Desiccation tolerance in the chlorophyte green alga Ulva compressa: does cell wall architecture contribute to ecological success?

The extracellular matrix of green algae

Green algae display a wide range of extracellular matrix (ECM) components that include various types of cell walls (CW), scales, crystalline glycoprotein coverings, hydrophobic compounds, and complex gels or mucilage. Recently, new information derived from genomic/transcriptomic screening, advanced biochemical analyses, immunocytochemical studies, and ecophysiology has significantly enhanced and refined our understanding of the green algal ECM. In the later diverging charophyte group of green algae, the CW and other ECM components provide insight into the evolution of plants and the ways the ECM modulates during environmental stress. Chlorophytes produce diverse ECM components, many of which have been exploited for various uses in medicine, food, and biofuel production. This review highlights major advances in ECM studies of green algae.

Temporal and spatial dynamics of tropical macroalgal contributions to blue carbon

Blue carbon ecosystems are a vital part of nature-based climate solutions due to their capacity to store and sequester carbon, but often exclude macroalgal beds even though they can form highly productive coastal ecosystems. Recent estimates of macroalgal contributions to global carbon sequestration are derived primarily from temperate kelp forests, while tropical macroalgal carbon stock in living biomass is still unclear. Here, using Singapore as a case study, we integrate field surveys and remote sensing data to estimate living macroalgal carbon stock. Results show that macroalgae in Singapore account for up to 650 Mg C biomass stock, which is greater than the aboveground carbon found in seagrass meadows but lower than that in mangrove forests. Ulva and Sargassum dominate macroalgal assemblages and biomass along the coast, with both genera exhibiting distinct spatio-temporal variation. The annual range of macroalgal biomass carbon is estimated to be 450 Mg C yr^-1, or 0.77 Mg C ha^-1 yr^-1. Noting the uncertainties of the fate of macroalgal biomass carbon, we estimate the potential sequestration rate and find that it is comparable to mature terrestrial ecosystems such as tropical grasslands and temperate forests. This study demonstrates that macroalgal seasonality allows for a consistent amount of biomass carbon to either be exported and eventually sequestered, or harvested for utilization on an annual basis. These findings on macroalgal growth patterns and their considerable contributions to tropical coastal carbon pool add to the growing support for macroalgae to be formally included in blue carbon assessments.

Physiological and multi-omics responses of Neoporphyra haitanensis to dehydration-rehydration cycles

BACKGROUND

Seaweeds in the upper intertidal zone experience extreme desiccation during low tide, followed by rapid rehydration during high tide. Porphyra sensu lato are typical upper intertidal seaweeds. Therefore, it is valuable to investigate the adaptive mechanisms of seaweed in response to dehydration-rehydration stress.

RESULTS

A reduction in photosynthetic capacity and cell shrinkage were observed when N. haitanensis was dehydrated, and such changes were ameliorated once rehydrated. And the rate and extent of rehydration were affected by the air flow speed, water content before rehydration, and storage temperature and time. Rapid dehydration at high air-flow speed and storage at - 20 °C with water content of 10% caused less damage to N. haitanensis and better-protected cell activity. Moreover, proteomic and metabolomic analyses revealed the abundance members of the differentially expressed proteins (DEPs) and differentially abundant metabolites (DAMs) mainly involved in antioxidant system and osmotic regulation. The ascorbic acid-glutathione coupled with polyamine antioxidant system was enhanced in the dehydration response of N. haitanensis. The increased soluble sugar content, the accumulated polyols, but hardly changed (iso)floridoside and insignificant amount of sucrose during dehydration indicated that polyols as energetically cheaper organic osmolytes might help resist desiccation. Interestingly, the recovery of DAMs and DEPs upon rehydration was fast.

CONCLUSIONS

Our research results revealed that rapid dehydration and storage at - 20 °C were beneficial for recovery of N. haitanensis. And the strategy to resist dehydration was strongly directed toward antioxidant activation and osmotic regulation. This work provided valuable insights into physiological changes and adaptative mechanism in desiccation, which can be applied for seaweed farming.

Terrestrial Green Algae Show Higher Tolerance to Dehydration than Do Their Aquatic Sister-Species

Diverse algae possess the ability to recover from extreme desiccation without forming specialized resting structures. Green algal genera such as Tetradesmus (Sphaeropleales, Chlorophyceae) contain temperate terrestrial, desert, and aquatic species, providing an opportunity to compare physiological traits associated with the transition to land in closely related taxa. We subjected six species from distinct habitats to three dehydration treatments varying in relative humidity (RH 5%, 65%, 80%) followed by short- and long-term rehydration. We tested the capacity of the algae to recover from dehydration using the effective quantum yield of photosystem II as a proxy for physiological activity. The degree of recovery was dependent both on the habitat of origin and the dehydration scenario, with terrestrial, but not aquatic, species recovering from dehydration. Distinct strains of each species responded similarly to dehydration and rehydration, with the exception of one aquatic strain that recovered from the mildest dehydration treatment. Cell ultrastructure was uniformly maintained in both aquatic and desert species during dehydration and rehydration, but staining with an amphiphilic styryl dye indicated damage to the plasma membrane from osmotically induced water loss in the aquatic species. These analyses demonstrate that terrestrial Tetradesmus possess a vegetative desiccation tolerance phenotype, making these species ideal for comparative omics studies.

Comparative Transcriptome Analysis Provides Insights into Response of Ulva compressa to Fluctuating Salinity Conditions

Ulva compressa, a green tide-forming species, can adapt to hypo-salinity conditions, such as estuaries and brackish lakes. To understand the underlying molecular mechanisms of hypo-salinity stress tolerance, transcriptome-wide gene expression profiles in U. compressa were created using digital gene expression profiles. The RNA-seq data were analyzed based on the comparison of differently expressed genes involved in specific pathways under hypo-salinity and recovery conditions. The up-regulation of genes in photosynthesis and glycolysis pathways may contribute to the recovery of photosynthesis and energy metabolism, which could provide sufficient energy for the tolerance under long-term hyposaline stress. Multiple strategies, such as ion transportation and osmolytes metabolism, were performed to maintain the osmotic homeostasis. Additionally, several long noncoding RNA were differently expressed during the stress, which could play important roles in the osmotolerance. Our work will serve as an essential foundation for the understanding of the tolerance mechanism of U. compressa under the fluctuating salinity conditions.

Seaweed cellulose scaffolds derived from green macroalgae for tissue engineering

Extracellular matrix (ECM) provides structural support for cell growth, attachments and proliferation, which greatly impact cell fate. Marine macroalgae species Ulva sp. and Cladophora sp. were selected for their structural variations, porous and fibrous respectively, and evaluated as alternative ECM candidates. Decellularization-recellularization approach was used to fabricate seaweed cellulose-based scaffolds for in-vitro mammalian cell growth. Both scaffolds were confirmed nontoxic to fibroblasts, indicated by high viability for up to 40 days in culture. Each seaweed cellulose structure demonstrated distinct impact on cell behavior and proliferation rates. The Cladophora sp. scaffold promoted elongated cells spreading along its fibers' axis, and a gradual linear cell growth, while the Ulva sp. porous surface, facilitated rapid cell growth in all directions, reaching saturation at week 3. As such, seaweed-cellulose is an environmentally, biocompatible novel biomaterial, with structural variations that hold a great potential for diverse biomedical applications, while promoting aquaculture and ecological agenda.

An individual alginate lyase is effective in the disruption of Laminaria digitata recalcitrant cell wall

In the present study, 199 pre-selected Carbohydrate-Active enZymes (CAZymes) and sulfatases were assessed, either alone or in combination, to evaluate their capacity to disrupt Laminaria digitata cell wall, with the consequent release of interesting nutritional compounds. A previously characterized individual alginate lyase, belonging to the family 7 of polysaccharide lyases (PL7) and produced by Saccharophagus degradans, was shown to be the most efficient in the in vitro degradation of L. digitata cell wall. The alginate lyase treatment, compared to the control, released up to 7.11 g/L of reducing sugars (p < 0.001) and 8.59 mmol/100 g dried alga of monosaccharides (p < 0.001), and reduced cell wall fluorescence intensity by 39.1% after staining with Calcofluor White (p = 0.001). The hydrolysis of gel-forming polymer alginate by the alginate lyase treatment could prevent the trapping of fatty acids and release beneficial monounsaturated fatty acids, particularly 18:1c9 (p < 0.001), to the extracellular medium. However, no liberation of proteins (p > 0.170) or pigments (p > 0.070) was observed. Overall, these results show the ability of an individual alginate lyase, from PL7 family, to partially degrade L. digitata cell wall under physiological conditions. Therefore, this CAZyme can potentially improve the bioavailability of L. digitata bioactive compounds for monogastric diets, with further application in feed industry.

Advances in Understanding of Desiccation Tolerance of Lichens and Lichen-Forming Algae

Lichens are symbiotic associations (holobionts) established between fungi (mycobionts) and certain groups of cyanobacteria or unicellular green algae (photobionts). This symbiotic association has been essential in the colonization of terrestrial dry habitats. Lichens possess key mechanisms involved in desiccation tolerance (DT) that are constitutively present such as high amounts of polyols, LEA proteins, HSPs, a powerful antioxidant system, thylakoidal oligogalactolipids, etc. This strategy allows them to be always ready to survive drastic changes in their water content. However, several studies indicate that at least some protective mechanisms require a minimal time to be induced, such as the induction of the antioxidant system, the activation of non-photochemical quenching including the de-epoxidation of violaxanthin to zeaxanthin, lipid membrane remodeling, changes in the proportions of polyols, ultrastructural changes, marked polysaccharide remodeling of the cell wall, etc. Although DT in lichens is achieved mainly through constitutive mechanisms, the induction of protection mechanisms might allow them to face desiccation stress in a better condition. The proportion and relevance of constitutive and inducible DT mechanisms seem to be related to the ecology at which lichens are adapted to.

X-ray Fluorescence Techniques in Determining the Habitat Preferences of Species-Ulva pilifera (Ulvales, Chlorophyta) from Montenegro Case Study

The paper presents four new sites where bright green Ulva thalli were found inhabiting freshwater (a river, a ditch, the Milet Canal) and marine (on the rocky shore of the Adriatic Sea) habitats in Montenegro. The aims of this study were to determine, for the first time, whether specimens of Ulva pilifera collected in Montenegro are phylogenetically and morphologically the same species as the one occurring in Europe. Using total reflection X-ray fluorescence (TXRF) and wavelength dispersive X-ray fluorescence (WDXRF) techniques it assessed the elemental composition of their thalli and its influence to colonise new habitats. Elements: Al, As, Ba, Br, Ca, Cl, Cr, Cu, Fe, Hf, I, K, Mg, Mn, Na, Ni, P, Pb, Rb, S, Si, Sr, Ti, V, and Zn were determined. The highest elemental concentrations were found for Ca = 16.3% (using WDXRF) and for Sr = 292 ppm (using TXRF) in the Ulva thalli. Ulva pilifera analysed from Montenegro, based on classical morphological methods and molecular techniques, are closely related to the same species from inland and coastal waters throughout Europe. The analysis of trace elements showed that the metal content in Ulva thalli is correlated with the trace elements in water and sediments. Ulva pilifera fits numerous features that make it one of the bioindicators of marine pollution, thanks to its worldwide distribution and capacity to accumulate trace elements.

Callose deposition is essential for the completion of cytokinesis in the unicellular alga Penium margaritaceum

Cytokinesis in land plants involves the formation of a cell plate that develops into the new cell wall. Callose, a β-1,3 glucan, accumulates at later stages of cell plate development, presumably to stabilize this delicate membrane network during expansion. Cytokinetic callose is considered specific to multicellular plant species, because it has not been detected in unicellular algae. Here we present callose at the cytokinesis junction of the unicellular charophyte, Penium margaritaceum Callose deposition at the division plane of P. margaritaceum showed distinct, spatiotemporal patterns likely representing distinct roles of this polymer in cytokinesis. Pharmacological inhibition of callose deposition by endosidin 7 resulted in cytokinesis defects, consistent with the essential role for this polymer in P. margaritaceum cell division. Cell wall deposition at the isthmus zone was also affected by the absence of callose, demonstrating the dynamic nature of new wall assembly in P. margaritaceum The identification of candidate callose synthase genes provides molecular evidence for callose biosynthesis in P. margaritaceum The evolutionary implications of cytokinetic callose in this unicellular zygnematopycean alga is discussed in the context of the conquest of land by plants.This article has an associated First Person interview with the first author of the paper.

Evo-physio: on stress responses and the earliest land plants

Embryophytes (land plants) can be found in almost any habitat on the Earth's surface. All of this ecologically diverse embryophytic flora arose from algae through a singular evolutionary event. Traits that were, by their nature, indispensable for the singular conquest of land by plants were those that are key for overcoming terrestrial stressors. Not surprisingly, the biology of land plant cells is shaped by a core signaling network that connects environmental cues, such as stressors, to the appropriate responses-which, thus, modulate growth and physiology. When did this network emerge? Was it already present when plant terrestrialization was in its infancy? A comparative approach between land plants and their algal relatives, the streptophyte algae, allows us to tackle such questions and resolve parts of the biology of the earliest land plants. Exploring the biology of the earliest land plants might shed light on exactly how they overcame the challenges of terrestrialization. Here, we outline the approaches and rationale underlying comparative analyses towards inferring the genetic toolkit for the stress response that aided the earliest land plants in their conquest of land.

Effects of air exposure on desiccation and photosynthetic performance of Cymodocea nodosa with and without epiphytes and Ulva rigida in comparison, under laboratory conditions

The seagrass Cymodocea nodosa and the chlorophyte Ulva rigida growing in the upper sublittoral zones of the Mediterranean Sea are exposed to air during low tides. We compared the desiccation coefficient (k), and the photosynthetic performance (ΔF/Fm') of C. nodosa leaves with and without epiphytes to that of U. rigida. The recovery ability of these two species was assessed by ΔF/Fm', after re-immersion. The desiccation coefficient (k) significantly (p < 0.01) varied among the three macrophyte materials, while the lowest k values were measured both in C. nodosa leaves with epiphytes and U. rigida. ΔF/Fm' significantly declined with decreasing RWC for all materials, while significant differences in the physiological response were observed between U. rigida and the other two materials. Thallus pieces of U. rigida showed higher desiccation tolerance compared to C. nodosa leaves, with its ΔF/Fm' under 75% degree of dehydration reaching to its initial values thirty (30) minutes after re-immersion, when C. nodosa only reached the 50%. This study provides valuable information on possible changes in the two species distribution under different desiccation scenarios.

Sieve elements rapidly develop 'nacreous walls' following injury - a common wounding response?

Thick glistening cell walls occur in sieve tubes of all major land plant taxa. Historically, these 'nacreous walls' have been considered a diagnostic feature of sieve elements; they represent a conundrum, though, in the context of the widely accepted pressure-flow theory as they severely constrict sieve tubes. We employed the cucurbit Gerrardanthus macrorhizus as a model to study nacreous walls in sieve elements by standard and in situ confocal microscopy and electron microscopy, focusing on changes in functional sieve tubes that occur when prepared for microscopic observation. Over 90% of sieve elements in tissue sections processed for microscopy by standard methods exhibit nacreous walls. Sieve elements in whole, live plants that were actively transporting as shown by phloem-mobile tracers, lacked nacreous walls and exhibited open lumina of circular cross-sections instead, an appropriate structure for Münch-type mass flow of the cell contents. Puncturing of transporting sieve elements with micropipettes triggered the rapid (<1 min) development of nacreous walls that occluded the cell lumen almost completely. We conclude that nacreous walls are preparation artefacts rather than structural features of transporting sieve elements. Nacreous walls in land plants resemble the reversibly swellable walls found in various algae, suggesting that they may function in turgor buffering, the amelioration of osmotic stress, wounding-induced sieve tube occlusion, and possibly local defence responses of the phloem.

Insight into transketolase of Pyropia haitanensis under desiccation stress based on integrative analysis of omics and transformation

BACKGROUND

Pyropia haitanensis, distributes in the intertidal zone, can tolerate water losses exceeding 90%. However, the mechanisms enabling P. haitanensis to survive harsh conditions remain uncharacterized. To elucidate the mechanism underlying P. haitanensis desiccation tolerance, we completed an integrated analysis of its transcriptome and proteome as well as transgenic Chlamydomonas reinhardtii carrying a P. haitanensis gene.

RESULTS

P. haitanensis rapidly adjusted its physiological activities to compensate for water losses up to 60%, after which, photosynthesis, antioxidant systems, chaperones, and cytoskeleton were activated to response to severe desiccation stress. The integrative analysis suggested that transketolase (TKL) was affected by all desiccation treatments. Transgenic C. reinhardtii cells overexpressed PhTKL grew better than the wild-type cells in response to osmotic stress.

CONCLUSION

P. haitanensis quickly establishes acclimatory homeostasis regarding its transcriptome and proteome to ensure its thalli can recover after being rehydrated. Additionally, PhTKL is vital for P. haitanensis desiccation tolerance. The present data may provide new insights for the breeding of algae and plants exhibiting enhanced desiccation tolerance.

Quantitative assessment of photosynthetic activity of Chlorella (Class Trebouxiophyceae) adsorbed onto soil by using fluorescence imaging

In the present study, we evaluate our previously developed non-destructive soil algal toxicity method using species from a different class of algae; Class Trebouxiophyceae (Chlorella vulgaris and Chlorella sorokiniana), and directly measure the photosynthetic activity of these species adsorbed onto the soil as a new toxicity endpoint. This study shows that non-destructive soil algal toxicity method is applicable to non-specific test species, including those of Class Trebouxiophyceae as well as Class Chlorophyceae (Chlorococcum infusionum and Chlamydomonas reinhardtii). Furthermore, by performing photosynthesis image analysis, we verify that it is possible to measure the photosynthetic activity of soil algae Chlorella vulgaris adsorbed onto soils without the need to extract algal cells from the soil. We propose that the non-destructive soil algal toxicity method represents a novel technique for 1) evaluating pollutants in soil using non-specific algae and 2) conveniently and rapidly assessing the photosynthetic activity of soil algae Chlorella vulgaris adsorbed onto soil as a new toxicity endpoint.

Interaction of Photoprotective and Acclimation Mechanisms in Ulva rigida (Chlorophyta) in Response to Diurnal Changes in Solar Radiation in Southern Chile

Species of the genus Ulva (Chlorophyta) are regarded as opportunistic organisms, which efficiently adjust their metabolism to the prevailing environmental conditions. In this study, changes in chlorophyll-a fluorescence-based photoinhibition of photosynthesis, electron transport rates, photosynthetic pigments, lipid peroxidation, total phenolic compounds, and antioxidant metabolism were investigated during a diurnal cycle of natural solar radiation in summer (for 12 h) under two treatments: photosynthetically active radiation (PAR: 400-700 nm) and PAR+ ultraviolet (UV) radiation (280-700 nm). In the presence of PAR alone, Ulva rigida showed dynamic photoinhibition, and photosynthetic parameters and pigment concentrations decreased with the intensification of the radiation. On the other hand, under PAR+UV conditions a substantial decline up to 43% was detected and an incomplete fluorescence recovery, also, P-I curve values remained low in relation to the initial condition. The phenolic compounds increased their concentration only in UV radiation treatments without showing a correlation with the antioxidant activity. The enzimatic activity of superoxide dismutase (SOD) and ascorbate peroxidase (APX) increased over 2-fold respect at initial values during the onset of light intensity. In contrast, catalase (CAT) increased its activity rapidly in response to the radiation stress to reach maxima at 10 a.m. and decreasing during solar. The present study suggests that U. rigida is capable of acclimating to natural radiation stress relies on a concerted action of various physiological mechanisms that act at different times of the day and under different levels of environmental stress.

Species composition, diversity, and distribution of the genus Ulva along the coast of Jeju Island, Korea based on molecular phylogenetic analysis

Species diversity in the genus Ulva remains understudied worldwide. Using molecular analyses we investigated the species composition, diversity, distribution, and relative frequencies of the genus Ulva along the entire coast of Jeju Island, off the southern tip of Korea. Species identification was performed for 215 samples collected from 23 sites, based on comprehensive phylogenetic and model-based species delimitation analyses using the sequences of two molecular markers, chloroplast elongation factor Tu (tufA) and nuclear rDNA internal transcribed spacer (ITS). We identified 193 specimens as nine Ulva species, 14 specimens as Blidingia spp., and eight samples undetermined, based on the combined analysis of tufA and ITS phylogenies. Two model-based approaches generally supported nine groups of Ulva species. Previously documented species complex, such as U. ohnoi-U. spinulosa and U. procera-U. linza showed discordant relationships between the two phylogenies. The occurrence of U. torta on Jeju Island was first observed, despite its existence on the mainland previously reported. Ulva australis [16 of 23 sites; 34.4% (relative frequency)], U. ohnoi (16; 21.9%), and U. procera (11; 14%) were found to be the predominant species. Our study highlights that molecular analysis is critical for species delimitation in the genus Ulva and provides fundamental information for an understanding of green-tide assemblages on the "biological hotspot" coastal ecosystem, Jeju Island in Korea. This study will also help to monitor and manage local green tides at the areas that are currently encountering rapid climate changes.

A Supercomplex, of Approximately 720 kDa and Composed of Both Photosystem Reaction Centers, Dissipates Excess Energy by PSI in Green Macroalgae Under Salt Stress

The thylakoid membranes of plants play a critical role in electron transfer and energy fixation, and are highly dynamic. So far, studies on the thylakoid membranes have mainly focused on microalgae and higher plants, yet very little information is available on the macroalgal thylakoids. Here, we studied the structure and organization of the thylakoid membranes in Ulva prolifera, a representative species of the green macroalgae. We found that U. prolifera had few but long loosely stacked membranes which lack the conventional grana found in higher plants. However, the thylakoid membrane complexes demonstrate lateral heterogeneity. Moreover, we found a supercomplex composed of PSII, light-harvesting complex II (LHCII) and PSI from U. prolifera under salt stress. The supercomplex is approximately 720 kDa, and includes the two important photoprotection proteins, the PSII S subunit (PsbS) and the light-harvesting complex stress-related protein (LhcSR), as well as xanthophyll cycle pigments (violaxanthin, antheraxanthin and zeaxanthin). Time-resolved fluorescence analysis suggested that, in the supercomplex, excitation energy could efficiently be transferred from PSII to PSI, even when PSII was inhibited, a function which disappeared when the supercomplex was incubated in mild detergent. We suggest that the supercomplex might be an important mechanism to dissipate excess energy by PSI in green macroalgae under salt stress.

Land-based drip-irrigated culture of Ulva compressa: The effect of culture platform design and nutrient concentration on biomass production and protein content

This work developed a laboratory prototype methodology for cost-effective, water-sparing drip-irrigation of seaweeds, as a model for larger-scale, on-land commercial units, which we envision as semi-automated, inexpensive polyethylene sheet-covered bow-framed greenhouses with sloping plastic covered floors, water-collecting sumps, and pumped recycling of culture media into overhead low-pressure drip emitters. Water droplets form on the continually wetted interior plastic surfaces of these types of greenhouses scattering incoming solar radiation to illuminate around and within the vertically-stacked culture platforms. Concentrated media formulations applied through foliar application optimize nutrient uptake by the seaweeds to improve growth and protein content of the cultured biomass. An additional attribute is that seaweed growth can be accelerated by addition of anthropogenic CO₂-containing industrial flue gases piped into the head-space of the greenhouse to reuse and recycle CO₂ into useful algal biomass. This demonstration tested three different drip culture platform designs (horizontal, vertical and slanted) and four increasing fertilizer media concentrations (in seawater) for growth, areal productivity, and thallus protein content of wild-collected Ulva compressa biomass, against fully-submerged controls. Cool White fluorescent lights provided 150–200 μmol photon m^-2 s^-1 illumination on a 12/12 hr day/night cycle. Interactive effects we tested using a four-level single factorial randomized block framework (p<0.05). Growth rates and biomass of the drip irrigation designs were 3–9% day^-1 and 5–18 g m^-2 day^-1 (d.w.) respectively, whereas the fully-submerged control group grew better at 8–11% per day with of 20–30 g m^-2 day^-1, indicating further optimization of the drip irrigation methodology is needed to improve growth and biomass production. Results demonstrated that protein content of Ulva biomass grown using the vertically-oriented drip culture platform and 2x fertilizer concentrations (42:16:36 N:P:K) was 27% d.w., approximating the similarly-fertilized control group. The drip methodology was found to significantly improve gas and nutrient mass transfer through the seaweed thalli, and overall, the labor- and-energy-saving methodology would use a calculated 20% of the seawater required for conventional on-land tank-based tumble culture.

A siphonous morphology affects light-harvesting modulation in the intertidal green macroalga Bryopsis corticulans (Ulvophyceae)

The macroalga Bryopsis corticulans relies on a sustained protective NPQ and a peculiar body architecture to efficiently adapt to the extreme light changes of intertidal shores. During low tides, intertidal algae experience prolonged high light stress. Efficient dissipation of excess light energy, measured as non-photochemical quenching (NPQ) of chlorophyll fluorescence, is therefore required to avoid photodamage. Light-harvesting regulation was studied in the intertidal macroalga Bryopsis corticulans, during high light and air exposure. Photosynthetic capacity and NPQ kinetics were assessed in different filament layers of the algal tufts and in intact chloroplasts to unravel the nature of NPQ in this siphonous green alga. We found that the morphology and pigment composition of the B. corticulans body provides functional segregation between surface sunlit filaments (protective state) and those that are underneath and undergo severe light attenuation (light-harvesting state). In the surface filaments, very high and sustained NPQ gradually formed. NPQ induction was triggered by the formation of transthylakoid proton gradient and independent of the xanthophyll cycle. PsbS and LHCSR proteins seem not to be active in the NPQ mechanism activated by this alga. Our results show that B. corticulans endures excess light energy pressure through a sustained protective NPQ, not related to photodamage, as revealed by the unusually quick restoration of photosystem II (PSII) function in the dark. This might suggest either the occurrence of transient PSII photoinactivation or a fast rate of PSII repair cycle.

Localisation and substrate specificities of transglycanases in charophyte algae relate to development and morphology

Cell wall-modifying enzymes have been previously investigated in charophyte green algae (CGA) in cultures of uniform age, giving limited insight into their roles. Therefore, we investigated the in situ localisation and specificity of enzymes acting on hemicelluloses in CGA genera of different morphologies and developmental stages. In vivo transglycosylation between xyloglucan and an endogenous donor in filamentous Klebsormidium and Zygnema was observed in longitudinal cell walls of young (1 month) but not old cells (1 year), suggesting that it has a role in cell growth. By contrast, in parenchymatous Chara, transglycanase action occurred in all cell planes. In Klebsormidium and Zygnema, the location of enzyme action mainly occurred in regions where xyloglucans and mannans, and to a lesser extent mixed-linkage β-glucan (MLG), were present, indicating predominantly xyloglucan:xyloglucan endotransglucosylase (XET) activity. Novel transglycosylation activities between xyloglucan and xylan, and xyloglucan and galactomannan were identified in vitro in both genera. Our results show that several cell wall-modifying enzymes are present in CGA, and that differences in morphology and cell age are related to enzyme localisation and specificity. This indicates an evolutionary significance of cell wall modifications, as similar changes are known in their immediate descendants, the land plants. This article has an associated First Person interview with the first author of the paper.

The terrestrial green macroalga Prasiola calophylla (Trebouxiophyceae, Chlorophyta): ecophysiological performance under water-limiting conditions

The phylogenetic placement of Prasiola calophylla, from an anthropogenic habitat previously shown to contain a novel UV sunscreen compound, was confirmed by analysis of its rbcL gene. This alga has the capacity to tolerate strong water-limiting conditions. The photosynthetic performance and ultrastructural changes under desiccation and osmotic stress were investigated. Freshly harvested thalli showed an effective quantum yield of PSII [Y(II)] of 0.52 ± 0.06 that decreased to ∼60% of the initial value at 3000 mM sorbitol, and 4000 mM sorbitol led to a complete loss of Y(II). The Y(II) of thalli exposed to controlled desiccating conditions at 60% relative humidity (RH) ceased within 240 min, whereas zero values were reached after 120 min at 20% RH. All investigated samples completely recovered Y(II) within ∼100 min after rehydration. Relative electron transport rates (rETR) were temperature dependent, increasing from 5, 10, to 25 °C but strongly declining at 45 °C. Transmission electron microscopy of samples desiccated for 2.5 h showed an electron dense appearance of the entire cytoplasm when compared to control samples. Thylakoid membranes were still visible in desiccated cells, corroborating the ability to recover. Control and desiccated cells contained numerous storage lipids and starch grains, providing reserves. Overall, P. calophylla showed a high capacity to cope with water-limiting conditions on a physiological and structural basis. A lipophilic outer layer of the cell walls might contribute to reduce water evaporation in this poikilohydric organism.

Increased anthropogenic pressure decreases species richness in tropical intertidal reefs

Multiple human stressors affect tropical intertidal sandstone reefs, but little is known about their biodiversity and the environmental impacts of these stressors. In the present study, multiple anthropogenic pressures were integrated using the relative environmental pressure index (REPI) and related to benthic community structure across an intertidal gradient in five sandstone reefs in the tropical South Atlantic coast. Greater species richness and diversity were noted in the low intertidal zones. There was a negative relationship between REPI and species richness, suggesting that increasing anthropogenic pressure has decreased benthic richness. The factors associated with the loss of richness were jetties built to control erosion, urban areas, beachfront kiosks and restaurants, fish markets, and storm sewers with illegal sewage connections. Our results highlight the need for better infrastructure planning and rigorous monitoring of coastal urban areas, since the large influence of multiple human pressures in these reefs leads to biodiversity losses.

Localization and Quantification of Callose in the Streptophyte Green Algae Zygnema and Klebsormidium: Correlation with Desiccation Tolerance

Freshwater green algae started to colonize terrestrial habitats about 460 million years ago, giving rise to the evolution of land plants. Today, several streptophyte green algae occur in aero-terrestrial habitats with unpredictable fluctuations in water availability, serving as ideal models for investigating desiccation tolerance. We tested the hypothesis that callose, a β-d-1,3-glucan, is incorporated specifically in strained areas of the cell wall due to cellular water loss, implicating a contribution to desiccation tolerance. In the early diverging genus Klebsormidium, callose was drastically increased already after 30 min of desiccation stress. Localization studies demonstrated an increase in callose in the undulating cross cell walls during cellular water loss, allowing a regulated shrinkage and expansion after rehydration. This correlates with a high desiccation tolerance demonstrated by a full recovery of the photosynthetic yield visualized at the subcellular level by Imaging-PAM. Furthermore, abundant callose in terminal cell walls might facilitate cell detachment to release dispersal units. In contrast, in the late diverging Zygnema, the callose content did not change upon desiccation for up to 3.5 h and was primarily localized in the corners between individual cells and at terminal cells. While these callose deposits still imply reduction of mechanical damage, the photosynthetic yield did not recover fully in the investigated young cultures of Zygnema upon rehydration. The abundance and specific localization of callose correlates with the higher desiccation tolerance in Klebsormidium when compared with Zygnema.

Introduction to desiccation biology: from old borders to new frontiers

A special issue reviews the recent progress made in our understanding of desiccation tolerance across various plant and animal kingdoms. It has been known for a long time that seeds can survive near absolute protoplasmic dehydration through air drying and complete germination upon rehydration because of their desiccation tolerance. This property is present both in prokaryotes and eukaryotes across all life kingdoms. These dry organisms suspend their metabolism when dry, are extremely tolerant to acute environmental stresses and are relatively stable during long periods of desiccation. Studies aiming at understanding the mechanisms of survival in the dry state have emerged during the past 40 years, moving from in vitro to genomic models and comparative genomics, and from a view that tolerance is an all-or-nothing phenomenon to a quantitative trait. With the prospect of global climate change, understanding the mechanisms of desiccation tolerance appears to be a promising avenue as a prelude to engineering crops for improved drought tolerance. Understanding desiccation is also useful for seed banks that rely on dehydration tolerance to preserve plant genetic resources in the form of these propagules. Articles in this special issue explore the recent progress in our understanding of desiccation tolerance, including the evolutionary mechanisms that have been adopted across various plant (algae, lichens, seeds, resurrection plants) and animal model systems (Caenorhabditis elegans, brine shrimp). We propose that the term desiccation biology defines the discipline dedicated to understand the desiccation tolerance in living organisms as well as the limits and time constraints thereof.