The odd behaviour of carbonic anhydrase in the terrestrial cyanobacterium Nostoc flagelliforme during hydration-dehydration cycles

Reading and surviving the harsh conditions in desert biological soil crust: The cyanobacterial viewpoint

Biological soil crusts (BSCs) are found in drylands, cover ∼12% of the Earth's surface in arid and semi-arid lands and their destruction is considered an important promoter of desertification. These crusts are formed by the adhesion of soil particles to polysaccharides excreted mostly by filamentous cyanobacteria, which are the pioneers and main primary producers in BSCs. Desert BSCs survive in one of the harshest environments on Earth, and are exposed to daily fluctuations of extreme conditions. The cyanobacteria inhabiting these habitats must precisely read the changing conditions and predict, for example, the forthcoming desiccation. Moreover, they evolved a comprehensive regulation of multiple adaptation strategies to enhance their stress tolerance. Here we focus on what distinguishes cyanobacteria able to revive after dehydration from those that cannot. While important progress has been made in our understanding of physiological, biochemical and omics aspects, clarification of the sensing, signal transduction and responses enabling desiccation tolerance are just emerging. We plot the trajectory of current research and open questions ranging from general strategies and regulatory adaptations in the hydration/desiccation cycle, to recent advances in our understanding of photosynthetic adaptation. The acquired knowledge provides new insights to mitigate desertification and improve plant productivity under drought conditions.

Weak red light plays an important role in awakening the photosynthetic machinery following desiccation in the subaerial cyanobacterium Nostoc flagelliforme

The subaerial cyanobacterium Nostoc flagelliforme can survive for years in the desiccated state and light exposure may stimulate photosynthetic recovery during rehydration. However, the influence of light quality on photosynthetic recovery and the underlying mechanism remain unresolved. Exposure of field collected N. flagelliforme to light intensity ≥2 μmol photons m^-2 s^-1 showed that the speed of photosystem II (PSII) recovery was in the following order: red > green > blue ≈ violet light. Decreasing the light intensity showed that weak red light stimulated PSII recovery during rehydration. The chlorophyll fluorescence transient and oxygen evolution activity indicated that the oxygen evolution complex (OEC) was the activated site triggered by weak red light. The damaged D1 protein accumulated in the thylakoid membrane during dehydration and is degraded and resynthesized during dark rehydration. PsbO interaction with the thylakoid membrane was induced by weak red light. Thus, weak red light plays an important role in triggering OEC photoactivation and the formation of functional PSII during rehydration. In its arid habitats, weak red light could stimulate the awakening of dormant N. flagelliforme after absorbing water from nighttime dew or rain to maximize growth during the early daylight hours of the dry season.

In Synechococcus sp. competition for energy between assimilation and acquisition of C and those of N only occurs when growth is light limited

The carbon-concentrating mechanisms (CCMs) of cyanobacteria counteract the low CO2 affinity and CO2:O2 selectivities of the Rubisco of these photolithotrophs and the relatively low oceanic CO2 availability. CCMs have a significant energy cost; if light is limiting, the use of N sources whose assimilation demands less energy could permit a greater investment of energy into CCMs and inorganic C (Ci) assimilation. To test this, we cultured Synechococcus sp. UTEX LB 2380 under either N or energy limitation, in the presence of NO3- or NH4+. When growth was energy-limited, NH4+-grown cells had a 1.2-fold higher growth rate, 1.3-fold higher dissolved inorganic carbon (DIC)-saturated photosynthetic rate, 19% higher linear electron transfer, 80% higher photosynthetic 1/K1/2(DIC), 2.0-fold greater slope of the linear part of the photosynthesis versus DIC curve, 3.5-fold larger intracellular Ci pool, and 2.3-fold higher Zn quota than NO3--grown cells. When energy was not limiting growth, there were not differences between NH4+- and NO3--grown cells, except for higher linear electron transfer and larger intracellular Ci pool.We conclude that, when energy limits growth, cells that use the cheaper N source divert energy from N assimilation to C acquisition and assimilation; this does not happen when energy is not limiting.

Effect of carbonic anhydrase on silicate weathering and carbonate formation at present day CO₂ concentrations compared to primordial values

It is widely recognized that carbonic anhydrase (CA) participates in silicate weathering and carbonate formation. Nevertheless, it is still not known if the magnitude of the effect produced by CA on surface rock evolution changes or not. In this work, CA gene expression from Bacillus mucilaginosus and the effects of recombination protein on wollastonite dissolution and carbonate formation under different conditions are explored. Real-time fluorescent quantitative PCR was used to explore the correlation between CA gene expression and sufficiency or deficiency in calcium and CO₂ concentration. The results show that the expression of CA genes is negatively correlated with both CO₂ concentration and ease of obtaining soluble calcium. A pure form of the protein of interest (CA) is obtained by cloning, heterologous expression, and purification. The results from tests of the recombination protein on wollastonite dissolution and carbonate formation at different levels of CO₂ concentration show that the magnitudes of the effects of CA and CO₂ concentration are negatively correlated. These results suggest that the effects of microbial CA in relation to silicate weathering and carbonate formation may have increased importance at the modern atmospheric CO₂ concentration compared to 3 billion years ago.

Dynamic cyanobacterial response to hydration and dehydration in a desert biological soil crust

Biological soil crusts (BSCs) cover extensive portions of the earth's deserts. In order to survive desiccation cycles and utilize short periods of activity during infrequent precipitation, crust microorganisms must rely on the unique capabilities of vegetative cells to enter a dormant state and be poised for rapid resuscitation upon wetting. To elucidate the key events involved in the exit from dormancy, we performed a wetting experiment of a BSC and followed the response of the dominant cyanobacterium, Microcoleus vaginatus, in situ using a whole-genome transcriptional time course that included two diel cycles. Immediate, but transient, induction of DNA repair and regulatory genes signaled the hydration event. Recovery of photosynthesis occurred within 1 h, accompanied by upregulation of anabolic pathways. Onset of desiccation was characterized by the induction of genes for oxidative and photo-oxidative stress responses, osmotic stress response and the synthesis of C and N storage polymers. Early expression of genes for the production of exopolysaccharides, additional storage molecules and genes for membrane unsaturation occurred before drying and hints at preparedness for desiccation. We also observed signatures of preparation for future precipitation, notably the expression of genes for anaplerotic reactions in drying crusts, and the stable maintenance of mRNA through dormancy. These data shed light on possible synchronization between this cyanobacterium and its environment, and provides key mechanistic insights into its metabolism in situ that may be used to predict its response to climate, and or, land-use driven perturbations.

Integrase-directed recovery of functional genes from genomic libraries

Large population sizes, rapid growth and 3.8 billion years of evolution firmly establish microorganisms as a major source of the planet's biological and genetic diversity. However, up to 99% of the microorganisms in a given environment cannot be cultured. Culture-independent methods that directly access the genetic potential of an environmental sample can unveil new proteins with diverse functions, but the sequencing of random DNA can generate enormous amounts of extraneous data. Integrons are recombination systems that accumulate open reading frames (gene cassettes), many of which code for functional proteins with enormous adaptive potential. Some integrons harbor hundreds of gene cassettes and evidence suggests that the gene cassette pool may be limitless in size. Accessing this genetic pool has been hampered since sequence-based techniques, such as hybridization or PCR, often recover only partial genes or a small subset of those present in the sample. Here, a three-plasmid genetic strategy for the sequence-independent recovery of gene cassettes from genomic libraries is described and its use by retrieving functional gene cassettes from the chromosomal integron of Vibrio vulnificus ATCC 27562 is demonstrated. By manipulating the natural activity of integrons, we can gain access to the caches of functional genes amassed by these structures.