Relative expression analysis of light-harvesting genes in the freshwater alga Lympha mucosa (Batrachospermales,Rhodophyta)

Members of the freshwater red algal order Batrachospermales are often described as shade-adapted. Nevertheless, recent ecophysiological studies have demonstrated species-level differences in acclimation to a range of irradiances. Lympha mucosa occurs in open and shaded portions of temperate streams and is abundant during summer months, suggesting it tolerates high and low irradiances. Specimens of L. mucosa were collected from open (sun-acclimated) or shaded (shade-acclimated) sites and exposed to low (<20 μmol photons · m⁻² · s⁻¹) or high (220 μmol photon · m⁻² · s⁻¹) light for 72 h to examine mechanisms of photoacclimation at the transcriptional level. High-throughput sequence data were used to design specific primers for genes involved with light harvesting and these were quantified with qPCR. The greatest significant difference in transcript abundances was observed in the psaA gene (Photosystem I P700 apoprotein), and site-type had an effect on these responses. Shade-acclimated thalli were 22-fold down-regulated at high light, whereas sun-acclimated thalli were only 5-fold down-regulated. Another gene involved with Photosystem I (petF ferredoxin) was down-regulated at high light, but only individuals from the shaded site were significantly different (4-fold). In thalli from both sites, cpeA (Phycoerythrin alpha chain) was down-regulated at high light. Although not statistically significant, patterns consistent with previous physiological and transcriptomic studies were uncovered, namely the inverse response of transcriptional activity in genes that encode phycobiliproteins. In support of previous ecophysiological studies of freshwater red algae, these data indicate significant transcriptional changes involving Photosystem I and phycobiliprotein synthesis are required to tolerate and grow at various irradiances.