Nitrogen Starvation-Induced Chlorosis inSynechococcus PCC 7942. Low-Level Photosynthesis As a Mechanism of Long-Term Survival

Abstract

Cells of the non-diazotrophic cyanobacterium Synechococcus sp. strain PCC 7942 acclimate to nitrogen deprivation by differentiating into non-pigmented resting cells, which are able to survive prolonged periods of starvation. In this study, the physiological properties of the long-term nitrogen-starved cells are investigated in an attempt to elucidate the mechanisms of maintenance of viability. Preservation of energetic homeostasis is based on a low level of residual photosynthesis; activities of photosystem II and photosystem I were approximately 0.1% of activities of vegetatively growing cells. The low levels of photosystem I activity were measured by a novel colorimetric assay developed from the activity staining of ferredoxin:NADP ϩ oxidoreductase. Photosystem II reaction centers, as determined by chlorophyll fluorescence measurements, exhibited normal properties, although the efficiency of light harvesting was significantly reduced compared with that of control cells. Long-term chlorotic cells carried out protein synthesis at a very low, but detectable level, as revealed by in vivo [35 S]methionine labeling and two-dimensional gel electrophoresis. In conjunction with the very low levels of total cellular protein contents, this implies a continuous protein turnover during chlorosis. Synthesis of components of the photosynthetic apparatus could be detected, whereas factors of the translational machinery were stringently downregulated. Beyond the massive loss of protein during acclimation to nitrogen deprivation, two proteins that were identified as SomA and SomB accumulated due to an induced expression following nitrogen reduction.

Acclimation of unicellular cyanobacteria to macronutrient deficiency: emergence of a complex network of cellular responses

“…the activity observed in growing cells (Sauer et al, 2001). Net oxygen evolution and anabolic activities, however, were not detected under these conditions.…”

“…At the level of gene expression and protein synthesis, an overall reduction in the rate of protein synthesis is observed upon starvation (Aldehni et al, 2003;Sauer et al, 2001). However, the synthesis of certain proteins, which are required for the acclimation process, is strongly enhanced upon nutrient step-down.…”

“…Transmission electron micrographs of Synechococcus elongatus PCC 7942 cells grown in nutrient-sufficient medium (top) or starved for nitrogen (bottom). Chlorosis results from incubation in combined nitrogen-free medium for 7 days as described by Sauer et al (2001). Note the granular white structures in the cytoplasm in the chlorotic cell due to the accumulation of glycogen as well as the almost complete degradation of the intracellular membrane system, the thylakoids.…”

“…Further studies are required to elucidate the detailed regulation of Dps expression and to find out whether additional DNA-binding proteins with protective functions are present. Additional proteins that strongly accumulate in Synechococcus under conditions of nitrogen and sulfur starvation include the outer-membrane porins SomA/SomB (Sauer et al, 2001;K. Forchhammer, unpublished), which might enhance the ability of the organisms to scavenge nutrients from their surroundings.…”

Applicability of the MTT assay for measuring viability of cyanobacteria and algae, specifically for Microcystis aeruginosa (Chroococcales, Cyanobacteria)

“…(Agusti & Sanchez 2002). Methods based on membrane permeability like Evan's Blue failed to differentiate dead cells under some stresses such as CO 2 limitation (Vardi et al 1999) or nitrogen starvation (Sauer et al 2001) or, as in this study, exposure to heat and CuSO 4 . MTT also had its limitations as an assay.…”