Diatoms possess an impressive capacity for rapidly inducible thermal dissipation of excess absorbed energy (qE), provided by the xanthophyll diatoxanthin and Lhcx proteins. By knocking out the Lhcx1 and Lhcx2 genes individually in Phaeodactylum tricornutum strain 4 and complementing the knockout lines with different Lhcx proteins, multiple mutants with varying qE capacities are obtained, ranging from zero to high values. We demonstrate that qE is entirely dependent on the concerted action of diatoxanthin and Lhcx proteins, with Lhcx1, Lhcx2 and Lhcx3 having similar functions. Moreover, we establish a clear link between Lhcx1/2/3 mediated inducible thermal energy dissipation and a reduction in the functional absorption cross-section of photosystem II. This regulation of the functional absorption cross-section can be tuned by altered Lhcx protein expression in response to environmental conditions. Our results provide a holistic understanding of the rapidly inducible thermal energy dissipation process and its mechanistic implications in diatoms.

Photosynthesis is a unique process that allows independent colonization of the land by plants and of the oceans by phytoplankton. Although the photosynthesis process is well understood in plants, we are still unlocking the mechanisms evolved by phytoplankton to achieve extremely efficient photosynthesis. Here, we combine biochemical, structural and in vivo physiological studies to unravel the structure of the plastid in diatoms, prominent marine eukaryotes. Biochemical and immunolocalization analyses reveal segregation of photosynthetic complexes in the loosely stacked thylakoid membranes typical of diatoms. Separation of photosystems within subdomains minimizes their physical contacts, as required for improved light utilization. Chloroplast 3D reconstruction and in vivo spectroscopy show that these subdomains are interconnected, ensuring fast equilibration of electron carriers for efficient optimum photosynthesis. Thus, diatoms and plants have converged towards a similar functional distribution of the photosystems although via different thylakoid architectures, which likely evolved independently in the land and the ocean.

Aureochromes constitute a family of blue light (BL) receptors which are found exclusively in heterokont algae such as diatoms (Bacillariophyceae) and yellow-green algae (Xanthophyceae). Previous studies on the diatom Phaeodactylum tricornutum indicate that the formation of a high light acclimated phenotype is mediated by the absorption of BL and that aureochromes might play an important role in this process. P. tricornutum possesses four genes encoding aureochromes. In this study we confirm the nuclear localisation of the PtAUREO1a, 1b and 2 proteins. Furthermore we studied the physiology of light quality acclimation in genetically transformed P. tricornutum cell lines with reduced expression of the aureochrome 1a gene. The results demonstrate that the AUREO1a protein has a distinct function in light acclimation. However, rather unexpectedly AUREO1a seems to repress high light acclimation which resulted in a state of ‘hyper’ high light acclimation in aureo1a silenced strains. This was indicated by characteristic changes of several photosynthetic parameters, including increased maximum photosynthesis rates, decreased chlorophyll a contents per cell and increased values of non-photochemical quenching in AUREO1a silenced strains compared to wild type cultures. Strikingly, AUREO1a silenced strains exhibited phenotypic differences compared to wild type cells during cultivation under BL as well as under red light (RL) conditions. Therefore, AUREO1a might influence the RL signalling process, suggesting an interaction of AUREO1a with RL perception pathways.

Current efforts to technically use microalgae focus on the generation of fuels with a molecular structure identical to crude oil based products. Here we suggest a different approach for the utilization of algae by translating the unique molecular structures of algae oil fatty acids into higher value chemical intermediates and materials. A crude extract from a microalga, the diatom Phaeodactylum tricornutum, was obtained as a multicomponent mixture containing amongst others unsaturated fatty acid (16:1, 18:1, and 20:5) phosphocholine triglycerides. Exposure of this crude algae oil to CO and methanol with the known catalyst precursor [{1,2-(tBu2 PCH2)2C6H4}Pd(OTf)](OTf) resulted in isomerization/methoxycarbonylation of the unsaturated fatty acids into a mixture of linear 1,17- and 1,19-diesters in high purity (>99 %). Polycondensation with a mixture of the corresponding diols yielded a novel mixed polyester-17/19.17/19 with an advantageously high melting and crystallization temperature.

Reverse genetics techniques are powerful tools for studying gene functions. In the model diatom Phaeodactylum tricornutum, RNAi-mediated knockdown of genes still is the most commonly used reverse genetics technique. Due to the diploidic life cycle missing reproduction in lab cultures, many commonly used techniques to create knockout instead of knockdown lines are not applicable in P. tricornutum. These limitations can be overcome by using genome editing approaches like TALEN (Transcription activator-like effector nucleases), and/or CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats), allowing the introduction of targeted mutagenesis events. Both techniques have recently been adapted exemplarily for diatoms, however, no concise guidelines exist yet for routine utilization of these tools and the subsequent characterization of the mutants. We therefore have adapted a cost-effective TALEN generation system previously established for mammalian cells for the use in P. tricornutum, allowing the assembly of TALENs in about two weeks. We further provide protocols for: a) choosing a TALEN target site in order to avoid potentially ineffective and/or off-target prone TALEN constructs, b) efficient transformation of P. tricornutum with both TALEN constructs, utilizing two antibiotics resistance markers, c) effective screening of the transformants. In order to test our system we chose the blue-light dependent transcription factor Aureochrome 1a (PtAureo1a) as a target gene due to the known phenotype of previously characterized P. tricornutum RNAi knockdown strains. Our TALEN approach appears to be highly efficient: targeted mutation events were detected in 50% of all transformants obtained, whereas 21% of the transformants were found to be bi-allelic knockout lines. Furthermore, most TALEN transformed cell lines were found to be genetically homogeneous without the need for re-plating, which greatly facilitates the screening process..