AP2/EREBP transcription factors are part of gene regulatory networks and integrate metabolic, hormonal and environmental signals in stress acclimation and retrograde signalling

Growth of five aeroterrestrial green algal strains (Trebouxiophyceae) in response to changing water availabilities—caused by osmotic (ionic) and matric (desiccation) stresses—was investigated in comparison with a freshwater and a marine strain. All investigated algae displayed good growth under brackish conditions while four out of the five aeroterrestrial strains even grew well under full marine conditions (28–40 psu). The comparison between growth responses in liquid medium, on solid agarose, and on glass fiber filters at 100% air humidity indicated a broad growth tolerance of aeroterrestrial algae towards diminished water availability. While two aeroterrestrial strains even grew better on solid medium which mimics natural biofilm conditions, the aquatic strains showed significant growth inhibition under matric stress. Except Stichococcus sp., which contained the C6-polyol sorbitol, all other aeroterrestrial green algae investigated synthesized and accumulated the C5-polyol ribitol in response to osmotic stress. Using ¹³C NMR spectroscopy and HPLC, it could be verified that ribitol functions as an osmotically regulated organic solute. This is the first proof of ribitol in free-living aeroterrestrial green algae. The biochemical capability to synthesize polyols under environmental stress conditions seems to support algal life outside aquatic habitats.     

宁夏沙坡头地区藻类及其分布

通过野外采样,室内培养、观察、鉴定,研究了沙坡头地区藻类多样性和分布特点,并为当地藻类资源利用提出建议。研究发现沙坡头地区藻类植物共计40种(包括1变型),其中蓝藻17种,绿藻10种,硅藻9种,裸藻4种,全为普生种,陆地生境中种类最丰富。亚气及陆地生境中蓝藻在种类数(除灌溉林地结皮)、生物量方面占主导地位;贫瘠水体生境中硅藻占主要地位,营养较丰富水体中绿藻处于优势地位。另外,陆生生境中多以具鞘微鞘藻Microcoleus vaginatus为优势种,小席藻Phormidium tenue为主要种。     

Biodiversity of green algae covering artificial hard substrate surfaces in a suburban environment: a case study using molecular approaches

In Middle European suburban environments green algae often cover open surfaces of artificial hard substrates. Microscopy reveals the Apatococcus/Desmococcus morphotype predominant over smaller coccoid forms. Adverse conditions such as limited water availability connected with high PAR and UV irradiance may narrow the algal diversity to a few specialists in these subaerial habitats. We used rRNA gene cloning/sequencing from both DNA extracts of the biofilms without culturing as well as cultures, for the unambiguous determination of the algal composition and to assess the algal diversity more comprehensively. The culture independent approach revealed mainly just two genera (Apatococcus, Trebouxia) for all study sites and five molecular operational taxonomic units (OTUs) for a particular study site, which based on microscopic observation was the one with the highest morphological diversity. The culture approach, however, revealed seven additional OTUs from five genera (Chloroidium, Coccomyxa, Coenochloris, Pabia, Klebsormidium) and an unidentified trebouxiophyte lineage for that same site; only two OTUs were shared by both approaches. Two OTUs or species were recovered for which references have been isolated only from Antarctica so far. However, the internal transcribed spacer (ITS) sequence differences among them supported they are representing distinct populations of the same species. Within Apatococcus five clearly distinct groups of ITS sequences, each putatively representing a distinct species, were recovered with three or four such ITS types co‐occurring at the same study site. Except for the streptophyte Klebsormidium only members of Trebouxiophyceae were detected suggesting these algae may be particularly well‐adapted to subaerial habitats.     

Photosynthetic recovery following desiccation of desert green algae (Chlorophyta) and their aquatic relatives

Recent molecular data suggest that desert green algae have evolved from freshwater ancestors at least 14 times in three major classes (Chlorophyceae, Trebouxiophyceae and Charophyceae), offering a unique opportunity to study the adaptation of photosynthetic organisms to life on land in a comparative phylogenetic framework. We examined the photorecovery of phylogenetically matched desert and aquatic algae after desiccation in darkness and under illumination. Desert algae survived desiccation for at least 4 weeks when dried in darkness, and recovered high levels of photosynthetic quantum yield within 1 h of rehydration in darkness. However, when 4 weeks of desiccation was accompanied by illumination, three of six desert taxa lost their ability to recover quantum yield during rehydration in the dark. Aquatic algae, in contrast, recovered very little during dark rehydration following even just 24 h of desiccation. Re-illuminating rehydrated algae produced a nearly complete recovery of quantum yield in all desert and two of five aquatic taxa. These contrasts provide physiological evidence that desert green algae possess mechanisms for photosynthetic recovery after desiccation distinct from those in aquatic relatives, corroborating molecular evidence that they are not happenstance, short-term visitors from aquatic environments. Photosensitivity during desiccation among desert algae further suggests that they may reside in protected microsites within crusts, and species specificity of photosensitivity suggests that disturbances physically disrupting crusts could lead to shifts or losses of taxonomic diversity within these habitats.     

Origin and Early Evolution of Plant Body Symmetry and Gravity Responses

Land plant bodies exhibit both apical–basal and radial symmetry, and they are able to detect and respond to gravitational forces. These attributes were, likely important factors in the success of earliest plants on land. This study focuses on features of charophycean green algae likely to have been pre‐adaptive to early establishment of plant symmetry and gravitational responses, though most modern charophyceans occupy aquatic habitats where the buoyancy of water counteracts the effects of gravity. Trait mapping suggests that even the earliest‐divergent modern members of the streptophyte clade have bodies whose symmetry departs significantly from the spherical condition, and that cellular mechanisms defining aspects of radial symmetry and polarized tip growth originated early. Genes, cell biological approaches, and taxa are identified for which further exploration is likely to illuminate early evolution of plant body symmetry and gravity responses.     

兰州五泉山的藻类及其分布

以兰州五泉山为该地藻种资源库,对其中水生、陆生生境中藻类的种类多样性、群落结构、分布特点进行了研究。结果发现该地藻类植物65种(含4变种),包括蓝藻、绿藻、硅藻和红藻,其中硅藻种类最多(29种),其它依次为蓝藻(24种)、绿藻(11种)和红藻(1种)。水体中共42种,硅藻最多,有26种,其次蓝藻8种,绿藻7种,红藻1种,不同水体中优势种和亚优势种不同。土壤生境中发现20种,蓝藻13种,绿藻4种,硅藻3种,且非洲席藻和小球藻分为优势种和亚优势种。7个种类在水、陆两大生境都有分布,而且它们主要是丝状蓝藻。     

Streptophyte algae and the origin of embryophytes

Background

Land plants (embryophytes) evolved from streptophyte green algae, a small group of freshwater algae ranging from scaly, unicellular flagellates (Mesostigma) to complex, filamentous thalli with branching, cell differentiation and apical growth (Charales). Streptophyte algae and embryophytes form the division Streptophyta, whereas the remaining green algae are classified as Chlorophyta. The Charales (stoneworts) are often considered to be sister to land plants, suggesting progressive evolution towards cellular complexity within streptophyte green algae. Many cellular (e.g. phragmoplast, plasmodesmata, hexameric cellulose synthase, structure of flagellated cells, oogamous sexual reproduction with zygote retention) and physiological characters (e.g. type of photorespiration, phytochrome system) originated within streptophyte algae.

Recent Progress

Phylogenetic studies have demonstrated that Mesostigma (flagellate) and Chlorokybus (sarcinoid) form the earliest divergence within streptophytes, as sister to all other Streptophyta including embryophytes. The question whether Charales, Coleochaetales or Zygnematales are the sister to embryophytes is still (or, again) hotly debated. Projects to study genome evolution within streptophytes including protein families and polyadenylation signals have been initiated. In agreement with morphological and physiological features, many molecular traits believed to be specific for embryophytes have been shown to predate the Chlorophyta/Streptophyta split, or to have originated within streptophyte algae. Molecular phylogenies and the fossil record allow a detailed reconstruction of the early evolutionary events that led to the origin of true land plants, and shaped the current diversity and ecology of streptophyte green algae and their embryophyte descendants.

Conclusions

The Streptophyta/Chlorophyta divergence correlates with a remarkably conservative preference for freshwater/marine habitats, and the early freshwater adaptation of streptophyte algae was a major advantage for the earliest land plants, even before the origin of the embryo and the sporophyte generation. The complete genomes of a few key streptophyte algae taxa will be required for a better understanding of the colonization of terrestrial habitats by streptophytes.Key words: Chlorophyta, Streptophyta, Embryophyta, Charales, Coleochaetales, Zygnematales, viridiplant phylogeny, land plants, genome evolution, freshwater adaptation, sporophyte origin, diversification, extinction     

A freshwater analog for the production of Epiphyton‐like microfossils

Calcified microbial microfossils—often interpreted as cyanobacteria—were important components of Precambrian and Paleozoic limestones, but their paucity in modern marine environments complicates our ability to make conclusive interpretations about their taxonomic affinity and geologic significance. Freshwater spring‐associated limestones (e.g., travertine and tufa) serve as terrestrial analogs to investigate mineralization in and around aquatic biofilms on observable timescales. We document the diagenesis of calcite fabrics associated with the freshwater algae Oocardium stratum, an epiphytic colonial green algae (desmid) known for producing stalks of extracellular polymeric substances (EPS) and passively producing a bifurcating tubular calcite monocrystal. Bifurcating EPS stalks produced by Oocardium colonies can become calcified and preserved in ancient carbonate deposits. Calcified micritic EPS stalks have a filamentous morphology, show evidence of branching, and maintain uniformity in diameter thickness throughout the mm‐scale colony, much like the enigmatic calcimicrobe Epiphyton. We provide a mechanism by which calcification associated with a colonial semispherical micro‐organism produces microfossils that deceptively resemble filamentous forms. These findings have implications for the use of morphological traits when assigning taxonomic affinities to extinct microfossil groups and highlight the utility of calcifying freshwater modern environments to investigate microbial taphonomy.     

Quantification of algal biofilms colonising building materials: chlorophyll a measured by PAM-fluorometry as a biomass parameter

The aim of this study was to quantify algal colonisation on anthropogenic surfaces (viz. building facades and roof tiles) using chlorophyll a (chl a) as a specific biomarker. Chl a was estimated as the initial fluorescence F0 of 'dark adapted' algae using a pulse-modulated fluorometer (PAM-2000). Four isolates of aeroterrestrial green algae and one aquatic isolate were included in this study. The chl a concentration and F0 showed an exponential relationship in the tested range between 0 and 400 mg chl a m(-2). The relationship was linear at chl a concentrations <20 mg m(-2). Exponential and linear models are presented for the single isolates with large coefficients of determination (exponential: r2 > 0.94, linear: r2 > 0.92). The specific power of this fluorometric method is the detection of initial algal colonisation on surfaces in thin or young biofilms down to 3.5 mg chl a m(-2), which corresponds to an abundances of the investigated isolates between 0.2 and 1.5 million cells cm(-2).     

Green land: Multiple perspectives on green algal evolution and the earliest land plants

Green plants, broadly defined as green algae and the land plants (together, Viridiplantae), constitute the primary eukaryotic lineage that successfully colonized Earth's emergent landscape. Members of various clades of green plants have independently made the transition from fully aquatic to subaerial habitats many times throughout Earth's history. The transition, from unicells or simple filaments to complex multicellular plant bodies with functionally differentiated tissues and organs, was accompanied by innovations built upon a genetic and phenotypic toolkit that have served aquatic green phototrophs successfully for at least a billion years. These innovations opened an enormous array of new, drier places to live on the planet and resulted in a huge diversity of land plants that have dominated terrestrial ecosystems over the past 500 million years. This review examines the greening of the land from several perspectives, from paleontology to phylogenomics, to water stress responses and the genetic toolkit shared by green algae and plants, to the genomic evolution of the sporophyte generation. We summarize advances on disparate fronts in elucidating this important event in the evolution of the biosphere and the lacunae in our understanding of it. We present the process not as a step-by-step advancement from primitive green cells to an inevitable success of embryophytes, but rather as a process of adaptations and exaptations that allowed multiple clades of green plants, with various combinations of morphological and physiological terrestrialized traits, to become diverse and successful inhabitants of the land habitats of Earth.     

PHYLOGENETIC POSITION OF CHAETOSPHAERIDIUM (CHLOROPHYTA), A BASAL LINEAGE IN THE CHAROPHYCEAE INFERRED FROM 18S rDNA SEQUENCES

We sequenced the nuclear-encoded small-subunit ribosomal RNA gene (18S rDNA) of Chaetosphaeridium globosum (Nordst.) Klebahn, a microscopic freshwater epiphytic chlorophyte, to assess its phylogenetic affinities in the Chlorophyta. A phylogenetic analysis of a broad sampling of green algal taxa and Chaetosphaeridium confirmed that this alga is a member of the Charophyceae (Streptophyta) as earlier microscopical studies had suggested. However, more detailed phylogenetic analyses of the streptophyte lineage showed that contrary to expectations based on the ultrastructure of the zoospores, the presence of a unique type of setae, the oogamous mode of reproduction, and the occurrence of oscillatory rotations of the cytoplasm, Chaetosphaeridium and Coleochaete are not closely related and do not form a monophyletic clade. Instead, Chaetosphaeridium represents an early branch in the streptophyte lineage that had a near-simultaneous origin as the Charalean clade and a clade formed by all remaining streptophytes examined ( Klebsormidium, Coleochaete, Chlorokybus, Zygnematales, and bryophytes). All phylogenetic inference methods used (neighbor-joining analysis of Kimura distances, maximum likelihood, and maximum parsimony) resulted in essentially the same tree topology. No Group I introns were found in the 18S rDNA coding region of Chaetosphaeridium. Our molecular phylogenetic analysis of Chaetosphaeridium supports a recent cladistic classification of the Streptobionta by Kenrick and Crane in which Chaetosphaeridium is placed in a monotypic division and class, Chaetosphaeridiophyta and Chaetosphaeridiophyceae, respectively.     

To concentrate or ventilate? Carbon acquisition,isotope discrimination and physiological ecology of early land plant life forms

A comparative study has been made of the photosynthetic physiological ecology and carbon isotope discrimination characteristics for modern-day bryophytes and closely related algal groups. Firstly, the extent of bryophyte distribution and diversification as compared with more advanced land plant groups is considered. Secondly, measurements of instantaneous carbon isotope discrimination (Delta), photosynthetic CO(2) assimilation and electron transport rates were compared during the drying cycles. The extent of surface diffusion limitation (when wetted), internal conductance and water use efficiency (WUE) at optimal tissue water content (TWC) were derived for liverworts and a hornwort from contrasting habitats and with differing degrees of thallus ventilation (as intra-thalline cavities and internal airspaces). We also explore how the operation of a biophysical carbon-concentrating mechanism (CCM) tempers isotope discrimination characteristics in two other hornworts, as well as the green algae Coleochaete orbicularis and Chlamydomonas reinhardtii. The magnitude of Delta was compared for each life form over a drying curve and used to derive the surface liquid-phase conductance (when wetted) and internal conductance (at optimal TWC). The magnitude of external and internal conductances, and WUE, was higher for ventilated, compared with non-ventilated, liverworts and hornworts, but the values were similar within each group, suggesting that both factors have been optimized for each life form. For the hornworts, leakiness of the CCM was highest for Megaceros vincentianus and C. orbicularis (approx. 30%) and, at 5%, lowest in C. reinhardtii grown under ambient CO2 concentrations. Finally, evidence for the operation of a CCM in algae and hornworts is considered in terms of the probable role of the chloroplast pyrenoid, as the origins, structure and function of this enigmatic organelle are explored during the evolution of land plants.     

TWO CHLOROPLAST TRANSFER RNA INTRONS FOUND IN CHAETOSPHAERIDIUM SUPPORT A MONOPHYLETIC COLEOCHAETALES

Lewandowski, J. D. & Delwiche, C. F. Cell Biology and Molecular Genetics, University of Maryland, College Park, MD 20742 USA The evolutionary relationships of the algal genera Mesostigma and Chaetosphaeridium to other algae and land plants are currently controversial. A close evolutionary relationship between land plants and two orders of the charophycean algae, the Charales and Coleochaetales, is supported by morphological, ultrastructural, biochemical, genomic, and phylogenetic data. A number of phylogenetic analyses support a monophyletic Coleochaetales, with Coleochaete and Chaetosphaeridum as sister groups. Mesostigma was traditionally viewed as a member of the prasinophytes and has recently been considered as a lineage possibly basal to the charophycean algae, or sister to all green algae. By contrast, recent analyses of small subunit ribosomal RNA gene sequences have been interpreted as evidence of an alternative classification with Mesostigma forming a clade with Chaetosphaeridium to the exclusion of Coleochaete and other charophycean lineages. The shared presence of introns in two chloroplast tRNA genes (tRNAAla and tRNAIle) among charophytes Coleochaete and Nitella and the liverwort Marchantia supports a monophyletic group containing the Coleochaetales, the Charales, and land plants. Through isolation and sequence analysis of the tRNAAla and tRNAIle genes in Chaetosphaeridium, we have identified introns similar in sequence and position to those found in Coleochaete. These data and the published absence of these introns in Mesostigma lend new support to a monophyletic Coleochaetales including the genera Coleochaete and Chaetosphaeridium.     

The terrestrial biota prior to the origin of land plants (embryophytes): a review of the evidence

It is often assumed that life originated and diversified in the oceans prior to colonizing the land. However, environmental constraints in chemical evolution models point towards critical steps leading to the origin of life as having occurred in subaerial settings. The earliest fossil record does not include finds from terrestrial deposits, so much of our understanding about the presence of a terrestrial microbial cover prior to the Proterozoic is based on inference and geochemical proxies that indicate biospheric carbon cycling during the Archaean. Our assessment is that by 2.7 Ga, microbial ecosystems in terrestrial settings were driven by oxygen‐generating, photosynthetic cyanobacteria. Studies of modern organisms indicate that both the origin and primary diversification of the eukaryotes could have occurred in terrestrial settings, shortly after 2.0 Ga, but there is no direct fossil evidence of terrestrial eukaryotes until about 1.1 Ga. At this time, it appears that the diversity of life in non‐marine habitats exceeded that found in marine settings where sulphidic seas may have impaired eukaryotic physiology and retarded evolution. Geochemical proxies indicate the establishment of an extensive soil‐forming microbial cover by 850 Ma, and it is possible that a rise in atmospheric oxygen at this time was due to the evolutionary expansion of green algae into terrestrial habitats. Direct fossil evidence of the earliest terrestrial biotas in the Phanerozoic consists of problematical palynomorphs from the Cambro‐Ordovician of Laurentia. These indicate that the evolution of the first land plants (embryophytes) during the Middle Ordovician took place within a landscape that included aeroterrestrial algae which were actively adapting to selection in subaerial settings.     

Isolation and properties of fungi that lyse blue-green algae.

Of 70 pure microbial cultures isolated from aquatic habitats, soil, and air according to the ability to lyse live blue-green algae, 62 were fungi representing the genera Acremonium, Emericellopsis, and Verticillium. Algal-lysing fungi were isolated from all habitat types sampled. The remaining isolates comprised four bacteria and four streptomycetes. All isolates lysed Anabaena flos-aquae and, in most cases, several other filamentous and unicellular blue-green algae. The fungi generally showed greater activity than most other isolates towards a wider range of susceptible algae, including green algae in some cases. Acremonium and Emericellopsis isolates, but not Verticillium, also inhibited the growth of blue-green algae and gram-positive bacteria, but did not lyse the latter. Lysis of blue green algae by Acremonium and Emericellopsis spp. was associated with the formation of diffusible heat-stable extracellular factors which, evidence suggests, could be cephalosporin antibiotic(s). Blue-green algae were also lysed by pure cephalosporin C. The frequent isolation of lytic fungi from algal habitats suggests a possible natural algal-destroying role for such fungi, which might be exploitable for algal bloom control.     

Isolation and properties of fungi that lyse blue-green algae.

Of 70 pure microbial cultures isolated from aquatic habitats, soil, and air according to the ability to lyse live blue-green algae, 62 were fungi representing the genera Acremonium, Emericellopsis, and Verticillium. Algal-lysing fungi were isolated from all habitat types sampled. The remaining isolates comprised four bacteria and four streptomycetes. All isolates lysed Anabaena flos-aquae and, in most cases, several other filamentous and unicellular blue-green algae. The fungi generally showed greater activity than most other isolates towards a wider range of susceptible algae, including green algae in some cases. Acremonium and Emericellopsis isolates, but not Verticillium, also inhibited the growth of blue-green algae and gram-positive bacteria, but did not lyse the latter. Lysis of blue green algae by Acremonium and Emericellopsis spp. was associated with the formation of diffusible heat-stable extracellular factors which, evidence suggests, could be cephalosporin antibiotic(s). Blue-green algae were also lysed by pure cephalosporin C. The frequent isolation of lytic fungi from algal habitats suggests a possible natural algal-destroying role for such fungi, which might be exploitable for algal bloom control.     

Effect of temperature on chlorophyll stability of some subaerial blue-green algae

Chlorophyll stability index of eight subaerial blue-green algal species, collected from their natural habitats, i.e., bark of trees, soil and roof-tops and from cultures, has been determined. The algae from natural habitats showed greater chlorophyll stability compared to those algae from cultures. Among the natural algae, high chlorophyll stability was observed in the algae inhabiting adverse habitats. A slight modification in the method of Murty and Majumder (1962) for determination of chlorophyll stability index has been made. Here the total percentage loss in the amount of Chl alpha of heated sample in relation to its unheated sample has been considered as chlorophyll stability index.     

Origin of strigolactones in the green lineage

? The aims of this study were to investigate the appearance of strigolactones in the green lineage and to determine the primitive function of these molecules. ? We measured the strigolactone content of several isolated liverworts, mosses, charophyte and chlorophyte green algae using a sensitive biological assay and LC-MS/MS analyses. In parallel, sequence comparison of strigolactone-related genes and phylogenetic analyses were performed using available genomic data and newly sequenced expressed sequence tags. The primitive function of strigolactones was determined by exogenous application of the synthetic strigolactone analog, GR24, and by mutant phenotyping. ? Liverworts, the most basal Embryophytes and Charales, one of the closest green algal relatives to Embryophytes, produce strigolactones, whereas several other species of green algae do not. We showed that GR24 stimulates rhizoid elongation of Charales, liverworts and mosses, and rescues the phenotype of the strigolactone-deficient Ppccd8 mutant of Physcomitrella patens. ? These findings demonstrate that the first function of strigolactones was not to promote arbuscular mycorrhizal symbiosis. Rather, they suggest that the strigolactones appeared earlier in the streptophyte lineage to control rhizoid elongation. They may have been conserved in basal Embryophytes for this role and then recruited for the stimulation of colonization by glomeromycotan fungi.     

A suggested standardised method for testing photocatalytic inactivation of aeroterrestrial algal growth on TiO2-coated glass

Aeroterrestrial green algae form conspicuous biofilms on man-made surfaces. The self-cleaning properties of photocatalytic coatings prevent their growth and can probably replace biocides. The aim of this study was to develop a laboratory method to investigate the efficiency of photocatalytic materials against algal growth. Two algal isolates (“Chlorella” luteoviridis, SAG 2196, and Coccomyxa sp., SAG 2040) functioned well as model organisms because they grew on almost all test specimens at 100% humidity and low UVA radiation. With these species, we examined algal growth prevention using photocatalytic glass. No effects on algal growth were detected, although the coated surfaces were photocatalytically active and degraded methylene blue. Because their cells are protected well against photocatalytically generated hydroxyl radicals, aeroterrestrial algae survive various stress factors. Nevertheless, the newly developed experimental design may be useful for assessing the biological function of other photocatalytic materials or stress factors.