FIRST INDUCED PLASTID GENOME MUTATIONS IN AN ALGA WITH SECONDARY PLASTIDS: psbA MUTATIONS IN THE DIATOM PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE) REVEAL CONSEQUENCES ON THE REGULATION OF PHOTOSYNTHESIS¹

Abstract

Diatoms play a crucial role in the biochemistry and ecology of most aquatic ecosystems, especially because of their high photosynthetic productivity. They often have to cope with a fluctuating light climate and a punctuated exposure to excess light, which can be harmful for photosynthesis. To gain insight into the regulation of photosynthesis in diatoms, we generated and studied mutants of the diatom Phaeodactylum tricornutum Bohlin carrying functionally altered versions of the plastidic psbA gene encoding the D 1 protein of the PSII reaction center (PSII RC). All analyzed mutants feature an amino acid substitution in the vicinity of the QBbinding pocket of Dl. We characterized the photosynthetic capacity of the mutants in comparison to wildtype cells, focusing on the way they regulate their photochemistry as a function of light intensity. The results show that the mutations resulted in constitutive changes of PSII electron transport rates. The extent of the impairment varies between mutants depending on the proximity of the mutation to the QB-binding pocket and/or to the nonheme iron within the PSII RC. The effects of the mutations described here for P. tricornutum are similar to effects in cyanobacteria and green microalgae, emphasizing the conservation of the DI protein structure among photosynthetic organisms of different evolutionary origins.

Silencing of the Violaxanthin De-Epoxidase Gene in the Diatom Phaeodactylum tricornutum Reduces Diatoxanthin Synthesis and Non-Photochemical Quenching

“…We performed the same experiments as before, but cultivated P. tricornutum at 3 times higher light intensities (135 µmol photons⋅m −2 ⋅s −1 ). A similar irradiance, which is close to E k in P. tricornutum and other diatoms [27], [48], [61], was used previously to grow P. tricornutum in a number of photo-physiological studies [50], [60], [62], [63]. At these growth conditions, F v /F m was similar in the WT cells while the growth rate slightly increased (16%) compared to LL conditions (table 3).…”

“…As a consequence, the activity of the global pool of DDE enzymes responsible for the de-epoxidation of DD is likely to be indirectly regulated by ETR via the coupled generation of the transthylakoidal ΔpH. We indeed recently observed that a decreased ETR can indirectly alter the DD de-epoxidation in P. tricornutum ([48] and unpublished data). The activity of DDE pool therefore progressively increased with the light intensity until a maximum already before the saturation of photochemistry at E m was reached as observed for WT cells (Fig.…”

Vertical cell movement is a primary response of intertidal benthic biofilms to increasing light dose

“…It is well known that excess light can lead to photodamage by production of free radicals and superoxides, which may lead to protein breakdown in photosystem (PS)II reaction centres, e.g. the D1 dimer (Olaizola et al 1994, Materna et al 2009). Cells can hence prevent such damage through 2 processes, both of which effectively down-regulate photosynthetic activity.…”

High Light Acclimation in the Secondary Plastids Containing Diatom Phaeodactylum tricornutum is Triggered by the Redox State of the Plastoquinone Pool

“…The redox state of the PQ pool is of crucial importance in several photosynthetic regulatory processes, such as the PSII electron cycle (Onno Feikema et al, 2006;Lavaud et al, 2007), nonphotochemical fluorescence quenching (NPQ; Lavaud, 2007;Lavaud et al, 2007), and chlororespiration (Caron et al, 1987;Dijkman and Kroon, 2002;Grouneva et al, 2009). The dynamics of the PQ pool redox state as a function of light intensity differ between diatom species and between diatoms and land plants (Ruban et al, 2004;Lavaud, 2007;Materna et al, 2009). So far, no evidence for an involvement of the PQ pool in redox signaling in diatoms has been published.…”