Transcript patterns of chloroplast‐encoded genes in cultured Symbiodinium spp. (Dinophyceae): testing the influence of a light shift and diel periodicity

Abstract

Microalgae possess numerous cellular mechanisms specifically employed for acclimating the photosynthetic pathways to changes in the physical environment. Despite the importance of coral-dinoflagellate symbioses, little focus has been given as to how the symbiotic algae (Symbiodinium spp.) regulate the expression of their photosynthetic genes. This study used real-time PCR to investigate the transcript abundance of the plastid-encoded genes, psbA (encoding the D1 protein of photosystem II) and psaA (encoding the P700 protein in photosystem I), within the cultured Symbiodinium ITS-2 (internal transcribed spacer region) types A20 and A13. Transcript abundance was monitored during a low to high-light shift, as well as over a full diel light cycle. In addition, psaA was characterized in three isolates (A20, A13, and D4-5) and noted as another example of a dinoflagellate plastid gene encoded on a minicircle. In general, the overall incongruence of transcript patterns for both psbA and psaA between the Symbiodinium isolates and other models of transcriptionally controlled chloroplast gene expression (e.g., Pisum sativum [pea], Sinapis alba [mustard seedling], and Synechocystis sp. PCC 6803 [cyanobacteria]) suggests that Symbiodinium is reliant on posttranscriptional mechanisms for homeostatic regulation of its photosynthetic proteins.

Gene Expression Variation Resolves Species and Individual Strains among Coral-Associated Dinoflagellates within the GenusSymbiodinium

“…Here, overrepresentation of plastid genes cannot be attributed to light intensity differences because all cultures were reared under identical light conditions. In fact, although we might expect these genes to be regulated by light intensity in Symbiodinium as they are in other photosynthetic organisms ( Escoubas et al 1995 ; Pfannschmidt 2003 ), only minor (or no) changes in photosynthesis-related gene expression have been detected in cultures exposed to varying light levels ( McGinley et al 2013 ; Xiang et al 2015 ). Thus, we conclude that different species evolved unique expression levels among photosynthesis-related genes.…”

The engine of the reef: photobiology of the coral–algal symbiosis

“…In addition to the large amount of cellular DNA they contain, there are a number of well-known genetic peculiarities to dinoflagellates such as having permanently condensed chromosomes, few or no nucleosomes and reduced plastid genomes ( Hackett et al, 2004 ; Leggat et al, 2011b ). A real-time PCR study of Symbiodinium showed no effect of diurnal changes in light levels or transfer from low to high light, on transcript abundance of reaction center proteins of both PSI and PSII, suggesting that posttranscriptional processes may be important for regulating proteins ( McGinley et al, 2013 ). Most previous studies have focused on a small number of genes (e.g., Leggat et al, 2011a ; McGinley et al, 2013 ; Sorek et al, 2013 ), but the tools are now available for quantitative transcriptome-wide studies.…”

“…A real-time PCR study of Symbiodinium showed no effect of diurnal changes in light levels or transfer from low to high light, on transcript abundance of reaction center proteins of both PSI and PSII, suggesting that posttranscriptional processes may be important for regulating proteins ( McGinley et al, 2013 ). Most previous studies have focused on a small number of genes (e.g., Leggat et al, 2011a ; McGinley et al, 2013 ; Sorek et al, 2013 ), but the tools are now available for quantitative transcriptome-wide studies. A recent study using RNA-seq on thermotolerant and sensitive phylotypes of Symbiodinium in the same coral host showed no detectable change in gene expression after a short heat stress despite evidence of symbiosis breakdown ( Barshis et al, 2014 ).…”