Effect of desert soil algae on the stabilization of fine sands

Four filamentous cyanobacteria, Microcoleusvaginatus, Phormidium tenue,Scytonemajavanicum (Kutz.) and Nostoc sp., and asingle-celled green alga, Desmococcus olivaceus, allisolated from Shapotou (Ningxia Hui Autonomous Region of China), were batchcultured and inoculated onto unconsolidated sand in greenhouse and fieldexperiments. Their ability to reduce wind erosion in sands was quantified byusing a wind tunnel laboratory. The major factors related to cohesion of algalcrusts, such as biomass, species, species combinations, bioactivity, niche,growth phase of algae, moisture, thickness of the crusts, dust accretion(including dust content and manner of dust added) and other cryptogams(lichens,fungi and mosses) were studied. The best of the five species were M.vaginatus and P. tenue, while the best mix wasablend of 80% M. vaginatus and 5% each of P.tenue,S. javanicum,Nostocsp. and D. olivaceus. The threshold friction velocity wassignificantly increased by the presence of all of the cyanobacterial species,while the threshold impact velocity was notably increased only by thefilamentous species. Thick crusts were less easily eroded than thin crusts,while biomass was more effective than thickness. Dust was incorporated bestintoMicrocoleus crust when added in small amounts over time,and appeared to increase growth of the cyanobacterium as well as strengthen thecohesion of the crust. Microbial crust cohesion was mainly attributed to algalaggregation, while lichens, fungi and mosses affected more the soil structureand physico-chemical properties.     

Algal crust formation in the inland dune area,Laarder Wasmeer,the Netherlands

The aim of this study is to acquire insight in the initial vegetation development on active drifting sands in relation to geomorphological processes. For this purpose the algal vegetation and surface dynamics were monitored on a sand hill with active drifting sand, and on a bare slope within a terrain covered with Polytrichum piliferum located in the Laarder Wasmeer (The Netherlands).There is a successional development from an algal community dominated by the cyanobacterium Oscillatoria spp., through the crust in which initially the green alga Klebsormidium and later the cyanobacterium Synechococcus predominates, eventually succeeded by the green alga Zygogonium ericetorum. In this phase the sand is stabilized. The areal extent of algal crusts on the active drifting sand area is greatest during the winter, contrastingly highest biomass values are found during the summer and autumn in mature Zygogonium crusts.Substrate instability due to the action of wind clearly limits algal growth and vegetational development on the sand hill. The algal crust seldom reaches the stage of maturity. Near-flat ground is more easily stabilized. Unlike the algae found in coastal dune area and the initial pioneers in the Laarder Wasmeer area, the Zygogonium crust is water repellent when dry. Consequently increasing surface stability by algal crust development is accompanied by higher surface runoff on sloping areas.     

Response of desert biological soil crusts to alterations in precipitation frequency

Biological soil crusts, a community of cyanobacteria, lichens, and mosses that live on the soil surface, occur in deserts throughout the world. They are a critical component of desert ecosystems, as they are important contributors to soil fertility and stability. Future climate scenarios predict alteration of the timing and amount of precipitation in desert environments. Because biological soil crust organisms are only metabolically active when wet, and as soil surfaces dry quickly in deserts during late spring, summer, and early fall, the amount and timing of precipitation is likely to have significant impacts on the physiological functioning of these communities. Using the three dominant soil crust types found in the western United States, we applied three levels of precipitation frequency (50% below-average, average, and 50% above-average) while maintaining average precipitation amount (therefore changing both timing and size of applied events). We measured the impact of these treatments on photosynthetic performance (as indicated by dark-adapted quantum yield and chlorophyll a concentrations), nitrogenase activity, and the ability of these organisms to maintain concentrations of radiation-protective pigments (scytonemin, beta-carotene, echinenone, xanthophylls, and canthaxanthin). Increased precipitation frequency produced little response after 2.5 months exposure during spring (1 April–15 June) or summer (15 June–31 August). In contrast, most of the above variables had a large, negative response after exposure to increased precipitation frequency for 6 months spring–fall (1 April–31 October) treatment. The crusts dominated by the soil lichen Collema, being dark and protruding above the surface, dried the most rapidly, followed by the dark surface cyanobacterial crusts (Nostoc-Scytonema-Microcoleus), and then by the light cyanobacterial crusts (Microcoleus). This order reflected the magnitude of the observed response: crusts dominated by the lichen Collema showed the largest decline in quantum yield, chlorophyll a, and protective pigments; crusts dominated by Nostoc-Scytonema-Microcoleus showed an intermediate decline in these variables; and the crusts dominated by Microcoleus showed the least negative response. Most previous studies of crust response to radiation stress have been short-term laboratory studies, where organisms were watered and kept under moderate temperatures. Such conditions would give crust organisms access to ample carbon to respond to imposed stresses (e.g., production of UV-protective pigments, replacement of degraded chlorophyll). In contrast, our longer-term study showed that under field conditions of high air temperatures and frequent, small precipitation events, crust organisms appear unable to produce protective pigments in response to radiation stress, as they likely dried more quickly than when they received larger, less frequent events. Reduced activity time likely resulted in less carbon available to produce or repair chlorophyll a and/or protective pigments. Our findings may partially explain the global observation that soil lichen cover and richness declines as the frequency of summer rainfall increases.     

毛乌素沙地南缘沙丘生物结皮对凝结水形成和蒸发的影响

在水分极度匮乏的荒漠生态系统,凝结水是除降雨之外最重要的水分来源之一,它对荒漠生态系统结构、功能和过程的维持产生重要的影响。为探明半干旱沙区生物结皮表面的凝结水形成和蒸发特征,采用自制的微型蒸渗计(直径7 cm、高5 cm的PVC管)实验观测了不同类型地表(裸沙、浅灰色藻类结皮、黑褐色藻类结皮和苔藓结皮)对凝结水形成和蒸发的影响。结果表明:(1)观测期间共有20次凝结水形成记录,除降雨天气外,几乎每天都能观测到水分凝结现象;(2)不同类型地表凝结水总量依次为(1.998±0.075),(2.326±0.083),(2.790±0.058)和(3.416±0.068) mm,生物结皮表面的凝结水总量显著大于裸沙(P < 0.05);随生物结皮的发育,不同类型生物结皮表面的凝结水总量呈增加的趋势,凝结水总量之间差异显著(P < 0.05);观测期间不同类型地表日平均凝结水量依次为(0.100±0.003),(0.116±0.004),(0.140±0.002)和(0.171± 0.003) mm,不同类型地表日平均凝结水量之间差异极显著(P < 0.01);(3)凝结水形成过程的观测结果显示,凝结水19:00开始形成,23:00-凌晨1:00形成不明显,1:00-7:00继续形成,除浅灰色藻类结皮外,太阳升出后在黑褐色藻类结皮和苔藓结皮表面继续形成少量的凝结水;凝结水7:30开始蒸发,10:30到11:00之间结束蒸发,凝结水在裸沙和浅灰色藻类结皮中的保持时间显著大于黑褐色藻类结皮和苔藓结皮中的保持时间(P < 0.05);(4)凝结水的形成受大气温度、地表温度、空气相对湿度和大气地表温度差等气象因素的影响,但其形成过程不与某一个气象因素呈简单的线性关系。     

Discovery of the mineral brucite (magnesium hydroxide) in the tropical calcifying alga Polystrata dura (Peyssonneliales,Rhodophyta)

Red algae of the family Peyssonneliaceae typically form thin crusts impregnated with aragonite. Here, we report the first discovery of brucite in a thick red algal crust (~1 cm) formed by the peyssonnelioid species Polystrata dura from Papua New Guinea. Cells of P. dura were found to be infilled by the magnesium‐rich mineral brucite [Mg(OH)₂]; minor amounts of magnesite and calcite were also detected. We propose that cell infill may be associated with the development of thick (> ~5 mm) calcified red algal crusts, integral components of tropical biotic reefs. If brucite infill within the P. dura crust enhances resistance to dissolution similarly to crustose coralline algae that infill with dolomite, then these crusts would be more resilient to future ocean acidification than crusts without infill.     

Effects of sand burial on biomass,chlorophyll fluorescence and extracellular polysaccharides of man-made cyanobacterial crusts under experimental conditions

Soil cyanobacterial crusts occur throughout the world, especially in the semiarid and arid regions. It always encounters sand burial, which is an important feature of mobile sand dunes. A greenhouse study was conducted to determine the effects of sand burial on biomass, chlorophyll fluorescence and extracellular polysaccharides of man-made cyanobacterial crusts in six periods of time (0, 5, 10, 15, 20 and 30 d after burying) and at five depths (0, 0.2, 0.5, 1 and 2cm). The results indicated that with the increase of the burial time and burial depth extracellular polysaccharides content and Fv/Fm decreased correspondingly and there were no significant differences between 20 and 30 burial days under different burial depths. The degradation of chlorophyll a content appeared only at 20 and 30 burial days and there was also no significant difference between them under different burial depths. It was also observed a simultaneous decrease of the values of the Fv/Fm and the content of extracellular polysaccharides happened in the crusted cyanobacterium Microcoleus vaginatus Gom. It may suggest that there exists a relationship between extracellular polysaccharides and recovery of the activity of photosystem II (PS II) after rehydration.     

半荒漠藻结皮中藻类的种类组成和分布

对宁夏沙坡头半荒漠藻结皮中２２种藻（其中蓝藻１０种,绿藻４种,硅藻６种,裸藻２种）进行了分离、鉴定;利用土壤磨片和电子探针揭示了藻体与基质间的结合方式;通过光镜、扫描电镜观察,从８个层次研究了藻类在结皮中的分布,并发现它们在微米间也呈“层片”分布规律,藻结皮由表及里依次为无机矿物质保护层（ｃａ．０．０２ｍｍ）、富藻层（ｃａ．０．０２～２．５ｍｍ）及疏藻层（２．５～５．０ｍｍ）;藻丝皮由表及里依次为     

Diazotrophic Community Structure and Function in Two Successional Stages of Biological Soil Crusts from the Colorado Plateau and Chihuahuan Desert

The objective of this study was to characterize the community structure and activity of N₂-fixing microorganisms in mature and poorly developed biological soil crusts from both the Colorado Plateau and Chihuahuan Desert. Nitrogenase activity was approximately 10 and 2.5 times higher in mature crusts than in poorly developed crusts at the Colorado Plateau site and Chihuahuan Desert site, respectively. Analysis of nifH sequences by clone sequencing and the terminal restriction fragment length polymorphism technique indicated that the crust diazotrophic community was 80 to 90% heterocystous cyanobacteria most closely related to Nostoc spp. and that the composition of N₂-fixing species did not vary significantly between the poorly developed and mature crusts at either site. In contrast, the abundance of nifH sequences was approximately 7.5 times greater (per microgram of total DNA) in mature crusts than in poorly developed crusts at a given site as measured by quantitative PCR. 16S rRNA gene clone sequencing and microscopic analysis of the cyanobacterial community within both crust types demonstrated a transition from a Microcoleus vaginatus-dominated, poorly developed crust to mature crusts harboring a greater percentage of Nostoc and Scytonema spp. We hypothesize that ecological factors, such as soil instability and water stress, may constrain the growth of N₂-fixing microorganisms at our study sites and that the transition to a mature, nitrogen-producing crust initially requires bioengineering of the surface microenvironment by Microcoleus vaginatus.     

半荒漠藻结皮中藻类的种类组成和分布

对宁夏沙坡头半荒漠藻结皮中22种藻(其中蓝藻10种,绿藻4种,硅藻6种,裸藻2种)进行了分离、鉴定;利用土壤磨片和电子探针揭示了藻体与基质间的结合方式;通过光镜、扫描电镜观察,从8个层次研究了藻类在结皮中的分布,并发现它们在微米间也呈"层片"分布规律,藻结皮由表及里依次为无机矿物质保护层(ca.0.02mm)、富藻层(ca.0.02～2.5mm)及疏藻层(2.5～5.0mm);藻丝通过胶鞘中多糖、蛋白质与粘土矿物质直接结合,或直接伸入矿物质内而网络、粘结着砂粒和土粒.     

生物结皮粗糙特征——以古尔班通古特沙漠为例

摘要：空气动力粗糙度可以反映地表气流与下垫面的相互作用。古尔班通古特沙漠是我国最大的固定、半固定沙漠,其间广泛分布的生物结皮在稳定地表和改善环境方面意义重大。对未经扰动的4种类型生物结皮进行表面微形态观察,并通过风洞实验确定动力粗糙度Z0和摩阻风速u*,结果表明：（1）不同生物结皮类型,其组成和表面微形态等都具有明显差异。藻结皮以表面致密光滑为显著特征,由藻类分泌物和藻丝体粘结细粒物质所形成;地衣结皮表面藻类和真菌形成的叶状体匍匐沙面生长,呈现三维生长方式,形成有明显凹凸的壳状覆被;苔藓结皮以苔藓植物体密集丛生为特点,地上部分出现了茎叶分化,有一定的柔韧性。（2）就动力粗糙度的大小而言,是按地衣结皮>藻类－地衣结皮>苔藓结皮>藻结皮的顺序排列的,Z0平均值依次为（6.5890.850）mm、（4.1790.239）mm、（2.5420.357）mm和（0.3930.220）mm,与定床裸沙面的（0.0420.019）mm相比,生物结皮Z0值提高了10—150倍。随着风速的增大Z0值有所减小,其中以地衣结皮的减小趋势较为明显。（3）由风速廓线对比发现,四类生物结皮对气流阻滞作用的差异主要局限于4 cm以下的高度范围,风速越大这种差异也越大。各类生物结皮摩阻风速u*随风速增大而增大,其中藻结皮的增大速率明显低于其它三类结皮,说明藻结皮随风速增大的阻滞效应较其它三类结皮要差。（4）在净风条件下,地衣结皮具有最好的防风效果,其次为藻类－地衣结皮和苔藓结皮,藻结皮最差。当生物结皮破损后,床面结构和气流性质将发生变化,对空气动力学粗糙度和摩阻风速产生的影响将有待于进行更深入的研究。     

Ostracod reactions to non-toxic and toxic algae

Summary When fed algae continuously, laboratory ostracods had a life span of 21–28 days; without food, they were able to survive about 7 days. When exposed to thick suspensions of gas-vacuolate blue-green algae, survival rates were generally low. Twenty five % of our euplanktonic strains killed entire ostracod populations within 24–48 hrs; about 30% of other waterbloom strains were dangerous to ostracods causing either death or coordination loss. Some amphibious non-gas-vacuolated strains were also toxic to ostracods. Toxins from broken cells were not persistent in effect; ostracods surviving the initial shock often appeared recovered within 2–4 days. In di-algal systems, ostracods generally fed on a single species and did not always choose green algae over blue-green. Unicellular green algal strains (Ankistrodesmus, Scenedesmus, Kirchneriella) were generally preferred. Some chlorophycean forms (Zygnema, Hormidium, Trentepholia) were shunned in favor of bluegreens such as Tolypothrix and Westiella. Erect and branched strains (Fischerella, Westiella, Tolypothrix) were eaten before Anabaena and Nostoc with the latter genus being least preferred. Intact Nostoc colonies were usually not ingested, but some loosely growing, non-toxic strains of both Nostoc and Anabaena provided adequate food for ostracod populations indefinitely.     

古尔班通古特沙漠生物结皮不同发育阶段中藻类的变化

通过在古尔班通古特沙漠南缘相同的地貌部位,选择裸沙、藻结皮、地衣结皮和苔藓结皮4种不同演替阶段中的生物结皮,研究了藻类的种类组成、优势种和生物量的变化.结果表明:(1)在结皮的不同演替阶段,藻类种类组成不同,其常见物种有一定的差异,如裸沙中藻类常见种是脆杆藻2(Fragilaria sp.2)、威利颤藻(Oscillatoria willei)和奥克席藻(Phormidium okenii),藻结皮的常见种是小聚球藻(Synechococcus parvus)、颗粒常丝藻 (Tychonema granulatum)、韧氏席藻(Phormidium retzli);同时在不同发育阶段亦存在一些特有种.(2)在裸沙发育到成熟生物结皮的过程中,藻类的优势物种也发生相应的变化.裸沙、藻结皮、地衣结皮和苔藓结皮的优势种分别是脆杆藻1(Fragilaria sp.1)、具鞘微鞘藻(Microcoleus vaginatus)、具鞘微鞘藻、眼点伪枝藻(Scytonema ocellatum)或集球藻(Palmellococcus miniatus).(3)藻类生物量在生物结皮不同演替阶段差异极显著(P<0.01),在裸沙中藻类生物量最低,随着生物结皮的逐渐发育,藻类生物量明显升高,地衣结皮最高,约是裸沙的8.3倍,当发育至苔藓结皮时,藻类生物量又有所下降.(4)在裸沙中基本为松散的沙粒,随着生物结皮的演替,丝状种类占明显的优势,尤其是具鞘微鞘藻,另外真菌菌丝和苔藓假根分别在地衣结皮和苔藓结皮中起着重要作用.     

Development of artificially induced biological soil crusts in fields and their effects on top soil

Aims

Biological soil crusts (BSCs) could improve severe environment ecological conditions by increasing soil moisture, soil nitrogen concentration, and so on. In order to control desertification and recover the destroyed soil fertility utilizing a new means using BSCs, the soil surface was artificially inoculated with Microcoleus vaginatus and Scytonema javanicum. Relationships between the development of the artificially induced biological soil crusts and the distribution and dynamic changes of nitrogen and phosphorus in the soil crusts have been analyzed.

Methods

Crusts of different ages were investigated by measuring soil physical and chemical factors, such as moisture, pH, total and available N content, and total and available P, which were correlated with the depths of the crusts.

Results

This study found that the types of color, shape, and species components of the algal crusts increased with crust development. Soil moisture, total N, available N, and available P increased gradually with crust growth. Soil with crusts was wetter than the controlled naked sandy soil, and a significant correlation was observed between biomass and total nitrogen (r?=?0.946, P?=?0.015). Soil pH was lower than that of control. The scytonemin on the soil surface was exceptionally higher than the other pigments, and all the pigments were mainly distributed at the soil surface level. Though the crusts were mainly distributed on soil surface, the available P was mainly stored below the crust layer.

Conclusions

Pearson correlation tests indicated that artificially inoculated biological crusts could improve soil fertility and micro-environment of the top soil: The development of artificially induced BSCs was very well, and this was favorable to inducing the following crust succession.     

MICROENVIRONMENTS AND MICROSCALE PRODUCTIVITY OF CYANOBACTERIAL DESERT CRUSTS1

We used microsensors to characterize physicochemical microenvironments and photosynthesis occurring immediately after water saturation in two desert soil crusts from southeastern Utah, which were formed by the cyanobacteria Microcoleus vaginatus Gomont, Nostoc spp., and Scytonema sp. The light fields within the crusts presented steep vertical gradients in magnitude and spectral composition. Near-surface light-trapping zones were formed due to the scattering nature of the sand particles, but strong light attenuation resulted in euphotic zones only ca. 1 mm deep, which were progressively enriched in longer wavelengths with depth. Rates of gross photosynthesis (3.4–9.4 mmol O₂·m^?2·h^?1) and dark respiration (0.81–3.1 mmol O^?2·m^?2·h^?1) occurring within 1 to several mm from the surface were high enough to drive the formation of marked oxygen microenvironments that ranged from oxygen supersaturation to anoxia. The photosynthetic activity also resulted in localized pH values in excess of 10, 2–3 units above the soil pH. Differences in metabolic parameters and community structure between two types of crusts were consistent with a successional pattern, which could be partially explained on the basis of the microenvironments. We discuss the significance of high metabolic rates and the formation of microenvironments for the ecology of desert crusts, as well as the advantages and limitations of microsensor-based methods for crust investigation.     

Effects of biological soil crusts on seed germination of four endangered herbs in a xeric Florida shrubland during drought

Soil crusts of rosemary scrubs in south-central Florida were examined for effects on seed germination of four herbs that are killed by fire and must recruit from seed: Eryngium cuneifolium (Apiaceae), Hypericum cumulicola (Hypericaceae), Polygonella basiramia (Polygonaceae), and Paronychia chartacea ssp. chartacea (Caryophyllaceae). Biological soil crusts in these sites are dominated by algae, cyanobacteria, fungi, and bacteria. Because crusts can change soil stability, water, and nutrients, they can affect seed germination. A series of greenhouse and field experiments were designed to first examine the effects of crusts in isolation and then to determine their role in the context of other environmental factors – time since fire and distance to the dominant shrub in this system, Ceratiola ericoides. In the greenhouse experiment, germination in autoclaved crusts was dramatically reduced relative to germination in living crusts for all but P. basiramia. In four field experiments where crusts were left intact, disturbed (mechanically or by flaming), or completely removed, the effects of crusts were variable and species-specific, but were significant enough to impact aboveground population sizes. More germination was consistently observed in recently burned sites away from C. ericoides shrubs. Overall rates of germination were generally very low during this study, possibly as a result of seasonal droughts that could have reduced germination, increased seed dormancy, and/or decreased seed viability. The importance of water for germination was confirmed in an experiment with two watering regimes and three crust treatments designed to create a gradient of soil water availability. Germination was significantly greater in the high water treatment and unaffected by soil crust moisture. Dry years are not uncommon in scrub and the results of this study help us to understand how scrub herbs fare during drought and what role biological soil crusts play in germination.     

Changes in periphytic algal community structure as a consequence of short herbicide exposures

Effects of the triazine herbicides simazine and terbutryn on total biovolume and community structure of haptobenthic periphytic algal communities within in situ marsh enclosures are described. Levels of biovolume inhibition in excess of 98% relative to an untreated control were observed at all levels of terbutryn tested (0.01, 0.1 and 1.0 mg l^–1). No reduction in total biovolume was observed at 0.1 mg l^–1 simazine, with increasing inhibition (to 98%) at 1.0 and 5.0 mg l^–1. Following incidental enclosure flooding and removal of herbicide, increases in biovolume were observed in all but the highest treatment levels, with rates of colonization similar to that of the control.Pre-flood community structure of periphyton in simazine-treated enclosures was qualitatively similar to that of the control, while a small blue-green alga was abundant only in terbutryn-treated enclosures. After flooding, substratum colonization in most experimental enclosures was dominated by the diatom Cocconeis placentula, while this taxon accounted for about 25% of total biovolume on substrata from the control and 0.1 mg l^–1 simazine enclosures. It is concluded that periphyton successional processes, which normally lead to the development of a complex 3-dimensional mat, may be averted by short herbicide exposures.     

Effect of Humic Substances on the Growth of Microalgal Cultures

Treatment with humic substances (preparation Biomin) considerably increased viability of algae during long storage (6 months) in collection. The effect was greater with green algae (Scenedesmus and Chlorella) than with blue-green algae (Nostoc and Anabaena). In intensive cultures, lower Biomin concentrations (1–10 mg/l) increased the biomass production, protein, chlorophyll and carotenoid content, as well as -esterase and glutamate dehydrogenase activities, whereas the lipid and carbohydrate content was higher at 100 mg/l Biomin. A high Biomin concentration (1 g/l) exerted an inhibition. Biomin supply to the cultures in outdoor installation of Shetlik type was associated with a moderate stimulation of algal biomass accumulation. It was concluded that Biomin might be helpful in the improvement of algal viability and growth.__________From Fiziologiya Rastenii, Vol. 52, No. 3, 2005, pp. 463–466.Original English Text Copyright © 2005 by Pouneva.This article was submitted by the author in English.     

CO2 Exchange in soil algal crusts occurring in the trachypogon savannas of the Orinoco Llanos,Venezuela

CO₂ exchange between the atmosphere and soil algal crusts of the Trachypogon savannas of the Orinoco Llanos has been analyzed using an open gas exchange system. These savannas encompass a wide range of physiognomic types, from herbaceous communities to savanna woodlands. A maximum CO₂ flux of 0.207 mg m^-2 s^-1 was measured in the crusts of the Guanipa savannas, while in the other examined crusts (0.035–0.105 mg m^-2 s^-1) the flux was similar to values reported for terrestrial algae. The CO₂ flux data were statistically fitted to the photosynthetically active radiation by a logarithmic relationship, and the photosynthetic efficiencies of the crusts were compared. The activation energy calculated for the CO₂ fixation indicates that limitations by diffusion and photochemical processes were excluded in the Guanipa crusts (above 12 kcal mole^-1), whereas they were evident in the other crust studied. An optimum CO₂ incorporation as a function of the crust water potential was established and carbon gain strategies were proposed on the basis of the results and characteristics of the habitats.     

The composition and importance of the phytoneuston in two floodplain lakes in south-eastern Australia

Normans Lagoon and 3-Gum pond are small floodplain water bodies adjacent to the Murray River, south-eastern Australia, and often have a visible film/sheen across their surface. Since few studies have provided quantitative comparisons of the surface and subsurface layer communities of shallow freshwater lakes, we determined the contributions of the surface and subsurface populations to overall algal biomass when a surface film was visible, and when it was not visible. We examined the algae and cyanobacteria present at the air–water interface of each water body, and compared the findings with those for the water immediately below the surface, and for the overall water column. The algal groupings Trachelomonas spp., other Euglenophyceae (principally Euglena spp.), Chlorophyceae and Cyanobacteria usually comprised >95% of the measured biovolume within all samples. Samples from the air–water interface were considerably enriched (up to 200-fold) with respect to algal biovolume, whether or not a visible surface film was present, and elevated cell counts were observed within the air–water interface for motile organisms such as Trachelomonas spp. and green unicellular flagellates. The reverse was true for the cyanobacterium Planktolyngbya however, with greater concentrations occurring at depth. In terms of its contribution to the overall algal/cyanobacterial populations within each water body, the surface layer was found to be responsible for <1–20% of the biovolume over the entire water column. Multivariate statistical analysis confirmed there were significant differences between the communities of the air–water interface and those of the water below, and that these differences occurred both in the presence and absence of a visible surface film/sheen. Handling editor: D. Ryder     

Algal colonization and its ecophysiology on the fine sculptures of terracotta monuments of Bishnupur,West Bengal,India

The blackish brown crust on the fine carvings of seven terracotta monuments of Bishnupur were studied with regards to the presence of algal species. Forty five taxa of micro algae belonging to 36 species of cyanobacteria, 5 species of chlorophyta and 4 species of bacillariophyta were found in crusts of these monuments. Among the algal groups, cyanobacteria were the dominant population growing in all the monuments with maximum diversity of 30 species in Rasmancha. Lyngbya corticicola, Scytonema schmidtii and Nostoc microscopicum were the major taxa in all crust samples. The absorption ratio of chlorophyll: carotenoid: MAAs (mycosporine-like amino acid substances) and scytonemin showed scytonemin and MAAs were more abundant than the chlorophyll pigment in all the crust. The MAAs and scytonemin of the crusts were identified as Mycosporine glycine having absorption maxima at 310 nm with retention time (RT) 4.1 min and scytonine having absorption maxima 386 nm with RT 3.2 min respectively. Among the major crust cyanobacteria cellular carbohydrate of N. microscopicum was maximum and lowest in S. schmidtii. However, the extra polymeric substance released to the medium was more in L. corticicola and so intense that the medium become coloured. The slimes of the major crust cyanobacteria plug the fine carvings of the terracotta structures which led to disfiguration of fine architecture, act as a biofilm that facilitated growth of fungi, lichens and subsequently to mosses and small plants leading to create creaks in the monuments and deteriorate these archeologically important monuments.