As an important component of marine phytoplankton, diatoms must be able to cope with large changes in illumination on a daily basis. They have an active xanthophyll cycle and non-photochemical quenching (NPQ), but no homolog has been detected for the gene encoding the PsbS protein required for NPQ in plants. However, diatoms do have a branch of the light-harvesting complex superfamily, the Lhcx clade, which is most closely related to the LI818 (LhcSR) genes of the green alga Chlamydomonas, known to be upregulated in response to a variety of stresses. When cultures of the diatom T. pseudonana grown under low light (40 micromol photons m(-2) s(-1)) were exposed to high light stress (HL, 700 micromol photons m(-2) s(-1)), transcripts of three of these genes (Lhcx1, Lhcx4, Lhcx6) were transiently accumulated. The amount of Lhcx6 protein was low under low light, but increased continuously during 10h of HL exposure, then slowly dropped to background levels in the dark. However, HL had little effect on the Lhcx1 protein, which was present under low light and only doubled after HL exposure. Diatoxanthin levels increased throughout the HL period with no change in diadinoxanthin. The fraction of NPQ attributable to photoinhibitory quenching (qI) also increased throughout the HL exposure. Taken together, the Lhcx6 protein could be associated with diatoxanthin binding and play a direct role in excess energy dissipation via sustained quenching during acclimation to prolonged HL stress, while the Lhcx1 protein may play a more structural role in thylakoid membrane organization under all conditions.

No abstract

The diatom Thalassiosira pseudonana has genes for at least 30 members of the Lightharvesting Complex (LHC) superfamily dispersed over a number of chromosomes but often found in pairs. Since diatoms do not have genes for PsbS, which is essential for non-photochemical quenching in plants, we are investigating the possibility that several proteins related to the stress-induced LI818 proteins of green algae could be taking the place of PsbS in responding to high light stress. The expression patterns of these "Lhcx" genes under different light intensities were assayed by real-time RT-PCR and compared with several "standard" Lhcf genes encoding the major fucoxanthin Chl a/c proteins. Lhcx1/2 was up-regulated 4-5 times after 1 h high light (HL), then dropped 30% in the next 5 h. Lhcx4 was also upregulated, although not as strongly, and Lhcx5 was unaffected. An antibody specific for the C-terminus of Lhcx1 identified it as a 22 kDa protein, which remained elevated after 6 h HL.

The LI818 proteins and their Lhcx homologs in diatoms are a subgroup of the light‐harvesting (LHC) antenna family, suspected of being involved in photoprotection and stress resistance. In this work, we report that the transcription of three LI818–like genes in Thalassiosira pseudonana Hasle et Heimdal (Lhcx1, Lhcx5, and Lhcx6) was down‐regulated under iron or copper deprivation and when both trace metals were limiting, as was the case for Lhcf4, one of the standard light‐harvesting genes. By contrast, the protein encoded by Lhcx1 was clearly up‐regulated under iron limitation, suggesting that this gene is independently regulated at transcriptional and translational levels. In general, copper starvation had less effect on the expression of light‐harvesting protein genes than iron deprivation, reflecting the different roles of iron and copper in photosynthetic function, that is, as an essential part of the electron transport chain versus as a cofactor for enzymes required to deal with the reactive oxygen species that result from inhibition of electron flow. Our results suggest that the Lhcx1 protein may be involved in stabilizing the photosynthetic apparatus when decreased nonphotochemical quenching (NPQ) results from Fe deficiency.

bWhole-genome sequencing, transcriptomic analyses, and metabolic reconstruction were used to investigate Gordonia sp. strain KTR9's ability to catabolize a range of compounds, including explosives and steroids. Aspects of this mycolic acid-containing actinobacterium's catabolic potential were experimentally verified and compared with those of rhodococci and mycobacteria.