Cell Biology of Chromerids: Autotrophic Relatives to Apicomplexan Parasites - International Review of Cell and Molecular Biology - page 348

makes the photosynthesis of C. velia very efficient. It was shown that
violaxanthin and isofucoxanthin contribute significantly also to the light-
harvesting capacity of C. velia . In addition to that, violaxanthin is able to
quickly deepoxidize to zeaxanthin and thus effectively protect the cell
against high light by nonphotochemical quenching (NPQ), similar, for
instance, to diatoms ( Coesel et al., 2008 ). Consequently, cells grown under
high light show higher content of violaxanthin than those cultivated under
low-light conditions, and NPQ is doubled when compared to less-
illuminated cells ( Kotabov´ et al., 2011 ). When the alga is grown under a
light-dark cycle with a sinusoidal shape of light intensity, it is able to notably
increase the photosynthetic rates. The changes in illumination in C. velia are
reflected by stimulation of photorespiration and NPQ. It was also suggested
that under sinusoidal illumination in this coral-associated alga, oxygen-
consuming processes such as chlororespiration allow high CO 2 assimilation
rates ( Quigg et al., 2012 ).
Evolutionary history of light-harvesting complexes (LHC) composed of
nuclear-encoded proteins posttranslationally targeted to the plastid where
they function, which are involved in the capture of photons and transfer
of energy in photosynthesis, was recently investigated in C. velia . Phyloge-
netic analyses by Pan et al. (2012) showed that out of 23 LHC homologues
from C. velia , 17 formed one large and compact clade, while the rest
appeared in four other clades dispersed throughout the tree, all groups con-
taining homologues from algae with secondary plastids. Particularly, LHC
from dinoflagellates and diatoms, and fucoxanthin chlorophyll-binding pro-
teins formed a sister group to the major clade of LHC from C. velia . Only
three LHC proteins from the chromerid alga appeared in close proximity to
the red algal homologues ( Pan et al., 2012 ).
Plastid genomes of both chromerids, assembled using 454 sequence data
( Janouˇkovec et al., 2010 ), contain a truly astonishing number of differences.
C. velia possesses a highly divergent plastid genome, which is large in size
(120 kb), especially when its relatively low number of genes (80 protein-
coding genes) is taken into account. These genes are generally highly diver-
gent, AT-rich, and subject to the noncanonical UGA code ( Janouˇkovec
et al., 2010; Lang-Unnasch and Aiello, 1999; Moore et al., 2008; Wilson
and Williamson, 1997 ). Chromeran plastid is also known to polyuridylylate
its transcripts, as shown by the comparison of genomic sequences of psbC ,
psbB , and psaA genes with corresponding cDNA sequences. It has been
demonstrated by pulse-field gel electrophoresis and hybridization with
the psbA probe that a substantial fraction of the C. velia plastid genome exists