Heterologous synthesis of chlorophyll b in Nannochloropsis salina enhances growth and lipid production by increasing photosynthetic efficiency.

ABSTRACT

BACKGROUND: Chlorophylls play important roles in photosynthesis, and thus are critical for growth and related metabolic pathways in photosynthetic organisms. They are particularly important in microalgae, emerging as the next generation feedstock for biomass and biofuels. Nannochloropsis are industrial microalgae for these purposes, but are peculiar in that they lack accessory chlorophylls. In addition, the localization of heterologous proteins to the chloroplast of Nannochloropsis has not been fully studied, due to the secondary plastid surrounded by four membranes. This study addressed questions of correct localization and functional benefits of heterologous expression of chlorophyllide a oxygenase from Chlamydomonas (CrCAO) in Nannochloropsis. RESULTS: We cloned CrCAO from Chlamydomonas, which catalyzes oxidation of Chla producing Chlb, and overexpressed it in N. salina to reveal effects of the heterologous Chlb for photosynthesis, growth, and lipid production. For correct localization of CrCAO into the secondary plastid in N. salina, we added the signal-recognition sequence and the transit peptide (cloned from an endogenous chloroplast-localized protein) to the N terminus of CrCAO. We obtained two transformants that expressed CrCAO and produced Chlb. They showed improved growth under medium light (90 µmol/m2/s) conditions, and their photosynthetic efficiency was increased compared to WT. They also showed increased expression of certain photosynthetic proteins, accompanied by an increased maximum electron-transfer rate up to 15.8% and quantum yields up to 17%, likely supporting the faster growth. This improved growth resulted in increased biomass production, and more importantly lipid productivity particularly with medium light. CONCLUSIONS: We demonstrated beneficial effects of heterologous expression of CrCAO in Chlb-less organism N. salina, where the newly produced Chlb enhanced photosynthesis and growth. Accordingly, transformants showed improved production of biomass and lipids, important traits of microalgae from the industrial perspectives. Our transformants are the first Nannochloropsis cells that produced Chlb in the whole evolutionary path. We also succeeded in delivering a heterologous protein into the secondary plastid for the first time in Nannochloropsis. Taken together, our data showed that manipulation of photosynthetic pigments, including Chlb, can be employed in genetic improvements of microalgae for production of biofuels and other biomaterials.