Phycobilisome-Deficient Strains of Synechocystis sp. PCC 6803 Have Reduced Size and Require Carbon-Limiting Conditions to Exhibit Enhanced Productivity

David J Lea-Smith; Paolo Bombelli; John S Dennis; Stuart A Scott; Alison G Smith; Christopher J Howe

doi:10.1104/pp.114.237206

Phycobilisome-Deficient Strains of Synechocystis sp. PCC 6803 Have Reduced Size and Require Carbon-Limiting Conditions to Exhibit Enhanced Productivity

Plant Physiol. 2014 Jun;165(2):705-714. doi: 10.1104/pp.114.237206. Epub 2014 Apr 23.

Authors

David J Lea-Smith¹, Paolo Bombelli², John S Dennis², Stuart A Scott², Alison G Smith², Christopher J Howe²

Affiliations

¹ Department of Biochemistry, University of Cambridge, Cambridge CB2 1QW, United Kingdom (D.J.L.-S., P.B., C.J.H.);Department of Chemical Engineering and Biotechnology, University of Cambridge, Cambridge CB2 3RA, United Kingdom (J.S.D.);Department of Engineering, University of Cambridge, Cambridge CB2 1PZ, United Kingdom (S.A.S.); andDepartment of Plant Sciences, University of Cambridge, Cambridge CB2 3EA, United Kingdom (A.G.S.) djl63@cam.ac.uk.
² Department of Biochemistry, University of Cambridge, Cambridge CB2 1QW, United Kingdom (D.J.L.-S., P.B., C.J.H.);Department of Chemical Engineering and Biotechnology, University of Cambridge, Cambridge CB2 3RA, United Kingdom (J.S.D.);Department of Engineering, University of Cambridge, Cambridge CB2 1PZ, United Kingdom (S.A.S.); andDepartment of Plant Sciences, University of Cambridge, Cambridge CB2 3EA, United Kingdom (A.G.S.).

Abstract

Reducing excessive light harvesting in photosynthetic organisms may increase biomass yields by limiting photoinhibition and increasing light penetration in dense cultures. The cyanobacterium Synechocystis sp. PCC 6803 harvests light via the phycobilisome, which consists of an allophycocyanin core and six radiating rods, each with three phycocyanin (PC) discs. Via targeted gene disruption and alterations to the promoter region, three mutants with two (p_cpcT→C) and one (ΔCpcC1C2:p_cpcT→C) PC discs per rod or lacking PC (olive) were generated. Photoinhibition and chlorophyll levels decreased upon phycobilisome reduction, although greater penetration of white light was observed only in the PC-deficient mutant. In all strains cultured at high cell densities, most light was absorbed by the first 2 cm of the culture. Photosynthesis and respiration rates were also reduced in the ΔCpcC1C2:p_cpcT→C and olive mutants. Cell size was smaller in the p_cpcT→C and olive strains. Growth and biomass accumulation were similar between the wild-type and p_cpcT→C under a variety of conditions. Growth and biomass accumulation of the olive mutant were poorer in carbon-saturated cultures but improved in carbon-limited cultures at higher light intensities, as they did in the ΔCpcC1C2:p_cpcT→C mutant. This study shows that one PC disc per rod is sufficient for maximal light harvesting and biomass accumulation, except under conditions of high light and carbon limitation, and two or more are sufficient for maximal oxygen evolution. To our knowledge, this study is the first to measure light penetration in bulk cultures of cyanobacteria and offers important insights into photobioreactor design.