Recoverability of Microcystis aeruginosa and Pseudanabaena foetida Exposed to a Year-Long Dark Treatment

Abstract

Cyanobacteria are a significant primary producer and pioneer species that play a vital role in ecological reconstruction, especially in aquatic environments. Cyanobacteria have excellent recovery capacity from significant stress exposure and are thus suggested as bioreserves, even for space colonization programs. Few studies have been conducted on the recovery capacity after experiencing stress. Long-duration darkness or insufficient light is stressful for photosynthetic species, including cyanobacteria, and can cause chlorosis. Cyanobacterial recovery after extensive exposure to darkness has not yet been studied. In this experiment, Microcystis aeruginosa and Pseudanabaena foetida were subjected to a year-long darkness treatment, and the change in recovery capacity was measured in monthly samples. Cyanobacterial growth, chlorophyll-a concentration, oxidative stress, and photosynthetic capacity were evaluated. It was found that the rapid recovery capacity of the two species remained even after one year of darkness treatment. However, the H₂O₂ content of recovered samples of both M. aeruginosa and P. foetida experienced significant changes at six-seven months, although the photosynthetic capacity of both cyanobacteria species was maintained within the healthy range. The chlorophyll-a and carotenoid content of the recovered samples also changed with increasing darkness. The results showed that long-term dark treatment had time-dependent effects but different effects on M. aeruginosa and P. foetida. However, both cyanobacteria species can recover rapidly after one year of dark treatment.

Keywords: bioreserve; cyanobacterial growth; extensive darkness; photosynthetic ability; stress recovery.

Figure 1

Cell counts and chlorophyll fluorescence (ChF) analysis in Microcystis aeruginosa and P. foetida re-cultured after the different dark treatment periods. Optical density (A(i)) and cell count, the blue dotted line represents the result of linear regression. (A(ii)) changes in dark-treated M. aeruginosa and the linear regression relationship between growth rate and cell counts, the points encircled by the yellow circle are outliers in the data, and the blue dotted line represents the result of linear regression on cell counts and growth rate. (A(iii)). The Fv/Fm and NPQ values over time for M. aeruginosa (B(i)) and P. foetida (B(ii)). There is a large gap between the two species in the ratio of Fv/Fm to NPQ values, the yellow and red circles indicate the value ranges of two cyanobacteria species. (B(iii)). Error bars represent standard errors.

Figure 2

H₂O₂ concentration, chlorophyll-a, and carotenoid content in M. aeruginosa and P. foetida re-cultured following various dark treatment periods. H₂O₂ contents per one mL of M. aeruginosa (A(i)) and P. foetida (A(ii)) culture solution was measured and normalized by the total protein content to compare the H₂O₂ production for both species (A(iii)). Chl-a and carotenoid content in M. aeruginosa (B(i); C(i)) and P. foetida (B(ii); C(ii)), and the alteration of pigment per unit protein (B(iii); C(iii)), were also estimated in the same manner. Error bars represent standard errors. Different letters represent significant differences between dark treatment durations (one-way ANOVA, p < 0.05).

Figure 3

Supplementary experimental results and correlation analysis. The H₂O₂ levels in the two cyanobacteria were significantly different when grouped by their dark treatment period (A). The growth of re-cultured M. aeruginosa after a 13-month dark treatment period was slower compared to healthy cyanobacteria (B). Correlation analysis between specific parameters is presented in (C). Error bars represent standard errors. The differences were considered significant at (**) p < 0.01 (t-test). The correlation between parameters is expressed by the Pearson correlation coefficient r, in which r > 0 means positive correlation and r < 0 means negative correlation.

Figure 4

Phenotypic observation of experimental samples. (a–d). M. aeruginosa initial culture, one-month dark-treated culture, six-month dark-treated culture, and twelve-month dark-treated culture. (e–h). Microscope photos of M. aeruginosa cultures at initial culture, one-month dark-treated culture, six-month dark-treated culture, and twelve-month dark-treated culture. (i–l). P. foetida initial culture, one-month dark-treated culture, six-month dark-treated culture, and twelve-month dark-treated culture. (m–p). Microscope photos of P. foetida cultures at initial culture, one-month dark-treated culture, six-month dark-treated culture, and twelve-month dark-treated culture.