Flv3A facilitates O2 photoreduction and affects H2 photoproduction independently of Flv1A in diazotrophic Anabaena filaments

The model heterocyst-forming filamentous cyanobacterium, Anabaena sp. PCC 7120 (Anabaena) represents multicellular organisms capable of simultaneously performing oxygenic photosynthesis in vegetative cells and the O2-sensitive N2-fixation inside the heterocysts. The flavodiiron proteins (FDPs) have been shown to participate in photoprotection of photosynthesis by driving excess electrons to O2 (Mehler-like reaction). Here, we addressed the physiological relevance of the vegetative cell-specific Flv1A and Flv3A on the bioenergetic processes occurring in the diazotrophic Anabaena under variable CO2. We demonstrate that both Flv1A and Flv3A are required for proper induction of the Mehler-like reaction upon a sudden change in light intensity, which is likely important for the activation of carbon-concentrating mechanisms (CCM) and CO2 fixation. Nevertheless, Flv3A showed a more important role in photoprotection than Flv1A. Under low CO2 diazotrophic conditions, Flv3A is capable of mediating moderate O2 photoreduction, independently of Flv1A, but in coordination with Flv2 and Flv4. Strikingly, the lack of Flv3A resulted in strong downregulation of the heterocyst-specific uptake hydrogenase, which led to enhanced H2 photoproduction under both oxic and micro-oxic conditions. These results reveal a novel regulatory network between the Mehler-like reaction and the H2 metabolism, which is of great interest for future photobiological production of H2 in Anabaena.

The roles of carbonic anhydrases in сarbon concentrating mechanisms of aquatic photoautotrophs

The article surveys multiple roles of carbonic anhydrases (CAs) in inorganic carbon (Ci) acquisition by cyanobacteria, microalgae, and macrophytes under Ci limiting conditions. Slow Ci diffusion in aquatic environments imposes the need for carbon concentrating mechanisms (also named CO2 concentrating mechanisms, CCMs) in aquatic photoautotrophs to transport Ci against the gradient and ensure CO2 supply to photosynthesis. There are common requirements for efficient CCM functioning in cyanobacteria, algae, and aquatic angiosperms, including active transport of HCO3- to the Ci-concentrating compartment and CO2 generation from the HCO3- pool in the Rubisco-enriched subcompartment. Facilitating Ci diffusion in aqueous solutions and across lipid bilayers, CAs play essential roles in CCMs that are best studied in cyanobacteria, green algae, and diatoms. Roles of CAs in CCMs depend on their localization and include facilitation of active transmembrane Ci uptake by its supplying at the outer surface (Role 1) and removal at the inner surface (Role 2), as well as the acceleration of CO2 production from HCO3- near Rubisco (Role 3) in a special CO2-tight compartment, carboxysome in cyanobacteria or pyrenoid in microalgae. The compartmentalization of CAs is also critical because, if activated in the HCO3- –concentrating compartment, they can easily eliminate the Ci gradient created by CCMs.

The Coevolution of RuBisCO, Photorespiration, and Carbon Concentrating Mechanisms in Higher Plants

Ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (RuBisCO) is the carbon-fixing enzyme present in most photosynthetic organisms, converting CO2 into organic matter. Globally, photosynthetic efficiency in terrestrial plants has become increasingly challenged in recent decades due to a rapid increase in atmospheric CO2 and associated changes toward warmer and dryer environments. Well adapted for these new climatic conditions, the C4 photosynthetic pathway utilizes carbon concentrating mechanisms to increase CO2 concentrations surrounding RuBisCO, suppressing photorespiration from the oxygenase catalyzed reaction with O2. The energy efficiency of C3 photosynthesis, from which the C4 pathway evolved, is thought to rely critically on an uninterrupted supply of chloroplast CO2. Part of the homeostatic mechanism that maintains this constancy of supply involves the CO2 produced as a byproduct of photorespiration in a negative feedback loop. Analyzing the database of RuBisCO kinetic parameters, we suggest that in genera (Flaveria and Panicum) for which both C3 and C4 examples are available, the C4 pathway evolved only from C3 ancestors possessing much lower than the average carboxylase specificity relative to that of the oxygenase reaction (SC/O=SC/SO), and hence, the higher CO2 levels required for development of the photorespiratory CO2 pump (C2 photosynthesis) essential in the initial stages of C4 evolution, while in the later stage (final optimization phase in the Flaveria model) increased CO2 turnover may have occurred, which would have been supported by the higher CO2 levels. Otherwise, C4 RuBisCO kinetic traits remain little changed from the ancestral C3 species. At the opposite end of the spectrum, C3 plants (from Limonium) with higher than average SC/O, which may be associated with the ability of increased CO2, relative to O2, affinity to offset reduced photorespiration and chloroplast CO2 levels, can tolerate high stress environments. It is suggested that, instead of inherently constrained by its kinetic mechanism, RuBisCO possesses the extensive kinetic plasticity necessary for adaptation to changes in photorespiration that occur in the homeostatic regulation of CO2 supply under a broad range of abiotic environmental conditions.

Convergent evolution of gene regulatory networks underlying plant adaptations to dry environments

Plants transitioned from an aquatic to a terrestrial lifestyle during their evolution. On land, fluctuations on water availability in the environment became one of the major problems they encountered. The appearance of morpho-physiological adaptations to cope with and tolerate water loss from the cells was undeniably useful to survive on dry land. Some of these adaptations, such as carbon concentrating mechanisms (CCMs), desiccation tolerance (DT) and root impermeabilization, appeared in multiple plant lineages. Despite being crucial for evolution on land, it has been unclear how these adaptations convergently evolved in the various plant lineages. Recent advances on whole genome and transcriptome sequencing are revealing that co-option of genes and gene regulatory networks (GRNs) is a common feature underlying the convergent evolution of these adaptations. In this review we address how the study of CCMs and DT have provided insight into convergent evolution of GRNs underlying plant adaptation to dry environments, and how these insights could be applied to currently emerging understanding of evolution of root impermeabilization through different barrier cell types. We discuss examples of co-option, conservation, and innovation of genes and GRNs at the cell, tissue and organ levels revealed by recent phylogenomic (comparative genomic) and comparative transcriptomic studies.

A synthetic C4 shuttle via the β-hydroxyaspartate cycle in C3 plants

Plants depend on the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) for CO2 fixation. However, especially in C3 plants, photosynthetic yield is reduced by formation of 2-phosphoglycolate, a toxic oxygenation product of Rubisco, which needs to be recycled in a high-flux–demanding metabolic process called photorespiration. Canonical photorespiration dissipates energy and causes carbon and nitrogen losses. Reducing photorespiration through carbon-concentrating mechanisms, such as C4 photosynthesis, or bypassing photorespiration through metabolic engineering is expected to improve plant growth and yield. The β-hydroxyaspartate cycle (BHAC) is a recently described microbial pathway that converts glyoxylate, a metabolite of plant photorespiration, into oxaloacetate in a highly efficient carbon-, nitrogen-, and energy-conserving manner. Here, we engineered a functional BHAC in plant peroxisomes to create a photorespiratory bypass that is independent of 3-phosphoglycerate regeneration or decarboxylation of photorespiratory precursors. While efficient oxaloacetate conversion in Arabidopsis thaliana still masks the full potential of the BHAC, nitrogen conservation and accumulation of signature C4 metabolites demonstrate the proof of principle, opening the door to engineering a photorespiration-dependent synthetic carbon–concentrating mechanism in C3 plants.

Alkenone isotopes show evidence of active carbon concentrating mechanisms in coccolithophores as aqueous carbon dioxide concentrations fall below 7 µmol L⁻¹

Coccolithophores and other haptophyte algae acquire the carbon required for metabolic processes from the water in which they live. Whether carbon is actively moved across the cell membrane via a carbon concentrating mechanism, or passively through diffusion, is important for haptophyte biochemistry. The possible utilization of carbon concentrating mechanisms also has the potential to over-print one proxy method by which ancient atmospheric CO2 concentration is reconstructed using alkenone isotopes. Here I show that carbon concentrating mechanisms are likely used when aqueous carbon dioxide concentrations are below 7 µmol L−1. I compile published alkenone-based CO2 reconstructions from multiple sites over the Pleistocene and recalculate them using a common methodology, which allows comparison to be made with ice core CO2 records. Interrogating these records reveals that the relationship between proxy CO2 and ice core CO2 breaks down when local aqueous CO2 concentration falls below 7 µmol L−1. The recognition of this threshold explains why many alkenone-based CO2 records fail to accurately replicate ice core CO2 records, and it suggests the alkenone proxy is likely robust for much of the Cenozoic when this threshold was unlikely to be reached in much of the global ocean.

Bacterial adaptation by a transposition burst of an invading IS element

The impact of transposable elements on host fitness range from highly deleterious to beneficial, but their general importance for adaptive evolution remains debated. Here, we investigated whether IS elements are a major source of beneficial mutations during 400 generations of laboratory evolution of the cyanobacterium Acaryochloris marina strain CCMEE 5410, which has experienced a recent or on-going IS element expansion. The dynamics of adaptive evolution were highly repeatable among eight independent experimental populations and included beneficial mutations related to exopolysaccharide production and inorganic carbon concentrating mechanisms for photosynthetic carbon fixation. Most detected mutations were IS transposition events, but, surprisingly, the majority of these involved the copy-and-paste activity of only a single copy of an unclassified element (ISAm1) that has recently invaded the genome of A. marina strain CCMEE 5410. Our study reveals that the activity of a single transposase can fuel adaptation for at least several hundred generations.Impact statementA single transposable element can fuel adaptation to a novel environment for hundreds of generations without an apparent accumulation of a deleterious mutational load.

Alkenone isotopes show evidence of active carbon concentrating mechanisms in coccolithophores as aqueous carbon dioxide concentrations fall below 7 μmol L⁻¹

Coccolithophores and other haptophyte algae acquire the carbon required for metabolic processes from the water in which they live. Whether carbon is actively moved across the cell membrane via a carbon concentrating mechanism, or passively through diffusion, is important for haptophyte biochemistry. The possible utilisation of carbon concentrating mechanisms also has the potential to over-print one proxy method by which ancient atmospheric CO2 is reconstructed using alkenone isotopes. Here I show that carbon concentrating mechanisms are likely used when aqueous carbon dioxide concentrations are below 7 μmol L−1. I use published alkenone based CO2 reconstructions from multiple sites over the Pleistocene, which allows comparison to be made with ice core CO2 records. Interrogating these records reveal that the relationship between proxy- and ice core-CO2 breaks down when local aqueous CO2 concentration falls below 7 μmol L−1. The recognition of this threshold explains why many alkenone based CO2 records fail to accurately replicate ice core CO2 records, and suggests the alkenone proxy is likely robust for much of the Cenozoic when this threshold was unlikely to be reached in much of the global ocean.