increased and that of NADPH/NADP(+) decreased in KM2-9 as compared with TM202. These results demonstrated that the two cell type-specific ferredoxins differentially modulate electron flow around photosystem I