Temporal responses of wild-type pigmentation and RcaE-deficient strains of Fremyelladiplosiphon during light transitions

Abstract

A temporal study was conducted to evaluate the dynamic complementary chromatic adaptation (CCA) response of two Fremyella diplosiphon strains—wild-type pigmentation strain SF33 and an RcaE-deficient (ΔrcaE) strain, which lacks the photosensor that regulates CCA. SF33 and ΔrcaE cultures were monitored for 15 days after transition of green-light (GL) acclimated cultures to red light (RL) and vice versa. SF33 showed similar growth irrespective of the external light quality; however, a ΔrcaE strain grew slower than SF33 under both RL and GL. Chlorophyll a (chla) content increased in both strains over time and was not much different under RL and GL indicating that chla biosynthesis is not affected significantly by light quality or RcaE function. Phycoerythrin is the sole pigment to absorb GL, whereas several pigments, i.e., allophycocyanin, phycocyanin and chla, function under RL to drive photosynthesis. SF33 compensates for this by synthesizing a higher percentage of PE under GL. The final pigment distribution in the ΔrcaE mutant was found to be more different from SF33 under GL than under RL indicating that RcaE is needed for a transitional response to RL and RL-dependent repression of PE accumulation, yet RcaE is virtually critical for both transitioning to and a full adaptation to GL.

Acknowledgments

We thank Dr. Bagmi Pattanaik for critically reading and commenting on the manuscript. This research was supported by the USDepartment of Energy (Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, grant no. DE-FG02-91ER20021 to B.L.M.) and a CAREER award from the National Science Foundation (grant no. MCB-0643516 to B.L.M.).

Figures and Tables

Figure 1 Growth of wild-type pigmentation strain SF33 and RcaE-deficient mutant (ΔrcaE) strain of F. diplosiphon under shifts between green and red light. (A) Cell growth was estimated as scattering of light (i.e., optical density or O.D.) at 750 nm and (B) total cellular protein content (µg ml⁻¹) during 15 days of exposure to green light (GL) and red light (RL) after transition of RL-acclimated cultures to GL and vice versa.

Figure 2 Chlorophyll content in wild-type pigmentation strain SF33 and RcaE-deficient mutant (ΔrcaE) strain of F. diplosiphon under shifts between green and red light. Chlorophyll a (chla) concentration was determined during 15 days of exposure to green light (GL) and red light (RL) after transition of RL-acclimated cultures to GL and vice versa. Asterisks represent significant difference from 0 day (*p < 0.05), similar alphabets over the bars indicate homogenous mean group while a line over the bars show no significant difference (p > 0.05) between treatment at each time interval.

Figure 3 Phycobiliprotein content in SF33 wild-type pigmentation strain of F. diplosiphon under shifts between green and red light. Concentration of phycobiliproteins, including phycocyanin (PC), allophycocyanin (AP) and phycoerythrin (PE) were determined during 15 days of exposure to green light (GL) and red light (RL) after transition of RL-acclimated cultures to GL and vice versa. Similar alphabets over the bars represent no significant difference between pigment levels (p > 0.05).

Figure 4 Phycobiliprotein content in RcaE-deficient mutant (ΔrcaE) strain of F. diplosiphon under shifts between green and red light. Concentration of phycobiliproteins, including phycocyanin (PC), allophycocyanin (AP) and phycoerythrin (PE) were determined during 15 days of exposure to green light (GL) and red light (RL) after transition of RL-acclimated cultures to GL and vice versa. Similar alphabets over the bars represent no significant difference between pigment levels (p > 0.05).

Figure 5 Phycobiliprotein distributions in wild-type pigmentation strain SF33 and RcaE-deficient mutant (ΔrcaE) strain under shifts between green and red light. Percentage distribution of phycocyanin (PC), allophycocyanin (AP) and phycoerythrin (PE) were calculated for strains during 15 days of exposure to green light (GL) and red light (RL) after transition of RL-acclimated cultures to GL and vice versa.