Red/far-red light signals regulate the activity of the carbon-concentrating mechanism in cyanobacteria.

Desiccation-tolerant cyanobacteria can survive frequent hydration/dehydration cycles likely affecting inorganic carbon (Ci) levels. It was recently shown that red/far-red light serves as signal-preparing cells toward dehydration. Here, the effects of desiccation on Ci assimilation by Leptolyngbya ohadii isolated from Israel's Negev desert were investigated. Metabolomic investigations indicated a decline in ribulose-1,5-bisphosphate carboxylase/oxygenase carboxylation activity, and this was accelerated by far-red light. Far-red light negatively affected the Ci affinity of L. ohadii during desiccation and in liquid cultures. Similar effects were evident in the non-desiccation-tolerant cyanobacterium Synechocystis The Synechocystis Δcph1 mutant lacking the major phytochrome exhibited reduced photosynthetic Ci affinity when exposed to far-red light, whereas the mutant ΔsbtB lacking a Ci uptake inhibitory protein lost the far-red light inhibition. Collectively, these results suggest that red/far-red light perception likely via phytochromes regulates Ci uptake by cyanobacteria and that this mechanism contributes to desiccation tolerance in strains such as L. ohadii.

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.

Desert cyanobacteria prepare in advance for dehydration and rewetting: The role of light and temperature sensing.

Cyanobacteria inhabiting desert biological soil crusts must prepare towards dehydration, or their revival after rewetting is severely impaired. The mechanisms involved are unknown but signalling of forthcoming dehydration by dawn illumination was demonstrated. Accurate and reproducible simulation of desert conditions enabled examination of physiological activities and transcript profiles in a model organism, Leptolyngbya ohadii, in response to specific conditions. Exposure to far red light or lack of ground warming during dawn severely reduced revival after rewetting and altered the network of gene expression. The data implicated phytochromes in light and temperature sensing. Many genes were up- or down-regulated before water content decline, while others were strongly affected by the progression of dehydration and desiccation. Transcription continues during the desiccated phase but only barely during early rewetting, although photosynthetic activity was regained. Application of rifampicin with or without a preceding dehydration phase demonstrated that RNA is stabilized/protected during desiccation, possibly by intrinsically disordered proteins. We conclude that increasing light and temperature at dawn activates a network of genes that prepare the cells towards dehydration. Quick resumption of photosynthesis upon rewetting in contrast to the slow change in the transcript profile suggested that in addition to preparing towards dehydration the cells also prepare for forthcoming rewetting, during dehydration. Unravelling the presently unknown function of many responding genes will help to clarify the networks involved.

Dawn illumination prepares desert cyanobacteria for dehydration.

Desert biological soil crusts (BSC), among the harshest environments on Earth, are formed by the adhesion of soil particles to polysaccharides excreted mainly by filamentous cyanobacteria (see [1] and references therein). These species are the main primary producers in this habitat where they cope with various stressors including frequent hydration-dehydration cycles. Water is mainly provided as early-morning dew, followed by dehydration with rising temperatures and declining relative humidity. Earlier studies focused on community structure and cyanobacterial activities in various BSCs [1,2]. They identified genes present in dehydration-tolerant, but not -sensitive cyanobacteria [3], and suggested that abiotic conditions during natural dehydration (Figure 1A) are critical for the recovery upon rewetting. Inability of Leptolyngbya ohadii, which is abundant in the BSC examined here, to recover after rapid desiccation (Figure 1B) [4] suggested that the cells must prepare themselves toward forthcoming dehydration, but the nature of the signal involved was unknown. We show here that the rising dawn illumination, perceived by photo-sensors, serves as the signal inciting BSC-inhabiting cyanobacteria to prepare for forthcoming dehydration. L. ohadii filaments were exposed to simulated natural conditions from the morning of October 14th 2009, using our environmental chamber that enables accurate reproduction of BSC environment [4] (Supplemental Figure S1A). Samples were withdrawn at specific time points (Figure 1A), followed by RNA extraction and global transcript profiling (accession PRJNA391854). Four hours of dehydration led to up-regulation of 567 genes and down-regulation of 1597 (by more than 2-fold). Since BSC-inhabiting organisms have not been used as genetic models, the functions of 3258 (43.5% of the 7487 L. ohadii genes [3]) are unknown. Nevertheless, a pronounced rise in transcript levels of genes involved in carbon metabolism, transport, osmolyte production, energy dissipation and other cellular activities was observed. On the other hand, a declining transcript abundance for genes involved in light harvesting, photosynthetic metabolism, protein biosynthesis, cell division and other pathways was detected. The analysis unraveled clear distinctions between early- and late-responding genes. Supplemental Table S1 lists the 40 strongest differentially expressed genes verified by RT-qPCR and used in further analyses.

What distinguishes cyanobacteria able to revive after desiccation from those that cannot: the genome aspect.

Filamentous cyanobacteria are the main founders and primary producers in biological desert soil crusts (BSCs) and are likely equipped to cope with one of the harshest environmental conditions on earth including daily hydration/dehydration cycles, high irradiance and extreme temperatures. Here, we resolved and report on the genome sequence of Leptolyngbya ohadii, an important constituent of the BSC. Comparative genomics identified a set of genes present in desiccation-tolerant but not in dehydration-sensitive cyanobacteria. RT qPCR analyses showed that the transcript abundance of many of them is upregulated during desiccation in L. ohadii. In addition, we identified genes where the orthologs detected in desiccation-tolerant cyanobacteria differs substantially from that found in desiccation-sensitive cells. We present two examples, treS and fbpA (encoding trehalose synthase and fructose 1,6-bisphosphate aldolase respectively) where, in addition to the orthologs present in the desiccation-sensitive strains, the resistant cyanobacteria also possess genes with different predicted structures. We show that in both cases the two orthologs are transcribed during controlled dehydration of L. ohadii and discuss the genetic basis for the acclimation of cyanobacteria to the desiccation conditions in desert BSC.

Towards clarifying what distinguishes cyanobacteria able to resurrect after desiccation from those that cannot: The photosynthetic aspect.

Organisms inhabiting biological soil crusts (BSCs) are able to cope with extreme environmental conditions including daily hydration/dehydration cycles, high irradiance and extreme temperatures. The photosynthetic machinery, potentially the main source of damaging reactive oxygen species during cessation of CO(2) fixation in desiccating cells, must be protected to avoid sustained photodamage. We compared certain photosynthetic parameters and the response to excess light of BCS-inhabiting, desiccation-tolerant cyanobacteria Leptolyngbya ohadii and Nostoc reinholdii with those observed in the "model" organisms Nostoc sp. PCC 7120, able to resurrect after mild desiccation, and Synechococcus elongatus PCC 7942 and Synechocystis sp. PCC 6803 that are unable to recover from dehydration. Desiccation-tolerant strains exhibited a transient decline in the photosynthetic rate at light intensities corresponding to the inflection point in the PI curve relating the O(2) evolution rate to light intensity. They also exhibited a faster and larger loss of variable fluorescence and profoundly faster Q(A)(-) re-oxidation rates after exposure to high illumination. Finally, a smaller difference was found in the temperature of maximal thermoluminescence signal in the absence or presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) than observed in "model" cyanobacteria. These parameters indicate specific functional differences of photosystem II (PSII) between desiccation tolerant and sensitive cyanobacteria. We propose that exposure to excess irradiation activates a non-radiative electron recombination route inside PSII that minimizes formation of damaging singlet oxygen in the desiccation-tolerant cyanobacteria and thereby reduces photodamage.

Simulated soil crust conditions in a chamber system provide new insights on cyanobacterial acclimation to desiccation.

Environmental research often faces two major hurdles: (i) fluctuating spatial and temporal conditions and consequently large variability in the organisms' abundance and performance, and (ii) complex, costly logistics involved in field experiments. Measurements of physiological parameters or molecular analyses often represent single shot experiments. To study desiccation acclimation of filamentous cyanobacteria, the founders and main primary producers in desert biological soil crusts (BSC), we constructed an environmental chamber that can reproducibly and accurately simulate ambient conditions and measure microorganism performance. We show that recovery from desiccation of BSC cyanobacteria and Leptolyngbya ohadii isolated thereof are strongly affected by dehydration rate following morning dew. This effect is most pronounced in cells exposed to high light and temperature in the dry phase. Simultaneous measurements of water content, gas exchange and fluorescence were performed during dehydration. Photosynthetic performance measured by fluorescence begins declining when light intensity reaches values above 100 µmol photons m(-2) s(-1), even in fully hydrated cells. In contrast, photosynthetic rates measured using O2 evolution and CO2 uptake increased during rising irradiance to the point where the water content declined below ∼ 50%. Thus, fluorescence cannot serve as a reliable measure of photosynthesis in desert cyanobacteria. The effects of drying on gas exchange are discussed.