Low Light Requirements Research Articles

Photosynthetic light requirement near the theoretical minimum detected in Arctic microalgae

Photosynthesis is one of the most important biological processes on Earth, providing the main source of bioavailable energy, carbon, and oxygen via the use of sunlight. Despite this importance, the minimum light level sustaining photosynthesis and net growth of primary producers in the global ocean is still unknown. Here, we present measurements from the MOSAiC field campaign in the central Arctic Ocean that reveal the resumption of photosynthetic growth and algal biomass buildup under the ice pack at a daily average irradiance of not more than 0.04 ± 0.02 µmol photons m−2 s−1 in late March. This is at least one order of magnitude lower than previous estimates (0.3–5 µmol photons m−2 s−1) and near the theoretical minimum light requirement of photosynthesis (0.01 µmol photons m−2 s−1). Our findings are based on measurements of the temporal development of the under-ice light field and concurrent measurements of both chlorophyll a concentrations and potential net primary production underneath the sea ice at 86 °N. Such low light requirements suggest that euphotic zones where photosynthesis can occur in the world’s oceans may extend further in depth and time, with major implications for global productivity estimates.

Phylogeny of Saxifraga section Saxifraga subsection Arachnoideae (Saxifragaceae) and the origin of low elevation shade-dwelling species.

Saxifraga section Saxifraga subsection Arachnoideae is a lineage of 12 species distributed mainly in the European Alps. It is unusual in terms of ecological diversification by containing both high elevation species from exposed alpine habitats and low elevation species from shady habitats such as overhanging rocks and cave entrances. Our aims are to explore which of these habitat types is ancestral, and to identify the possible drivers of this remarkable ecological diversification. Using a Hybseq DNA-sequencing approach and a complete species sample we reconstructed and dated the phylogeny of subsection Arachnoideae. Using Landolt indicator values, this phylogenetic tree was used for the reconstruction of the evolution of temperature, light and soil pH requirements in this lineage. Diversification of subsection Arachnoideae started in the late Pliocene and continued through the Pleistocene. Both diversification among and within clades was largely allopatric, and species from shady habitats with low light requirements are distributed in well-known refugia. We hypothesize that low light requirements evolved when species persisting in cold-stage refugia were forced into marginal habitats by more competitive warm-stage vegetation. While we do not claim that such competition resulted in speciation, it very likely resulted in adaptive evolution.

Photosynthetic, Respirational, and Growth Responses of Six Benthic Diatoms from the Antarctic Peninsula as Functions of Salinity and Temperature Variations.

Temperature and salinity are some of the most influential abiotic parameters shaping biota in aquatic ecosystems. In recent decades, climate change has had a crucial impact on both factors—especially around the Antarctic Peninsula—with increasing air and water temperature leading to glacial melting and the accompanying freshwater increase in coastal areas. Antarctic soft and hard bottoms are typically inhabited by microphytobenthic communities, which are often dominated by benthic diatoms. Their physiology and primary production are assumed to be negatively affected by increased temperatures and lower salinity. In this study, six representative benthic diatom strains were isolated from different aquatic habitats at King George Island, Antarctic Peninsula, and comprehensively identified based on molecular markers and morphological traits. Photosynthesis, respiration, and growth response patterns were investigated as functions of varying light availability, temperature, and salinity. Photosynthesis–irradiance curve measurements pointed to low light requirements, as light-saturated photosynthesis was reached at <70 µmol photons m−2 s−1. The marine isolates exhibited the highest effective quantum yield between 25 and 45 SA (absolute salinity), but also tolerance to lower and higher salinities at 1 SA and 55 SA, respectively, and in a few cases even <100 SA. In contrast, the limnic isolates showed the highest effective quantum yield at salinities ranging from 1 SA to 20 SA. Almost all isolates exhibited high effective quantum yields between 1.5 °C and 25 °C, pointing to a broad temperature tolerance, which was supported by measurements of the short-term temperature-dependent photosynthesis. All studied Antarctic benthic diatoms showed activity patterns over a broader environmental range than they usually experience in situ. Therefore, it is likely that their high ecophysiological plasticity represents an important trait to cope with climate change in the Antarctic Peninsula.

Photosynthesis, Respiration, and Growth of Five Benthic Diatom Strains as a Function of Intermixing Processes of Coastal Peatlands with the Baltic Sea.

In light of climate change, renaturation of peatlands has become increasingly important, due to their function as carbon sinks. Renaturation processes in the Baltic Sea include removal of coastal protection measures thereby facilitating exchange processes between peatland and Baltic Sea water masses with inhabiting aquatic organisms, which suddenly face new environmental conditions. In this study, two Baltic Sea and three peatland benthic diatom strains were investigated for their ecophysiological response patterns as a function of numerous growth media, light, and temperature conditions. Results clearly showed growth stimulation for all five diatom strains when cultivated in peatland water-based media, with growth dependency on salinity for the Baltic Sea diatom isolates. Nutrient availability in the peatland water resulted in higher growth rates, and growth was further stimulated by the carbon-rich peatland water probably facilitating heterotrophic growth in Melosira nummuloides and two Planothidium sp. isolates. Photosynthesis parameters for all five diatom strains indicated low light requirements with light saturated photosynthesis at <70 µmol photons m−2 s−1 in combination with only minor photoinhibition as well as eurythermal traits with slightly higher temperature width for the peatland strains. Growth media composition did not affect photosynthetic rates.

Cantilever enhanced based photoacoustic detection of SF6 decomposition component SO2 using UV LED

PurposeThis paper aims to establish a photoacoustic detection system for SO2 using UV-LED and testify its feasibility for sensitive measurement. The work in this paper can avoid potential crossover interference in infrared (IR) range and also balance the capability and cost of feasible excitation for photoacoustic detection system.Design/methodology/approachIn this experimental work, a cantilever-enhanced–based photoacoustic SO2 detection system using an ultraviolet (UV) LED light source with a light power of 4 mW as the excitation was established.FindingsA feasible photoacoustic detection system for SO2 using UV-LED was established. Experimental results demonstrate that the detection limit of the system can reach the level of 0.667 ppm, which can serve as a reference for the application of PAS in insulation fault diagnosis.Originality/valueThis work investigated the potential of using ultraviolet photoacoustic spectroscopy to detect trace SO2, which provided an ideal replacement of infrared-laser-based detection system. In this paper, a photoacoustic detection system using LED with a low light power was established. Low light power requirement can expand the options of light sources accordingly. In this paper, the absorption characteristics of SO2 in the presented detection system and ultraviolet range were studied. And the detection limit of the presented system was given. Both of which can provide reference to SO2 detection in ambient SF6.

Ecophysiological, morphological, and biochemical traits of free-living Diplosphaera chodatii (Trebouxiophyceae) reveal adaptation to harsh environmental conditions

Single-celled green algae within the Trebouxiophyceae (Chlorophyta) are typical components of terrestrial habitats, which often exhibit harsh environmental conditions for these microorganisms. This study provides a detailed overview of the ecophysiological, biochemical, and ultrastructural traits of an alga living on tree bark. The alga was isolated from a cypress tree in the Botanical Garden of Innsbruck (Austria) and identified by morphology and molecular phylogeny as Diplosphaera chodatii. Transmission electron microscopy after high-pressure freezing (HPF) showed an excellent preservation of the ultrastructure. The cell wall was bilayered with a smooth inner layer and an outer layer of polysaccharides with a fuzzy hair-like appearance that could possibly act as cell-cell adhesion mechanism and hence as a structural precursor supporting biofilm formation together with the mucilage observed occasionally. The photosynthetic-irradiance curves of D. chodatii indicated low light requirements without photoinhibition at high photon flux densities (1580 μmol photons m−2 s−1) supported by growth rate measurements. D. chodatii showed a high desiccation tolerance, as 85% of its initial value was recovered after controlled desiccation at a relative humidity of ~10%. The alga contained the low molecular weight carbohydrates sucrose and sorbitol, which probably act as protective compounds against desiccation. In addition, a new but chemically not elucidated mycosporine-like amino acid was detected with a molecular mass of 332 g mol−1 and an absorption maximum of 324 nm. The presented data provide various traits which contribute to a better understanding of the adaptive mechanisms of D. chodatii to terrestrial habitats.

Fine-scale environmental heterogeneity and spatial niche partitioning among spring-flowering forest herbs.

Environmental heterogeneity influences plant distributions and diversity at several spatial scales. In temperate forests, fine-scale environmental variation may promote local coexistence among herbaceous species by allowing plants to spatially partition microsites within forest stands. Here we argue that shallow soils, low soil water-holding capacity and fertility, and reduced light near tree boles should favor short, shallow-rooted, evergreen species like Anemone acutiloba with low moisture, nutrient, and light requirements. Farther from trees, richer, deeper soils should favor taller, deeper-rooted herbs with greater moisture and nutrient demands, such as Sanguinaria canadensis and Trillium flexipes. We tested these hypotheses by mapping the fine-scale distributions of Anemone, Sanguinaria, and Trillium individuals within a 50 × 50 m plot, comparing local species' distributions with respect to soil depth and proximity to neighboring trees, and characterizing intraspecific and interspecific spatial associations. Local plant distributions were consistent with our predictions based on leaf height, physiology, and phenology. Anemone was found in microsites on shallower soils and closer to trees than either Sanguinaria or Trillium. In all three species, individual plants were spatially aggregated within 2 m, but spatially segregated from individuals of the other species beyond 2 m. Differential plant responses to fine-scale environmental heterogeneity and observed spatial associations suggest that local species-environment associations could facilitate coexistence. These findings illustrate how fine-scale environmental heterogeneity coupled with phenological and physiological differences likely contribute to spatial niche partitioning among spring-flowering forest herbs and maintain high local plant diversity within temperate forests.

First Report of Stem Rot Caused by Athelia rolfsii on Zamioculcas zamiifolia in Korea

HomePlant DiseaseVol. 103, No. 11First Report of Stem Rot Caused by Athelia rolfsii on Zamioculcas zamiifolia in Korea PreviousNext DISEASE NOTES OPENOpen Access licenseFirst Report of Stem Rot Caused by Athelia rolfsii on Zamioculcas zamiifolia in KoreaYunhee Seo, Mi-Jeong Park, Chang-Gi Back, and Jong-Han ParkYunhee Seohttp://orcid.org/0000-0002-5721-2974Horticultural and Herbal Crop Environment Division, National Institute of Horticultural and Herbal Science, RDA, Wanju 55365, Republic of KoreaSearch for more papers by this author, Mi-Jeong ParkHorticultural and Herbal Crop Environment Division, National Institute of Horticultural and Herbal Science, RDA, Wanju 55365, Republic of KoreaSearch for more papers by this author, Chang-Gi BackHorticultural and Herbal Crop Environment Division, National Institute of Horticultural and Herbal Science, RDA, Wanju 55365, Republic of KoreaSearch for more papers by this author, and Jong-Han Park†Corresponding author: J.-H. Park; E-mail Address: [email protected]Horticultural and Herbal Crop Environment Division, National Institute of Horticultural and Herbal Science, RDA, Wanju 55365, Republic of KoreaSearch for more papers by this authorAffiliationsAuthors and Affiliations Yunhee Seo Mi-Jeong Park Chang-Gi Back Jong-Han Park † Horticultural and Herbal Crop Environment Division, National Institute of Horticultural and Herbal Science, RDA, Wanju 55365, Republic of Korea Published Online:28 Aug 2019https://doi.org/10.1094/PDIS-01-19-0221-PDNAboutSectionsSupplemental ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmailWechat Zamioculcas zamiifolia (Lodd.) Engl., commonly known as the “ZZ plant,” belongs to the Araceae family. It is native to Africa, from Kenya to South Africa, and is considered an attractive indoor plant because of its dark green foliage and low light requirements. ZZ plants are also considered more effective than Sansevieria spp. in reducing the contaminants benzene, toluene, ethylbenzene, and xylene in indoor air (Toabaita et al. 2016). Additionally, the plants are tolerant to drought stress, have a unique appearance with minimal maintenance requirements, and are popular as ornamentals for display in offices and homes in Korea. In August 2016, ZZ plant samples from the Yongin nursery, Gyeonggi-do Province (37°12′01.8″N, 126°20′01.0″E) were submitted to the Horticultural and Herbal Crop Environment Division in Wanju, Korea. Initial symptoms included water-soaking of stems with lower leaf chlorosis followed by stem collapse and crown rot. White mycelia and brown sclerotia were observed on infected plant parts. Sclerotia were collected, surface disinfected with 1% sodium hypochlorite for 1 min followed by 70% ethanol for 1 min, rinsed twice in sterilized water, dried, and placed on acidified potato dextrose agar (PDA; 1 ml of lactic acid per liter of medium) plates and incubated at 28°C in the dark. White, cottony fungal colonies grew from the sclerotia that often formed mycelial fans. One hyphal-tip isolate was selected and transferred to a PDA plate. After 7 days of growth, 1- to 3-mm-diameter sclerotia formed on the fungal colonies and turned dark brown with age. DNA was extracted from the fungus for identification purposes, and the ITS region was amplified using primer pair ITS1/ITS4 (White et al. 1990). A 615-bp fragment was amplified and sequenced (MK418758). BLAST analysis of the PCR product showed 97% identity to Athelia rolfsii for the ITS gene (KT750883). Pathogenicity of the fungal isolates was done on 2-month-old healthy ZZ plants that were surface disinfected with 70% ethanol. Plants were potted in sterilized soil, and three were inoculated with A. rolfsii mycelial plugs. Three control plants were inoculated with PDA agar plugs only. Plants were maintained in a greenhouse at 25 to 28°C and 90% relative humidity. Ten days after inoculation, symptoms on inoculated plants included stem lesion with white mycelium, crown rot, and leaf chlorosis. Symptoms were similar to those observed on ZZ plants in commercial greenhouses. Control plants remained symptomless. The experiment was repeated twice, and A. rolfsii was reisolated from the artificially infected plants, thus fulfilling Koch’s postulates. This pathogen was previously reported on ZZ plant from Sri Lanka (Jegathambigai et al. 2010). This is believed to be the first report of stem rot caused by A. rolfsii on commercially grown ZZ plant in Korea.The author(s) declare no conflict of interest.

Drivers of invasive tree and shrub natural regeneration in temperate forests

We assessed drivers of ecological success along resource availability gradients for three invasive woody species: Prunus serotina Ehrh., Quercus rubra L. and Robinia pseudoacacia L. We aimed to check how much of invasion success, measured by invader biomass, is explained by propagule pressure and plant community invasibility. Using 3 years of observations from 372 study plots (100 m2 each) in temperate forests of Wielkopolski National Park (Poland) we investigated the hierarchy of predictors and partial dependencies using the random forest method. Our study indicated that propagule pressure explained more variance in success of invaders than invasibility—describing availability of resources and competitors in understory vegetation. We also found different responses of seedlings and saplings, connected with dependence on stored carbohydrates, which decreased seedling responses to resource availability gradients. However, resource availability (light and leaf litter predictors) had greater influence than predictors describing understory vegetation. Based on importance and response strength the species studied may be arranged by decreasing requirements for soil fertility and acidity: P. serotina < Q. rubra < R. pseudoacacia, whereas for light requirements and competition vulnerability the order is: P. serotina > Q. rubra > R. pseudoacacia. However, low light requirements of R. pseudoacacia may be biased by high proportion of sprouts supplied by parental trees. Results provide guidelines for effective management of invasive woody species in forest ecosystems and describe complex interactions between factors studied on ecological success of invaders.

Epilithic Chamaesiphon (Synechococcales, Cyanobacteria) species in mountain streams of the Alps\u2014interspecific differences in photo-physiological traits

Many alpine streams inhabit conspicuous epilithic biofilms on pebbles and rocks that are formed by members of the cyanobacterial genus Chamaesiphon (Synechococcales). In the Austrian Alps, some Chamaesiphon species can even overgrow up to 70% of the surface of river rocks, and hence they must play an important but still unstudied ecological role in the organic matter flux. Since photo-biological traits have not been investigated so far, photosynthetic features, pigments, and UV-sunscreen compounds were studied in three Chamaesiphon morphospecies (C. geitleri, C. polonicus, C. starmachii). These species form conspicuously differently colored spots on cobbles and boulders in the alpine streams. While C. polonicus typically forms red crusts on flat pebble conglomerate, C. geitleri and C. starmachii are characterized by dark brown and black biofilms in the field, respectively. Photosynthesis-irradiance (PE) curves indicate that all three Chamaesiphon species have different light requirements for photosynthesis, with C. starmachii and C. polonicus preferring high and low photon fluence rates, respectively, while C. geitleri takes a position in between. This low-light requirement of C. polonicus is also reflected in ca. ten-times lower chlorophyll a, zeaxanthin, and ß-carotene concentrations, as well as in a lack of the UV-sunscreen scytonemin. All Chamaesiphon morphospecies exhibit the mycosporine-like amino acid porphyra-334. The physiological and biochemical data indicate strong intraspecific differences in photosynthetic activity and pigment patterns, which explain well the distinct preferences of the three studied Chamaesiphon morphospecies for sun-exposed or shaded habitats.

Ecophysiological characterization and toxin profile of two strains of Cylindrospermopsis raciborskii isolated from a subtropical lagoon in Southern Brazil

In the present work, we attempted to characterize two isolates of Cylindrospermopsis raciborskii, LP1 and LP2, from Peri Lagoon, for their morphology, ecophysiology, and toxin profiles. The genetic identity of the isolates was confirmed by amplifying and sequencing 16S rRNA. The isolates showed different morphologies and significant differences in the length of trichomes. LP2 showed a trend for higher growth rates than LP1 at the different temperatures and N:P ratios. Both isolates showed low light requirements, but were able to tolerate irradiances of around 200 μmol photons m−2 s−1. LP2 showed higher concentrations of saxitoxin than LP1 and wider range of analogs, therefore being considered more toxic. These results support the hypothesis of ecotype selection for this species, which probably originated in response to environmental fluctuations in Peri Lagoon. Dominance during almost the entire year can be explained by the alternation of these ecotypes in the total biomass contribution according to their physiological advantages, contributing to the ecological success of this species.

Ecophysiological traits of various genotypes of a green key alga in biological soil crusts from the semi-arid Colorado Plateau, USA

Biological soil crusts (BSCs) represent a micro-ecosystem consisting of various microorganisms including algae in drylands worldwide. The green algal genus Klebsormidium is a typical member of such communities, and because of its filamentous morphology and sticky cells, it plays an ecological key role in the stabilization of soil surfaces. In the present study, we investigated for the first time the phylogeny and ecophysiological performance of five BSC Klebsormidium strains from the semi-arid Colorado Plateau, USA. The molecular phylogeny of rbcL sequences showed that these strains belong to two subclades, which have been described before from mainly humid habitats. During controlled dehydration, the effective quantum yield of photosystem II decreased during 370–430 min of exposure. After controlled rehydration, all strains recovered between 32.9 and 97.6% of the control, but with significant differences depending on the genetic lineage. All five isolates grew between 18.1 and 27.9 °C, with isolate-specific optimal growth temperatures. Similarly, all strains were grown under increasing photon fluence rates from 4 to 110 μmol photons m−2 s−1 with optima under the higher tested light levels. Two strains showed inhibition at higher photon fluence rates and hence low light requirements for growth. Photosynthesis under increasing photon fluence rates up to 314 μmol photons m−2 s−1 indicated again low light requirements, but with no photoinhibition. All results underline a pronounced plasticity of the ecophysiological traits of the investigated Klebsormidium strains, which allow these terrestrial algae to exploit the environmental gradients within BSCs in arid regions.

High light intensity stress as the limiting factor in micropropagation of sugar maple (Acer saccharum Marsh.)

Attempts of clonal propagation of sugar maple through conventional and micropropagation techniques have been largely unsuccessful and the basis for this longstanding problem has remained unknown. Typical symptoms of stress in in vitro cultures of sugar maple and its intrinsic tolerance to highly shaded growing conditions indicated that optimization of light intensity might overcome its recalcitrance. To test this hypothesis we evaluated the growth, morphology, chlorophyll content and various chlorophyll fluorescence parameters of in vitro plantlets cultured under three intensities (4, 16 and 40 µmol m−2 s−1) using two spectra of light (red-blue and full spectrum white). Higher light intensities increased fresh and dry mass, but had no significant effect on the number of nodes or plant height. Anthocyanin accumulation in tissues, reduction in chlorophyll content, and significantly increased physiological stress quantified by chlorophyll fluorescence parameters, indicated increasing levels of light induced damage to the tissues with increasing light intensities. The critical factor limiting sugar maple’s response to in vitro propagation was light intensity and the levels commonly used for in vitro propagation are not suitable for sugar maple. The current study demonstrates the unique low light requirement, between 4 and 16 µmol m−2 s−1, of this species to overcome the recalcitrance to in vitro propagation. Spectral differences in quality of light and its effect on in vitro growth and stress in cultures are also discussed. In general light induced stress symptoms begin to appear at lower intensities with RB spectrum as compared to the full spectrum lights. While this study is limited to maples, requirement of specific light intensity is likely the cause of recalcitrance in other woody species with similar ecological roles.

Growth of eight Gambierdiscus (Dinophyceae) species: Effects of temperature, salinity and irradiance

Little is known about how the growth of individual Gambierdiscus species responds to environmental factors. This study examined the effects of temperature (15–34°C), salinity (15–41) and irradiance (2–664μmolphotonsm−2s−1) on growth of Gambierdiscus: G. australes, G. belizeanus, G. caribaeus, G. carolinianus, G. carpenteri, G. pacificus and G. ruetzleri and one putative new species, Gambierdiscus ribotype 2. Depending on species, temperatures where maximum growth occurred varied between 26.5 and 31.1°C. The upper and lower thermal limits for all species were between 31–34°C and 15–21°C, respectively. The shapes of the temperature vs. growth curves indicated that even small differences of 1–2°C notably affected growth potentials. Salinities where maximum growth occurred varied between 24.7 and 35, while the lowest salinities supporting growth ranged from <14 to 20.9. These data indicated that Gambierdiscus species are more tolerant of lower salinities than is generally appreciated. Growth of all species began to decline markedly as salinities exceed 35.1–39.4. The highest salinity tested in this study (41), however, was lethal to only one species, Gambierdiscus ribotype 2. The combined salinity data indicated that differences in salinity regimes may affect relative species abundances and distributions, particularly when salinities are <20 and >35. All eight Gambierdiscus species were adapted to relatively low light conditions, exhibiting growth maxima at 50–230μmolphotonsm−2s−1 and requiring only 6–17μmolphotonsm−2s−1 to maintain growth. These low light requirements indicate that Gambierdiscus growth can occur up to 150m depth in tropical waters, with optimal light regimes often extending to 75m. The combined temperature, salinity and light requirements of Gambierdiscus can be used to define latitudinal ranges and species-specific habitats, as well as to inform predictive models.

Light, Temperature, and Desiccation Effects on Photosynthetic Activity, and Drought-Induced Ultrastructural Changes in the Green Alga Klebsormidium dissectum (Streptophyta) from a High Alpine Soil Crust

Members of the cosmopolitan green algal genus Klebsormidium (Klebsormidiales, Streptophyta) are typical components of terrestrial microbiotic communities such as biological soil crusts, which have many important ecological functions. In the present study, Klebsormidium dissectum (Gay) Ettl & Gärtner was isolated from a high alpine soil crust in the Tyrolean Alps, Austria. Physiological performance in terms of growth and photosynthesis was investigated under different controlled abiotic conditions and compared with ultrastructural changes under the treatments applied. K. dissectum showed very low light requirements as reflected in growth patterns and photosynthetic efficiency. Increasing temperatures from 5°C to 40°C led to different effects on respiratory oxygen consumption and photosynthetic oxygen evolution. While at low temperatures (5-10°C), respiration was not detectable or on a very low level, photosynthesis was relatively high, Reversely, at the highest temperature, respiration was unaffected, and photosynthesis strongly inhibited pointing to strong differences in temperature sensitivity between both physiological processes. Although photosynthetic performance of K. dissectum was strongly affected under short-term desiccation and recovered only partly after rehydration, this species was capable to survive even 3weeks at 5% relative air humidity. K. dissectum cells have a cell width of 5.6 ± 0.3μm and a cell length of 8.4 ± 2.0μm. Desiccated cells showed a strongly reduced cell width (46% of control) and cell length (65% of control). In addition, in desiccated cells, fewer mitochondria were stained by DIOC(6), and damaged plasma membranes were detected by FM 1-43 staining. High-pressure freeze fixation as well as chemical fixation allowed visualizing ultrastructural changes caused by desiccation. In such cells, the nucleus and chloroplast were still visibly intact, but the extremely thin cell walls (75-180nm) were substantially deformed. The cytoplasm appeared electron dense and mitochondria were altered. Although K. dissectum can be characterized as euryoecious species, all ecophysiological and ultrastructural data indicate susceptibility to desiccation. However, the steadily occurring fragmentation of filaments into smaller units leads to improved self protection and thus may represent a life strategy to better survive longer periods of drought in exposed alpine soil crusts.

Physical and chemical processes promoting dominance of the toxic cyanobacterium Cylindrospermopsis raciborskii

The freshwater cyanobacterium, Cylindrospermopsis raciborskii (Wo’oszyńska) Seenayya and Subba Raju is a common species in lakes and reservoirs globally. In some areas of the world it can produce cyto- and hepatotoxins (cylindrospermopsins, saxitoxins), making blooms of this species a serious health concern for humans. In the last 10–15 years, there has been a considerable body of research conducted on the ecology, physiology and toxin production of this species and this paper reviews these studies with a focus on the cylindrospermopsin (CYN)-producing strains. C. raciborskii has low light requirements, close to neutral buoyancy, and a wide temperature tolerance, giving it the capacity to grow in many lentic waterbodies. It also has a flexible strategy with respect to nitrogen (N) utilisation; being able to switch between utilising fixed and atmospheric N as sources of N fluctuate. Additionally this species has a high phosphate (DIP) affinity and storage capacity. Like many cyanobacteria, it also has the capacity to use dissolved organic phosphorus (DOP). Changes in nutrient concentrations, light levels and temperature have also been found to affect production of the toxin CYN by this species. However, optimal toxin production does not necessarily occur when growth rates are optimal. Additionally, different strains of C. raciborskii vary in their cell quota of CYN, making it difficult to predict toxin concentrations, based on C. raciborskii cell densities. In summary, the ecological flexibility of this organism means that controlling blooms of C. raciborskii is a difficult undertaking. However, improved understanding of factors promoting the species and toxin production by genetically capable strains will lead to improved predictive models of blooms.

A light-gated, potassium-selective glutamate receptor for the optical inhibition of neuronal firing

Genetically targeted light-activated ion channels and pumps make it possible to determine the role of specific neurons in neuronal circuits, information processing and behavior. Here, we describe the development of a K+-selective ionotropic glutamate receptor that reversibly inhibits neuronal activity in response to light in dissociated neurons and brain slice and reversibly suppresses behavior in zebrafish. The receptor is a chimera of the pore region of a K+-selective bacterial glutamate receptor and the ligand binding domain of the light-gated mammalian kainate receptor (iGluR6/GluK2). This hyperpolarizing light-gated channel, HyLighter, is turned on by a brief light pulse at one wavelength and turned off by a pulse at a second wavelength. The control is obtained at moderate intensity. After optical activation, the photo-current and optical silencing of activity persist in the dark for extended periods. The low light requirement and bi-stability of HyLighter represent advantages for the dissection of neural circuitry.

Comparative studies on photosynthesis and phosphate metabolism of Cylindrospermopsis raciborskii with Microcystis aeruginosa and Aphanizomenon flos-aquae

The physiological differences for three bloom-forming cyanobacteria ( Cylindrospermopsis raciborskii, Microcystis aeruginosa, and Aphanizomenon flos-aquae) were investigated. In comparison with M. aeruginosa and A. flos-aquae, C. raciborskii exhibited a significantly higher concentration of carotenoids, higher values in maximum photosynthesis rate ( P m), apparent photosynthetic efficieny ( a), and maximum electron transport rate (ETR max) during the growth period. In addition, higher extracellular alkaline phosphatase activities and lower light compensation point ( I c) were also detected in C. raciborskii ( p < 0.05, ANOVA). Therefore, it is suggested that the higher photosynthetic activities, more effective uptake and utilization to phosphate, and low light requirements might play important roles in the occurrence and invasive behavior of C. raciborskii.

Photosynthetic responses of subtidal seagrasses to a daily light cycle in Torres Strait: A comparative study

In this study, we examined the photosynthetic responses of five common seagrass species from a typical mixed meadow in Torres Strait at a depth of 5–7 m using pulse amplitude modulated (PAM) fluorometry. The photosynthetic response of each species was measured every 2 h throughout a single daily light cycle from dawn (6 am) to dusk (6 pm). PAM fluorometry was used to generate rapid light curves from which measures of electron transport rate (ETR max ), photosynthetic efficiency ( α), saturating irradiance ( E k ) and light-adapted quantum yield (Δ F/ F′ m ) were derived for each species. The amount of light absorbed by leaves (absorption factor) was also determined for each species. Similar diurnal patterns were recorded among species with 3–4 fold increases in maximal electron rate from dawn to midday and a maintenance of ETR max in the afternoon that would allow an optimal use of low light by all species. Differences in photosynthetic responses to changes in the daily light regime were also evident with Syringodium isoetifolium showing the highest photosynthetic rates and saturating irradiances suggesting a competitive advantage over other species under conditions of high light. In contrast Halophila ovalis, Halophila decipiens and Halophila spinulosa were characterised by comparatively low photosynthetic rates and minimum light requirements (i.e. low E k ) typical of shade adaptation. The structural makeup of each species may explain the observed differences with large, structurally complex species such as Syringodium isoetifolium and Cymodocea serrulata showing high photosynthetic effciciencies ( α) and therefore high-light-adapted traits (e.g. high ETR max and E k ) compared with the smaller Halophila species positioned lower in the canopy. For the smaller Halophila species these shade-adapted traits are features that optimise their survival during low-light conditions. Knowledge of these characteristics and responses improves our understanding of the underlying causes of changes in seagrass biomass, growth and survival that occur when modifications in light quantity and quality arise from anthropogenic and climatic disturbances that commonly occur in Torres Strait.

Comparing after‐ripening response and germination requirements of Conyza canadensis and C. bonariensis (Asteraceae) through logistic functions

SummaryGermination requirements and after‐ripening effects during one year of dry storage at 15/5 and 25/15°C (day/night) were compared for Conyza bonariensis and C. canadensis (Asteraceae). A logistic function was fitted to the results from tests over time in various incubation conditions, using three populations of each species as replicates. Time required for response to dry storage was measured by using a new method, the third derivative of the logistic function. Therefore, a point when major germination was achieved could be detected, without having to rely on maximum germination (which is uncertain), individual data points or any subjectively chosen limit. Fresh seeds of both species were dependent on light for germination and after‐ripening was mainly manifested by increasing germination in darkness. Low dormancy status and light requirement might indicate that soil cultivations should rapidly reduce the seedbanks of these species, although fecundity and wind dispersal will affect population levels. The species differed in their germination response, with C. bonariensis germinating at lower temperatures than C. canadensis. This seemingly counter‐intuitive result may explain the prevention of fatal germination of C. canadensis in cold conditions and its behaviour as a summer annual in northern climates, while C. bonariensis is restricted to warmer parts of the world.