Light and Dehydration but Not Temperature Drive Photosynthetic Adaptations of Basal Streptophytes (Hormidiella, Streptosarcina and Streptofilum) Living in Terrestrial Habitats.

Streptophyte algae are the ancestors of land plants, and several classes contain taxa that are adapted to an aero-terrestrial lifestyle. In this study, four basal terrestrial streptophytes from the class Klebsormidiophyceae, including Hormidiella parvula; two species of the newly described genus Streptosarcina (S. costaricana and S. arenaria); and the newly described Streptofilum capillatum were investigated for their responses to radiation, desiccation and temperature stress conditions. All the strains showed low-light adaptation (Ik < 70 µmol photons m-2 s-1) but differed in photoprotective capacities (such as non-photochemical quenching). Acclimation to enhanced photon fluence rates (160 µmol photons m-2 s-1) increased photosynthetic performance in H. parvula and S. costaricana but not in S. arenaria, showing that low-light adaptation is a constitutive trait for S. arenaria. This lower-light adaptation of S. arenaria was coupled with a higher desiccation tolerance, providing further evidence that dehydration is a selective force shaping species occurrence in low light. For protection against ultraviolet radiation, all species synthesised and accumulated different amounts of mycosporine-like amino acids (MAAs). Biochemically, MAAs synthesised by Hormidiella and Streptosarcina were similar to MAAs from closely related Klebsormidium spp. but differed in retention time and spectral characteristics in S. capillatum. Unlike the different radiation and dehydration tolerances, Hormidiella, Streptosarcina and Streptofilum displayed preferences for similar thermal conditions. These species showed a temperature dependence of photosynthesis similar to respiration, contrasting with Klebsormidium spp. and highlighting an interspecific diversity in thermal requirements, which could regulate species distributions under temperature changes.

Formation of lipid bodies and changes in fatty acid composition upon pre-akinete formation in Arctic and Antarctic Zygnema (Zygnematophyceae, Streptophyta) strains.

Filamentous green algae of the genus Zygnema (Zygnematophyceae, Streptophyta) are key components of polar hydro-terrestrial mats where they face various stressors including UV irradiation, freezing, desiccation and osmotic stress. Their vegetative cells can develop into pre-akinetes, i.e. reserve-rich, mature cells. We investigated lipid accumulation and fatty acid (FA) composition upon pre-akinete formation in an Arctic and an Antarctic Zygnema strain using transmission electron microscopy and gas chromatography coupled with mass spectrometry. Pre-akinetes formed after 9 weeks of cultivation in nitrogen-free medium, which was accompanied by massive accumulation of lipid bodies. The composition of FAs was similar in both strains, and α-linolenic acid (C18:3) dominated in young vegetative cells. Pre-akinete formation coincided with a significant change in FA composition. Oleic (C18:1) and linoleic (C18:2) acid increased the most (up to 17- and 8-fold, respectively). Small amounts of long-chain polyunsaturated FAs were also detected, e.g. arachidonic (C20:4) and eicosapentaenoic (C20:5) acid. Pre-akinetes exposed to desiccation at 86% relative humidity were able to recover maximum quantum yield of photosystem II, but desiccation had no major effect on FA composition. The results are discussed with regard to the capability of Zygnema spp. to thrive in extreme conditions.

Evaluating the combined effects of pretilachlor and UV-B on two Azolla species.

The present study assessed the comparative responses of two agronomic species of Azolla (A.microphylla and A. pinnata) exposed to man-made and natural stressors by evaluating biomass accumulation, pigments (chlorophyll a and b and carotenoid contents), photosynthetic activity and nitrogen metabolism. The study was carried out in field where two species of Azolla were cultured and treated with various concentrations (5, 10 and 20 µg ml(-1)) of herbicide; pretilachlor [2-chloro-2,6-diethyl-N-(2-propoxyethyl) acetanilide] and enhanced levels (UV-B1: ambient +2.2 kJ m(-2) day(-1) and UV-B2: ambient +4.4 kJ m(-2) day(-1)) of UV-B, alone as well as in combination. Biomass accumulation, photosynthetic pigments; chlorophyll a, b and carotenoids, photosynthetic oxygen yield and photosynthetic electron transport activities i.e. photosystem II (PS II) and photosystem I (PS I) in both the species declined with the increasing doses of pretilachlor and UV-B radiation, which further declined when applied in combination. The lower doses (5 and 10 µg ml(-1)) of pretilachlor and UV-B (UV-B1 and UV-B2) alone, damaged mainly the oxidation side of PS II, whereas higher dose (20 µg ml(-1)) of pretilachlor alone and in combination with UV-B1 and UV-B2 caused damage to PS II reaction centre and beyond this towards the reduction side. A significant enhancement in respiration was also noticed in fronds of both the Azolla species following pretilachlor and UV-B treatment, hence indicating strong damaging effect. The nitrate assimilating enzymes - nitrate reductase and nitrite reductase and ammonium assimilating enzymes - glutamine synthetase and glutamate synthase were also severely affected when treated either with pretilachlor and/or UV-B while glutamate dehydrogenase exhibited a stimulatory response. The study suggests that both the species of Azolla showed considerable damage under pretilachlor and UV-B treatments alone, however, in combination the effect was more intense. Further, in comparison to A. pinnata, A. microphylla exhibited greater resistance against tested doses of both the stresses, either alone or in combination.

Living in biological soil crust communities of African deserts-Physiological traits of green algal Klebsormidium species (Streptophyta) to cope with desiccation, light and temperature gradients.

Green algae of the genus Klebsormidium (Klebsormidiales, Streptophyta) are typical members of biological soil crusts (BSCs) worldwide. The phylogeny and ecophysiology of Klebsormidium has been intensively studied in recent years, and a new lineage called superclade G, which was isolated from BSCs in arid southern Africa and comprising undescribed species, was reported. Three different African strains, that have previously been isolated from hot-desert BSCs and molecular-taxonomically characterized, were comparatively investigated. In addition, Klebsormidium subtilissimum from a cold-desert habitat (Alaska, USA, superclade E) was included in the study as well. Photosynthetic performance was measured under different controlled abiotic conditions, including dehydration and rehydration, as well as under a light and temperature gradient. All Klebsormidium strains exhibited optimum photosynthetic oxygen production at low photon fluence rates, but with no indication of photoinhibition under high light conditions pointing to flexible acclimation mechanisms of the photosynthetic apparatus. Respiration under lower temperatures was generally much less effective than photosynthesis, while the opposite was true for higher temperatures. The Klebsormidium strains tested showed a decrease and inhibition of the effective quantum yield during desiccation, however with different kinetics. While the single celled and small filamentous strains exhibited relatively fast inhibition, the uniserate filament forming isolates desiccated slower. Except one, all other strains fully recovered effective quantum yield after rehydration. The presented data provide an explanation for the regular occurrence of Klebsormidium strains or species in hot and cold deserts, which are characterized by low water availability and other stressful conditions.

Photosynthesis in extreme environments: responses to different light regimes in the Antarctic alga Koliella antarctica.

Antarctic algae play a fundamental role in polar ecosystem thanks to their ability to grow in an extreme environment characterized by low temperatures and variable illumination. Here, for prolonged periods, irradiation is extremely low and algae must be able to harvest light as efficiently as possible. On the other side, at low temperatures even dim irradiances can saturate photosynthesis and drive to the formation of reactive oxygen species. Colonization of this extreme environment necessarily required the optimization of photosynthesis regulation mechanisms by algal organisms. In order to investigate these adaptations we analyzed the time course of physiological and morphological responses to different irradiances in Koliella antarctica, a green microalga isolated from Ross Sea (Antarctica). Koliella antarctica not only modulates cell morphology and composition of its photosynthetic apparatus on a long-term acclimation, but also shows the ability of a very fast response to light fluctuations. Koliella antarctica controls the activity of two xanthophyll cycles. The first, involving lutein epoxide and lutein, may be important for the growth under very low irradiances. The second, involving conversion of violaxanthin to antheraxanthin and zeaxanthin, is relevant to induce a fast and particularly strong non-photochemical quenching, when the alga is exposed to higher light intensities. Globally K. antarctica thus shows the ability to activate a palette of responses of the photosynthetic apparatus optimized for survival in its natural extreme environment.

Ultrastructure of Cosmarium strains (Zygnematophyceae, Streptophyta) collected from various geographic locations shows species-specific differences both at optimal and stress temperatures.

Plant species collected from various climatic zones and stressed in vitro at various temperatures reveal changes in cellular ultrastructure which are in accordance with the climate at their sampling sites. This observation initiated the investigation to establish if stress at different temperatures may cause diverse extents of changes in the ultrastructure of microalgal strains originating from different geographic zones. The study revealed that the six Cosmarium strains demonstrated ultrastructural characteristics that were consistent with their source location under optimal, low and high temperature conditions, pointing to their preference to specific climatic niches. Interestingly, chloroplasts of all of the Cosmarium strains correspond to a sun-adapted type, which is concomitant with earlier statements that these strains are rendered as high-light adapted algae. The Cosmarium strains developed multiple ultrastructural responses which enabled them to cope with excessive temperatures, occasionally occurring in desmid natural habitats. The appearance of cubic membranes and increased number of plastoglobules may represent the first line in protection against high-temperature stress, which is accompanied by the alteration of protein synthesis and the appearance of stress granules in order to preserve cell homeostasis. However, the prolonged warm- or cold-temperature stress obviously initiated the programmed cell death, as concluded from the appearance of several ultrastructural features observed in all of the Cosmarium strains. The fair acclimation possibilities and the ability to undergo programmed cell death in order to save the population, certainly favor the cosmopolitan distribution of the genus Cosmarium.

UV-induced effects on growth, photosynthetic performance and sunscreen contents in different populations of the green alga Klebsormidium fluitans (Streptophyta) from alpine soil crusts.

Members of the green algal genus Klebsormidium (Klebsormidiales, Streptophyta) are typical components of biological soil crust communities worldwide, which exert important ecological functions. Klebsormidium fluitans (F. Gay) Lokhorst was isolated from an aeroterrestrial biofilm as well as from four different biological soil crusts along an elevational gradient between 600 and 2350 m in the Tyrolean and South Tyrolean Alps (Austria, Italy), which are characterised by seasonally high solar radiation. Since the UVtolerance of Klebsormidium has not been studied in detail, an ecophysiological and biochemical study was applied. The effects of controlled artificial ultraviolet radiation (UVR; <9 W m(-2) UV-A, <0.5 W m(-2) UV-B) on growth, photosynthetic performance and the capability to synthesise mycosporine-like amino acids (MAAs) as potential sunscreen compounds were comparatively investigated to evaluate physiological plasticity and possible ecotypic differentiation within this Klebsormidium species. Already under control conditions, the isolates showed significantly different growth rates ranging from 0.42 to 0.74 µm day(-1). The UVR effects on growth were isolate specific, with only two strains affected by the UV treatments. Although all photosynthetic and respiratory data indicated strain-specific differences under control conditions, UV-A and UV-B treatment led only to rather minor effects. All physiological results clearly point to a high UV tolerance in the K. fluitans strains studied, which can be explained by their biochemical capability to synthesize and accumulate a putative MAA after exposure to UV-A and UV-B. Using HPLC, a UV-absorbing compound with an absorption maximum at 324 nm could be identified in all strains. The steady-state concentrations of this Klebsormidium MAA under control conditions ranged from 0.09 to 0.93 mg g(-1) dry weight (DW). While UV-A led to a slight stimulation of MAA accumulation, exposure to UV-B was accompanied by a strong but strain-specific increase of this compound (5.34-12.02 mg(-1) DW), thus supporting its function as UV sunscreen. Although ecotypic differences in the UVR response patterns of the five K. fluitans strains occurred, this did not correlate with the altitude of the respective sampling location. All data indicate a generally high UV tolerance which surely contributes to the aeroterrestrial lifestyle of K. fluitans in soil crusts of the alpine regions of the European Alps.

Species-specific responses to combined thermal-irradiance stress in microalgae--"each is to its own".

This article is a highlight of the study by Stamenkovic and Hanelt in this issue of Photochemistry and Photobiology describing the high-irradiance photophysiology of several strains of freshwater chlorophyte microalgal genus, Cosmarium. These strains exhibit distinct differences in how combined thermal- high irradiance stress is managed which can be related to the temperature and irradiance conditions of their native habitat.

Protection strategies of Cosmarium strains (Zygnematophyceae, Streptophyta) isolated from various geographic regions against excessive photosynthetically active radiation.

Numerous in vitro investigations have suggested that macroalgae exhibit regular geographic and depth distribution patterns in accordance with the light and temperature predominance at their habitats; however, there have been only a few similar studies concerning microalgae. We examined the potential influence of irradiance on patterns of distribution of four Cosmarium strains isolated from various climatic zones and cultured long term (>15 years) under a constant temperature-light regime. All the Cosmarium strains demonstrated physiological responses that were consistent with the light intensity prevailing at their source location, confirming that these responses are genetically preserved, as concluded from chlorophyll fluorescence and oxygen evolution rates measurements. Addition of inhibitors of chloroplast-encoded protein synthesis (chloramphenicol and streptomycin) and violaxanthin de-epoxidase (dithiothreitol) indicated that the Cosmarium strains developed "sun- or shade-plant" protection strategies, in accordance with the climate at their sampling sites. The polar Cosmarium strains exhibited a "shade-plant strategy"-to suffer some photoinhibition, but acquire increasing protection from photoinhibited PSII centers, whereas the tropical strains displayed a "sun-plant strategy"-to counteract photoinhibition of PSII by a high rate of repair of photoinhibited PSII reaction centers and a high xanthophyll cycle turnover.

Changes in phenolic compounds and cellular ultrastructure of arctic and antarctic strains of Zygnema (Zygnematophyceae, Streptophyta) after exposure to experimentally enhanced UV to PAR ratio.

Ultraviolet (UV) radiation has become an important stress factor in polar regions due to anthropogenically induced ozone depletion. Although extensive research has been conducted on adaptations of polar organisms to this stress factor, few studies have focused on semi-terrestrial algae so far, in spite of their apparent vulnerability. This study investigates the effect of UV on two semi-terrestrial arctic strains (B, G) and one Antarctic strain (E) of the green alga Zygnema, isolated from Arctic and Antarctic habitats. Isolates of Zygnema were exposed to experimentally enhanced UV A and B (predominant UV A) to photosynthetic active radiation (PAR) ratio. The pigment content, photosynthetic performance and ultrastructure were studied by means of high-performance liquid chromatography (HPLC), chlorophyll a fluorescence and transmission electron microscopy (TEM). In addition, phylogenetic relationships of the investigated strains were characterised using rbcL sequences, which determined that the Antarctic isolate (E) and one of the Arctic isolates (B) were closely related, while G is a distinct lineage. The production of protective phenolic compounds was confirmed in all of the tested strains by HPLC analysis for both controls and UV-exposed samples. Moreover, in strain E, the content of phenolics increased significantly (p = 0.001) after UV treatment. Simultaneously, the maximum quantum yield of photosystem II photochemistry significantly decreased in UV-exposed strains E and G (p < 0.001), showing a clear stress response. The phenolics were most probably stored at the cell periphery in vacuoles and cytoplasmic bodies that appear as electron-dense particles when observed by TEM after high-pressure freeze fixation. While two strains reacted moderately on UV exposure in their ultrastructure, in strain G, damage was found in chloroplasts and mitochondria. Plastidal pigments and xanthophyll cycle pigments were investigated by HPLC analysis; UV A- and UV B-exposed samples had a higher deepoxidation state as controls, particularly evident in strain B. The results indicate that phenolics are involved in UV protection of Zygnema and also revealed different responses to UV stress across the three strains, suggesting that other protection mechanisms may be involved in these organisms.

Snow ball earth and the split of Streptophyta and Chlorophyta.

About 700 million years ago (Mya), the ancestor of all green plants evolved into two major groups: the Chlorophyta (many green algae) and the Streptophyta (some green algae and land plants = embryophytes). Both groups are separated by several morphological, physiological, and molecular characteristics, including different photorespiration pathways. The Chloropyhta/Streptophyta split was probably very important for the colonization of the terrestrial habitat because embryophytes, the descendants of streptophyte algae, today completely dominate the macrophyte flora of the terrestrial habitats. By contrast, in aquatic ecosystems macrophytes from brown, red, and green algae compete with embryophytes. In this opinion article, I argue that the Chlorophyta/Streptophyta split is related to glaciation events (snow ball earth states) in the Neoproterozoic and provide an explanation for the different photorespiration pathways.

Evolution of the biochemistry of the photorespiratory C2 cycle.

Oxygenic photosynthesis would not be possible without photorespiration in the present day O2 -rich atmosphere. It is now generally accepted that cyanobacteria-like prokaryotes first evolved oxygenic photosynthesis, which was later conveyed via endosymbiosis into a eukaryotic host, which then gave rise to the different groups of algae and streptophytes. For photosynthetic CO2 fixation, all these organisms use RubisCO, which catalyses both the carboxylation and the oxygenation of ribulose 1,5-bisphosphate. One of the reaction products of the oxygenase reaction, 2-phosphoglycolate (2PG), represents the starting point of the photorespiratory C2 cycle, which is considered largely responsible for recapturing organic carbon via conversion to the Calvin-Benson cycle (CBC) intermediate 3-phosphoglycerate, thereby detoxifying critical intermediates. Here we discuss possible scenarios for the evolution of this process toward the well-defined 2PG metabolism in extant plants. While the origin of the C2 cycle core enzymes can be clearly dated back towards the different endosymbiotic events, the evolutionary scenario that allowed the compartmentalised high flux photorespiratory cycle is uncertain, but probably occurred early during the algal radiation. The change in atmospheric CO2 /O2 ratios promoting the acquisition of different modes for inorganic carbon concentration mechanisms, as well as the evolutionary specialisation of peroxisomes, clearly had a dramatic impact on further aspects of land plant photorespiration.

Amino acid compositional shifts during streptophyte transitions to terrestrial habitats.

Across the streptophyte lineage, which includes charophycean algae and embryophytic plants, there have been at least four independent transitions to the terrestrial habitat. One of these involved the evolution of embryophytes (bryophytes and tracheophytes) from a charophycean ancestor, while others involved the earliest branching lineages, containing the monotypic genera Mesostigma and Chlorokybus, and within the Klebsormidiales and Zygnematales lineages. To overcome heat, water stress, and increased exposure to ultraviolet radiation, which must have accompanied these transitions, adaptive mechanisms would have been required. During periods of dehydration and/or desiccation, proteomes struggle to maintain adequate cytoplasmic solute concentrations. The increased usage of charged amino acids (DEHKR) may be one way of maintaining protein hydration, while increased use of aromatic residues (FHWY) protects proteins and nucleic acids by absorbing damaging UV, with both groups of residues thought to be important for the stabilization of protein structures. To test these hypotheses we examined amino acid sequences of orthologous proteins representing both mitochondrion- and plastid-encoded proteomes across streptophytic lineages. We compared relative differences within categories of amino acid residues and found consistent patterns of amino acid compositional fluxuation in extra-membranous regions that correspond with episodes of terrestrialization: positive change in usage frequency for residues with charged side-chains, and aromatic residues of the light-capturing chloroplast proteomes. We also found a general decrease in the usage frequency of hydrophobic, aliphatic, and small residues. These results suggest that amino acid compositional shifts in extra-membrane regions of plastid and mitochondrial proteins may represent biochemical adaptations that allowed green plants to colonize the land.