Vertical distribution of epibenthic freshwater cyanobacterial Synechococcus spp. strains depends on their ability for photoprotection

Abstract

Background: Epibenthic cyanobacteria often grow in environments where the fluctuation of light intensity and quality is extreme and frequent. Different strategies have been developed to cope with this problem depending on the distribution of cyanobacteria in the water column.

Principal findings: Here we provide an experimental proof that the light intensity plays an important role in the vertical distribution of seven, closely related, epibenthic Synechococcus spp. strains isolated from various water depths from the littoral zone of Lake Constance in Germany and cultivated under laboratory conditions. Pigment analysis revealed that the amount of chlorophyll a and total carotenoids decreased with the time of light stress exposure in three phycoerythrin-rich strains collected from 7.0 m water depth and remained low during the recovery phase. In contrast, a constant level of chlorophyll a and either constant or enhanced levels of carotenoids were assayed in phycocyanin-rich strains collected from 1.0 and 0.5 m water depths. Protein analysis revealed that while the amount of biliproteins remained constant in all strains during light stress and recovery, the amount of D1 protein from photosystem II reaction centre was strongly reduced under light stress conditions in strains from 7.0 m and 1.0 m water depth, but not in strains collected from 0.5 m depth.

Conclusion: Based on these data we propose that light intensity, in addition to light quality, is an important selective force in the vertical distribution of Synechococcus spp. strains, depending on their genetically fixed mechanisms for photoprotection.

Figure 1. Light intensity-dependent rates of photosynthetic oxygen evolution in different Synechococcus spp. strains.

Measurements were performed with 15 min dark adaptation, where the last 5 min were recorded, followed by 5 min exposure to light intensity of 8, 15, 22, 47, 60, 75, 97, 156, 420, 600, and 950 µmol photons·m⁻²·s⁻¹ and a final 5 min measurement in the dark to estimate cellular respiration. Gross photosynthesis rate was obtained by addition of dark respiration to the amount of oxygen produced in light. A light intensity of 200 µmol photons m⁻² s⁻¹ was chosen for further light stress experiments because at this light intensity photosynthesis is saturated in all strains investigated. Error bars represent standard errors.

Figure 2. Different Synechococcus spp. cultures after exposure to a light intensity of 200 µmol photons·m⁻²·s⁻¹ for 0 to 8 h.

Bars placed above individual kinetics represent the coloration of cultures prior to light stress exposure (0 h) in order to allow better comparison. The level of pigmentation was quantified (area and pixel values) with help of the ImageJ program . The maximal value was set as 100%.

Figure 3. Analysis of changes in pigment contents during light stress and recovery in low light pre-adapted PC-rich (left panels) and PE-rich (right panels) Synechococcus spp. strains.

(A) Kinetics of the amounts of total Chl a (upper panels) and total carotenoids (lower panels) in low light (8 to 10 µmol photons·m⁻²·s⁻¹)-pre-adapted strains after 1 to 8 h of light stress exposure (200 µmol photons·m⁻²·s⁻¹) and recovery phase (8 to 10 µmol photons·m⁻²·s⁻¹) of 1, 2 or 16 h. The 0 time values (control) were set as 100%. Grey bars above and below graphs indicate time periods with low irradiance. (B) Detailed analysis of changes in individual pigments between the initial state, during light stress (200 µmol photons·m⁻²·s⁻¹) and during recovery (8 to 10 µmol photons·m⁻²·s⁻¹). Time points in each period averaged for noise reduction. Error bars represent standard errors.

Figure 4. Immunoblot analysis using antibodies against the D1 protein of PSII reaction centre, PE and PC.

Low light-preadapted (8 to 10 µmol photons·m⁻²·s⁻¹) Synechococcus spp. strains were exposed to light stress (200 µmol photons·m⁻²·s⁻¹) for 8 h prior to recovery at low light conditions (8 to 10 µmol photons·m⁻²·s⁻¹) for 1, 2 and 16 h. Total protein was isolated and used for SDS-PAGE and immunoblotting as described in Materials and Methods. Gels were loaded on an equal protein basis (6 µg protein).