Light Regulation of LHCX Genes in the Benthic Diatom Seminavis robusta

Lander Blommaert,Koen Sabbe,Marie J J Huysman,Cristina M Osuna-Cruz,Bernard Lepetit,Klaas Vandepoele,Wim Vyverman,Emmelien Vancaester,Per Winge,Atle M Bones,Johann Lavaud,Sofie D’Hondt

doi:10.3389/fmars.2020.00192

Abstract

Intertidal benthic diatoms experience a highly variable light regime, which especially challenges these organisms to cope with excess light energy during low tide. Non-photochemical 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 dark-light transition. Exposure to 2,000 timol 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.

Highlights

Due to the complex interplay of diurnal and tidal cycles and weather conditions, the surface sediments of tidal flats experience highly variable light conditions
A HMMER search, with a profile based on annotated Light-Harvesting Complex X (LHCX) genes from T. pseudonana, P. tricornutum, F. cylindrus and P. multiseries, yielded 17 putative LHCX sequences in the draft S. robusta genome (SrLHCX), all with a calculated molecular weight (Mw) of about 20 kDa (Table 1), with exception of SrLHCX6a and SrLHCX6b, the latter being truncated on the C-terminus and possibly being a pseudogene, see Supplementary Figure S1
As LHCX proteins play a central role in the Non-Photochemical Quenching of chlorophyll a fluorescence (NPQ) mechanism of planktonic diatoms (Bailleul et al, 2010; Zhu and Green, 2010; Lepetit et al, 2013, 2017; Taddei et al, 2016) and light responsive LHCX-proteins have been observed in benthic diatom isolates and communities (Laviale et al, 2015; Blommaert et al, 2017), we investigated the presence of LHCX genes in the benthic diatom S. robusta and studied their transcriptional regulation during high light (HL) conditions and a darkness/low light (LL) transition

Summary

Introduction

Due to the complex interplay of diurnal and tidal cycles and weather conditions, the surface sediments of tidal flats experience highly variable light conditions. While the XC in benthic diatoms has been well-studied in natural communities (van Leeuwe et al, 2008; Jesus et al, 2009; Serôdio et al, 2012; Laviale et al, 2015) and more recently using unialgal isolates (Barnett et al, 2015; Blommaert et al, 2017), our current knowledge about LHCX proteins as an NPQ regulator is mostly based on studies of planktonic diatoms (Nymark et al, 2009, 2013; Zhu and Green, 2010; Zhu et al, 2010; Büchel, 2014; Lavaud and Goss, 2014; Valle et al, 2014; Dong et al, 2015; Goss and Lepetit, 2015; Ghazaryan et al, 2016; Grouneva et al, 2016; Lepetit et al, 2017; Taddei et al, 2018, 2016). The latter includes studies on the pennate model diatom Phaeodactylum tricornutum which to date has only been isolated from water samples from various coastal environments, but may have a benthic growth phase as well (De Martino et al, 2007)

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Results

Conclusion