Unusual features of the high light acclimation of Chromera velia

Abstract

In the present study, the high light (HL) acclimation of Chromera velia (Chromerida) was studied. HL-grown cells exhibited an increased cell volume and dry weight compared to cells grown at medium light (ML). The chlorophyll (Chl) a-specific absorption spectra ([Formula: see text]) of the HL cells showed an increased absorption efficiency over a wavelength range from 400 to 750 nm, possibly due to differences in the packaging of Chl a molecules. In HL cells, the size of the violaxanthin (V) cycle pigment pool was strongly increased. Despite a higher concentration of de-epoxidized V cycle pigments, non-photochemical quenching (NPQ) of the HL cells was slightly reduced compared to ML cells. The analysis of NPQ recovery during low light (LL) after a short illumination with excess light showed a fast NPQ relaxation and zeaxanthin epoxidation. Purification of the pigment-protein complexes demonstrated that the HL-synthesized V was associated with the chromera light-harvesting complex (CLH). However, the difference absorption spectrum of HL minus ML CLH, together with the 77 K fluorescence excitation spectra, suggested that the additional V was not protein bound but localized in a lipid phase associated with the CLH. The polypeptide analysis of the pigment-protein complexes showed that one out of three known LHCr proteins was associated in higher concentration with photosystem I in the HL cells, whereas in ML cells, it was enriched in the CLH fraction. In conclusion, the acclimation of C. velia to HL illumination shows features that are comparable to those of diatoms, while other characteristics more closely resemble those of higher plants and green algae.

Extensive gain and loss of photosystem I subunits in chromerid algae, photosynthetic relatives of apicomplexans

“…While the nature of PSII oligomerization in Aureococcus and Chondrus is unknown, monomeric PSII has been observed in Nannochloropsis oceanica 57 . Since the absence of the peripheral proteins in Chromera does not prevent efficient and adaptable photosynthesis 36 – 38 , Chromera must possess novel mechanisms, and possibly novel PSII subunits, to enable this efficiency. While no other taxon considered here lacks all of the four peripheral proteins missing in Chromera , eight other taxa – Fragilariopsis , Aureococcus , Vitrella , Nannochloropsis , Arabidopsis , Bigelowiella , and Euglena - lack one or more of them, suggesting that the protein infrastructure required for maintaining photosystem stability and light tolerance is evolutionarily flexible.…”

“…The absence of these genes, present in almost all other eukaryotes surveyed here, should have serious and extensive consequences for the structure and function of ATP synthase – for example, in Arabidopsis , knockout of atpD resulted in seedling lethality 65 . The electron transport chain still functions in Chromera and Vitrella , however, and, in the case of Chromera , does so efficiently 36 – 38 . It seems extremely likely, therefore, that Chromera and Vitrella merely possess ATP synthase subunits with highly divergent amino acid sequences that have eluded previous similarity-based searches and annotation efforts 66 .…”

“…Photosynthesis studies have revealed Chromera ’s high capacity for adaptation to different light conditions, allowing it to maintain high rates of photosynthesis and growth without succumbing to photoinhibition 36 – 38 . In order to expand our understanding of the relationship between photosystem structure and function in chromerid algae, we used biochemical methods to identify the components of PSI.…”

Violaxanthin inhibits nonphotochemical quenching in light‐harvesting antenna of Chromera velia

“…These loosely‐bound violaxanthin molecules are most likely situated on the protein periphery, where they are in equilibrium with the lipid phase. This could result in the antennae nonbound violaxanthin that has been already observed in lipid phase of C. velia thylakoids , which makes C. velia similar to diatoms where the lipid‐phase xanthophyll (diadinoxanthin) has been already identified . The presence of protein unbound violaxanthin in the antennae of higher plants is still questionable and requires further research .…”

The Organellar Genomes of Chromera and Vitrella, the Phototrophic Relatives of Apicomplexan Parasites

“…Because transcripts are not oligouridylylated in the apicoplast of Plasmodium, an intriguing link between the oligoU-tailed transcripts of phototrophic genes in Chromera and the loss of photosynthesis in Apicomplexa was recently proposed (13). Despite a greatly mutated plastid genome (31) and a reduced set of photosystem proteins (H. Esson, A. Horák, P. Dufková, R. Sobotka, P. Komenda & M. Oborník, unpublished data), Chromera is highly efficient at photosynthesis-likely because it can acclimatize to different light conditions (3,42,43,48,64) and because nonphotochemical quenching protects it very effectively from excessive radiation (43,48). The main light-harvesting complex of Chromera contains chlorophyll a, violaxanthin, and a yet-unidentified carbonyl isofucoxanthin-related carotenoid, and its energy transfer from carotenoid to chlorophyll a is more efficient than any seen among the investigated FCP (fucoxanthin chlorophyll protein)-like proteins (14).…”