Light Regulation of LHCX Genes in the Benthic Diatom Seminavis robusta

Abstract

Intertidal benthic diatoms experience a highly variable light regime, which especially challenges these organisms to cope with excess light energy during low tide. Nonphotochemical quenching of chlorophyll fluorescence (NPQ) is one of the most rapid mechanisms diatoms possess to dissipate excess energy. Its capacity is mainly defined by the xanthophyll cycle (XC) and Light-Harvesting Complex X (LHCX) proteins. Whereas the XC and its relation to NPQ have been relatively well-studied in both planktonic and benthic diatoms, our current knowledge about LHCX proteins and their potential involvement in NPQ regulation is largely restricted to planktonic diatoms. While recent studies using immuno-blotting have revealed the presence of light regulated LHCX proteins in benthic diatom communities and isolates, nothing is as yet known about the diversity, identity and transcriptional regulation or function of these proteins. We identified LHCX genes in the draft genome of the model benthic diatom Seminavis robusta and followed their transcriptional regulation during a day/night cycle and during exposure to high light conditions. The S. robusta genome contains 17 LHCX sequences, which is much more than in the sequenced planktonic model diatoms (4-5), but similar to the number of LHCX genes in the sea ice associated diatom Fragilariopsis cylindrus. LHCX diversification in both species, however, appears to have occurred independently. Interestingly, the S. robusta genome contains LHCX genes that are related to the LHCX6 of the model centric diatom Thalassiosira pseudonana, which are lacking in the well-studied pennate model diatom Phaeodactylum tricornutum. All investigated LHCX genes, with exception of SrLHCX6, were upregulated during the daily darklight transition. Exposure to 2,000 µmol photons m −2 s −1 , furthermore, increased transcription of all investigated LHCX genes. Our data suggest that the diversification and involvement of several light regulated LHCX genes in the photophysiology of S. robusta may represent an adaptation to the complex and highly variable light environment this benthic diatom species can be exposed to.

The Vertical Migratory Rhythm of Intertidal Microphytobenthos in Sediment Depends on the Light Photoperiod, Intensity, and Spectrum: Evidence for a Positive Effect of Blue Wavelengths

“…The fact that N. spartinetensis shows a much smaller size than G. fasciola, and probably a different migratory behavior (see Underwood et al, 2005) is an additional asset. The complementary use of the epipelic model Seminavis robusta (Blommaert et al, 2017(Blommaert et al, , 2018, which genome has been sequenced, will allow to search for molecular players, such as timekeepers (Annunziataa et al, 2019), photoreceptors (Jaubert et al, 2017), and diurnal-responsive genes (see Meyer et al, 2003;Blommaert et al, 2020), of the rhythmic migratory light-tuned response of intertidal epipelic MPB. JL performed samples and data analyses.…”

Diurnal transcript profiling of the diatomSeminavis robustareveals adaptations to a benthic lifestyle

“…Fig. 7) [41,59]. In contrast to pigment synthesis phasing at dawn, we observed a sharp peak in expression of LHCF, LHCR and LHCY genes at ZT6, likely in preparation for chloroplast division [10,38] (Fig.…”

“…In addition, all 73 expressed S. robusta light harvesting complex antenna proteins (LHC) showed significant diel rhythms. Phylogenetic analysis classified LHC genes into four distinct LHC clades: the light-harvesting fucoxanthin chl a/c LHCF, LHCR and LHCY and the photoprotective LHCX ( Figure 4B,C and Supplemental Figure 6 ) (Blommaert et al, 2020; Nymark et al, 2013). In contrast to pigment synthesis phasing at dawn, we observed a sharp peak in expression of LHCF, LHCR and LHCY genes at ZT6, likely in preparation for chloroplast division (Smith et al, 2016; Gillard et al, 2008) (Figure 4C, Supplemental Figure 7).…”

“…In contrast to pigment synthesis phasing at dawn, we observed a sharp peak in expression of LHCF, LHCR and LHCY genes at ZT6, likely in preparation for chloroplast division (Smith et al, 2016; Gillard et al, 2008) (Figure 4C, Supplemental Figure 7). An expansion of photoprotective LHCX is thought to allow benthic diatoms to cope with exposure to highly variable irradiances (Blommaert et al, 2020). Here, members of the LHCX family showed an intense joint expression peak at ZT2 the first day but lacked a second-day peak in expression, even though experimental conditions were equal to the first day.…”

“…Prolonged darkness followed by illumination causes synchronization in cell cycle progression, chloroplast repositioning, pigment synthesis and sexual reproduction [38,39]. Furthermore, S. robusta responds to high irradiances both by downwards migration into the sediment and the expression of photoprotective LHCX genes [40,41]. The recent generation of a reference genome assembly and the construction of a transcriptome atlas that spans various environmental conditions, biotic interactions and life cycle stages represents an important resource for comparative genomic studies, allowing to identify genomic features associated with the lifestyle of obligate benthic diatoms [33].…”