Iron superoxide dismutase protects against chilling damage in the cyanobacterium synechococcus species PCC7942

Abstract

A strain of Synechococcus sp. PCC7942 lacking functional Fe superoxide dismutase (SOD), designated sodB-, was characterized by its growth rate, photosynthetic pigments, inhibition of photosynthetic electron transport activity, and total SOD activity at 0 degrees C, 10 degrees C, 17 degrees C, and 27 degrees C in moderate light. At 27 degrees C, the sodB- and wild-type strains had similar growth rates, chlorophyll and carotenoid contents, and cyclic photosynthetic electron transport activity. The sodB- strain was more sensitive to chilling stress at 17 degrees C than the wild type, indicating a role for FeSOD in protection against photooxidative damage during moderate chilling in light. However, both the wild-type and sodB- strains exhibited similar chilling damage at 0 degrees C and 10 degrees C, indicating that the FeSOD does not provide protection against severe chilling stress in light. Total SOD activity was lower in the sodB- strain than in the wild type at 17 degrees C and 27 degrees C. Total SOD activity decreased with decreasing temperature in both strains but more so in the wild type. Total SOD activity was equal in the two strains when assayed at 0 degrees C.

Figure 1

Growth rates of wild-type and sodB⁻ strains. Growth curves measured at A₇₅₀ are shown here for 10°C, 17°C, and 27°C. Each point is the mean of four to seven samples. Error bars are ±sd. Open symbols, Wild type; closed symbols, sodB⁻.

Figure 2

P700 oxidation extent. ΔA₈₂₀ was used to measure the relative amount of photooxidizable P700. All data are percentages of starting values. Each point is the mean of four to eight samples. Error bars are ±sd. Open symbols, Wild type; closed symbols, sodB⁻.

Figure 3

P700 oxidation rate. The ΔA₈₂₀ over the initial 7.5 to 10 ms after the onset of the actinic flash was used to calculate the initial rate of P700 oxidation. Each point is the mean of four to eight samples and all data are percentages of initial values. Error bars are ±sd. DBMIB (25 μm) was added to all samples to prevent competing re-reduction by the cyclic and noncyclic pathways. Open symbols, Wild type; closed symbols, sodB⁻.

Figure 4

P700 re-reduction rate with DCMU. The ΔA₈₂₀ over the initial 30 to 300 ms after the actinic flash was used to calculate the initial rate of P700 re-reduction. Each point is the mean of four to eight samples and all data are percentages of initial values. Error bars are ±sd. The addition of 25 μm DCMU blocked input from PSII. The data shown represent the rates of cyclic electron transport. Open symbols, Wild type; closed symbols, sodB⁻.

Figure 5

Spectrophotometric measurements of phycocyanin in whole cells. Phycocyanin concentrations are shown for wild-type and sodB⁻ strains as measured by A₆₂₅ and A₆₇₈. Each point is the mean of four to eight samples. Error bars are ±sd. Open symbols, Wild type; closed symbols, sodB⁻.

Figure 6

Spectrophotometric measurements of acetone-extractable chlorophyll a. Chlorophyll (Chl) was extracted into 100% acetone. Extractable chlorophyll concentrations are shown for wild type and sodB⁻ as measured by A₆₆₃. Each point is the mean of four to eight samples. Error bars are ±sd. Open symbols, Wild type; closed symbols, sod⁻.

Figure 7

Carotenoid-to-chlorophyll ratios. Chlorophyll a and carotenoids were extracted into 100% acetone. The ratio of blue absorbance (400–520 nm) to red absorbance (650–690 nm) was calculated as a relative indicator of the carotenoid-to-chlorophyll a ratio. Each point is the mean of four to eight samples. Error bars are ±sd. For comparison, purified chlorophyll a from A. nidulans yielded a blue-to-red ratio of 4.53. Open symbols, Wild type; closed symbols, sodB⁻.

Figure 8

Total SOD activity in cell extracts. SOD activity was measured in crude cell extracts as inhibition of the NBT to formazan conversion. Data points are means of three to six samples; error bars are ±sd. Open symbols, Wild type; closed symbols, sodB⁻.