Influence of Lhcx proteins on acclimation to dynamic light and low iron conditions in the diatom Phaeodactylum tricornutum

Diatoms are unicellular photosynthetic eukaryotes that are abundant in fresh-, brackish- and saltwater throughout the world and are responsible for a large proportion of the world’s carbon fixation. Although sunlight is their energy source, excessive illumination constitutes a risk for the formation of reactive oxygen species that can potentially damage the cells. Fluctuations in light intensity and nutrient availability are stress factors present in all habitats to varying degrees. A major limiting resource for growth in the open ocean is iron, the lack of which renders the cells more light sensitive. In contrast, coastal areas are among the most productive habitats due to high nutrient levels, but vertical water mixing and tidal-driven light fluctuations are common and can quickly impact the cellular light absorption. Many diatoms are well-adapted to nutrient limitations and light fluctuations, which is presumed to be one reason for their great success in nature. The energy-dependent fluorescence quenching (qE) is one of the most important photoprotective mechanisms to cope with excess absorbed light and allows for its controlled thermal dissipation. However, the underlying molecular mechanisms in diatoms have hitherto not been resolved in detail. In this thesis, we investigate various aspects of qE in the diatom Phaeodactylum tricornutum. With our newly established selection marker, we were able to introduce Lhcx1, Lhcx2, Lhcx3 and Lhcx4, respectively, in an Lhcx1-knockout line. This demonstrates, for the first time, the key role of Lhcx in the qE process. We also show that qE is characterized by reduction in the functional antenna cross section of photosystem II (σ(PSII)). Through the regression of σ(PSII) vs. the quantum yield of NPQ, we provide strong evidence for uncoupled antenna complexes being the site of heat dissipation as well as for a dimeric organization of PSII in vivo. Previously, Lhcx proteins were hypothesized to shift to a heat-dissipative state after acidification of the thylakoid lumen via protonation of acidic amino acid residues, similar to the related green-algal LhcSR. Via site-directed mutagenesis of the proposed protonation- and pigment binding sites, we can exclude the direct activation of Lhcx by protonation; our experiments suggest that there are interactions between Lhcx and xanthophyll cycle pigments. Furthermore, by knocking out Lhcx2, we establish that it is the major effector in the acclimation of qE under iron depletion. Lhcx2 might also be involved in acclimation of the photosynthesis by enhancement of the energy flow to photosystem I (PSI). This could allow the cells to reduce their PSI:PSII ratio under iron deficiency. In conclusion, we show that diatoms react to environmental stressors due to the presence of multiple Lhcx isoforms that enable them to sense various triggers and help to fine-tune the photosynthetic apparatus.