Unexpected capacity for organic carbon assimilation by Thermosynechococcus elongatus, a crucial photosynthetic model organism

Abstract

Genetic modification of key residues of photosystems is essential to identify functionally crucial processes by spectroscopic and crystallographic investigation; the required protein stability favours use of thermophilic species. The currently unique thermophilic photosynthetic model organism is the cyanobacterial genus Thermosynechococcus. We report the ability of Thermosynechococcus elongatus to assimilate organic carbon, specifically D-fructose. Growth in the presence of a photosynthesis inhibitor opens the door towards crucial amino acid substitutions in photosystems by the rescue of otherwise lethal mutations. Yet depression of batch-culture growth after 7 days implies that additional developments are needed.Keywords: carbon metabolism; cyanobacteria; mixotrophy and photoheterotrophy; photosystem II Two of the crucial protein complexes for life on earth are the photosystems (PS) of oxygenic photosynthesis, PSI and PSII (for review see [1]). Specifically the light-driven water splitting (water oxidation) by PSII is a hot spot of topical research, inter alia because PSII serves as a 'role model' for water oxidation in synthetic system for sustainable production of nonfossil fuels [2,3]. Thermophilic cyanobacteria play a key role in the endeavour to obtain a thorough mechanistic understanding of photosynthetic water oxidation. Their superior stability of the protein complexes has facilitated crystallisation and structure elucidation [4], first of PSI [5] and then of intact PSII [6][7][8]. Notably, crystallisation of intact PSII containing the active site of water oxidation, a proteinbound manganese-calcium-oxo cluster, has been achieved only for two closely related thermophilic cyanobacteria, specifically Thermosynechococcus elongatus and Thermosynechococcus vulcanus. Moreover, the superior thermostability of purified PSII from thermophilic cyanobacteria is highly favourable for spectroscopic investigations. However, there has been a serious drawback regarding the use of thermophilic cyanobacteria in photosynthesis research: These organisms are mostly believed to be incapable of growing in the absence of a functional PSII, rendering genetic modification of key amino acid residues impossible. The unexpected ability of T. elongatus to use external carbon sources for growing photoheterotrophically, without light-driven water oxidation by PSII, is the subject of our study.For many decades, most cyanobacteria including all thermophilic species such as T. elongatus have been considered as obligate photolithoautotrophic prokaryotes using light, H 2 O and solely CO 2 as energy, electron and carbon source respectively [9]. Just for a few mesophilic species, for example, Synechocystis sp. PCC 6803, the capacity for metabolisation of organic carbon sources, namely D-glucose, has been well known for a long time [10]. This ability of Synechocystis Abbreviations DCMU, 3-(3,4-dichlorophenyl)-1,1-dimethylurea; LAHG, light-activated heterotrophic growth; NDH-1 complex, bacterial-type I NAD(P)H-quinone oxidoreductase; OD 750 , ...

Cytochrome cMdownscales photosynthesis under photomixotrophy inSynechocystissp. PCC 6803

“…The effect of importing and metabolising organic carbon on the bioenergetics properties of cyanobacteria over a long-term period is not fully understood. Previous studies have focused on the cellular changes following relatively short-term (from 10 min to 24 h) exposure to organic carbon (Lee et al 2007, Takahashi et al 2008, Haimovich-Dayan et al 2011, Zilliges and Dau 2016. The majority of these reports suggest partial inhibition of photosynthetic activity, whereas some studies demonstrate increased net photosynthesis under air-level CO 2 after 2 h exposure to 10 mM glucose (Haimovich-Dayan et al 2011).…”

Thermophilic cyanobacteria—exciting, yet challenging biotechnological chassis

“…Historically, the interest in thermophilic cyanobacteria in the context of biology and biotechnology stems from the robustness of their light-harvesting apparatus (Zilliges and Dau 2016;Zhang and Reisner 2020). This made Thermosynechococcus elongatus (nowadays Thermosynechococcus vestitus) and T. vulcanus model organisms for the structural studies of photosynthesis, and especially photosystems II and I (PS II and PS I).…”

Heterotrophy among Cyanobacteria

“…Zilliges and Dau investigated different mono- and disaccharides for their potential to induce photomixotrophic or photoheterotrophic growth of Themosynechococcus elongatus . 50 mM glucose and 50 mM fructose and to a lesser extent 50 mM galactose, 35 mM lactose, and 35 mM sucrose act as substrates, while 75 mM arabinose, 35 mM maltose, and 35 mM trehalose do not significantly affect growth.…”