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

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 (pcpcT→C) and one (ƊCpcC1C2:pcpcT→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 ƊCpcC1C2:pcpcT→C and olive mutants. Cell size was smaller in the pcpcT→C and olive strains. Growth and biomass accumulation were similar between the wild-type and pcpcT→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 ƊCpcC1C2:pcpcT→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.

A Novel Mechanism, Linked to Cell Density, Largely Controls Cell Division in Synechocystis

“…Faster doubling times of 6 and 5.6 h were reported by Hihara et al (2001) and Zavrel et al (2015), respectively; however, in the case of the latter study, these division rates could not be maintained for more 120 h. Many studies have attempted to identify growth-limiting factors in Synechocystis (e.g. nutrients and light; Kim et al, 2011;Lea-Smith et al, 2014;Burnap, 2015;Touloupakis et al, 2015;van Alphen and Hellingwerf, 2015). Understanding of the factors controlling the limitation of Synechocystis growth would facilitate the use of this strain as a cell factory (Yu et al, 2013) for the production of biomass (Joseph et al, 2014), pigments (Sekar and Chandramohan, 2008), secondary metabolite natural products (Frommeyer et al, 2016), biofuel (Dexter and Fu, 2009;Baebprasert et al, 2011;Liu et al, 2011), and other high-value compounds.…”

“…Considering that the metabolism does not control the rate of cell division, we tested whether already known factors were responsible, such as (1) nutrient starvation (Lazazzera, 2000;Berla and Pakrasi, 2012), (2) quorum sensing (Sharif et al, 2008), and (3) reduction of light quality and intensity available to the cells due to high cell density (self-shading; Raven and Kübler, 2002;Ort and Melis, 2011;Lea-Smith et al, 2014).…”

Hydrocarbons Are Essential for Optimal Cell Size, Division, and Growth of Cyanobacteria

“…Photoinhibition, photosynthesis, and respiration were determined according to Lea-Smith et al (2014). Photosynthetic O 2 evolution rates and O 2 depletion rates (respiration) were determined on cell cultures at OD 750nm = ;0.5 (;2.3 nmol Chl ml 21 in Synechocystis or ;4 nmol Chl ml 21 in Synechococcus) using an oxygen electrode system (Hansatech) maintained at 30°C.…”

“…Absorbance measurements on whole cells were performed according to Lea-Smith et al (2014). Cultures were harvested during the exponential growth phase at an OD 750nm = ;0.4.…”

Thermococcus kodakarensis : the key to affordable biohydrogen production

“…PCC 6803, in contrast, is similar in size (2 µm) to T. kodakarensis, and one may wonder how it is able to accommodate up to 10 times more chromosomes [57,518].…”