We have investigated if the heterologous expression of a functional green alga plastocyanin in the diatom Phaeodactylum tricornutum can improve photosynthetic activity and cell growth. Previous in vitro assays showed that a single‐mutant of the plastocyanin from the green algae Chlamydomonas reinhardtii is effective in reducing P. tricornutum photosystem I. In this study, in vivo assays with P. tricornutum strains expressing this plastocyanin indicate that even the relatively low intracellular concentrations of holo‐plastocyanin detected (≈4 μM) are enough to promote an increased growth (up to 60%) under iron‐deficient conditions as compared with the WT strain, measured as higher cell densities, content in pigments and active photosystem I, global photosynthetic rates per cell, and even cell volume. In addition, the presence of plastocyanin as an additional photosynthetic electron carrier seems to decrease the over‐reduction of the plastoquinone pool. Consequently, it promotes an improvement in the maximum quantum yield of both photosystem II and I, together with a decrease in the acceptor side photoinhibition of photosystem II—also associated to a reduced oxidative stress—a decrease in the peroxidation of membrane lipids in the choroplast, and a lower degree of limitation on the donor side of photosystem I. Thus the heterologous plastocyanin appears to act as a functional electron carrier, alternative to the native cytochrome c6, under iron‐limiting conditions.

In cyanobacteria and most green algae of the eukaryotic green lineage, the copper-protein plastocyanin alternatively replaces the heme-protein cytochrome c  6 as the soluble electron carrier from cytochrome f to photosystem I. The functional and structural equivalence of “green” plastocyanin and cytochrome c  6 has been well established, representing an example of convergent evolution of two unrelated proteins. However, plants only produce plastocyanin, despite having evolved from green algae. On the other hand, cytochrome c  6 is the only soluble donor available in most species of the red lineage of photosynthetic organisms, which includes, among others, red algae and diatoms. Interestingly, plastocyanin genes have been identified in oceanic diatoms, probably acquired by horizontal gene transfer from green algae. However, the mechanisms that regulate the expression of a functional plastocyanin in diatoms are still unclear. In the green eukaryotic lineage, the transfer of electrons from cytochrome f to photosystem I has been characterized in depth. The conclusion is that in the green line this process involves strong electrostatic interactions between partners, which ensures a high affinity and an efficient electron transfer, at the cost of limiting the turnover of the process. In the red lineage, recent kinetic and structural modelling data suggest a different strategy, based on weaker electrostatic interactions between partners, with lower affinity and less efficient electron transfer, but favouring instead the protein exchange and the turnover of the process. Finally, in diatoms the interaction of the acquired green-type plastocyanin with both cytochrome f and photosystem I may not yet be optimized.