ROLE OF SETTLEMENT DENSITY ON GAMETOPHYTE GROWTH AND REPRODUCTION IN THE KELPS PTERYGOPHORA CALIFORNICA AND MACROCYSTIS PYRIFERA (PHAEOPHYCEAE)1

Laboratory studies were used to examine how variation in the density of spore settlement influences gametophyte growth, reproduction, and subsequent sporophyte production in the kelps Pterygophora californica Ruprecht and Macrocystis pyrifera (L.) C. Ag. In still (non-aerated) cultures, egg maturation in both species was delayed when spores were seeded at densities 300 · mm^?2. Although the density at which this inhibition was first observed was similar for both species, the age at which their eggs matured was not. P. californica females reached sexual maturity an average of 4 days (or ～ 30%) sooner than did M, pyrifera. As observed previously in field experiments, per capita sporophyte production was negatively density dependent for both species when seeded at spore densities of 10 · mm^?2. Total sporophyte production (i.e. number · cm^?2) for both species, however, was greatest at intermediate densities of spore settlement (～ 50 spores · mm^?2). In contrast, total sporophyte production by P. californica steadily increased with increasing spore density in aerated cultures; highest sporophyte density was observed on slides seeded at a density of 1000 spores · mm^?2. Preliminary experiments with P. californica involving manipulation of aeration and nutrients indicate that inhibition of gametophyte growth and reproduction at higher densities of spore settlement in non-aerated cultures was probably caused by nutrient limitation.     

Wet and dry density of Bacillus anthracis and other Bacillus species

Aims: To determine the wet and dry density of spores of Bacillus anthracis and compare these values with the densities of other Bacillus species grown and sporulated under similar conditions. Methods and Results: We prepared and studied spores from several Bacillus species, including four virulent and three attenuated strains of B. anthracis, two Bacillus species commonly used to simulate B. anthracis (Bacillus atrophaeus and Bacillus subtilis) and four close neighbours (Bacillus cereus, Bacillus megaterium, Bacillus thuringiensis and Bacillus stearothermophilus), using identical media, protocols and instruments. We determined the wet densities of all spores by measuring their buoyant density in gradients of Percoll and their dry density in gradients of two organic solvents, one of high and the other of low chemical density. The wet density of different strains of B. anthracis fell into two different groups. One group comprised strains of B. anthracis producing spores with densities between 1·162 and 1·165 g ml^?1 and the other group included strains whose spores showed higher density values between 1·174 and 1·186 g ml^?1. Both Bacillus atrophaeus and B. subtilis were denser than all the B. anthracis spores studied. Interestingly and in spite of the significant differences in wet density, the dry densities of all spore species and strains were similar. In addition, we correlated the spore density with spore volume derived from measurements made by electron microscopy analysis. There was a strong correlation (R² = 0·95) between density and volume for the spores of all strains and species studied. Conclusions: The data presented here indicate that the two commonly used simulants of B. anthracis, B. atrophaeus and B. subtilis were considerably denser and smaller than all B. anthracis spores studied and hence, these simulants could behave aerodynamically different than B. anthracis. Bacillus thuringiensis had spore density and volume within the range observed for the various strains of B. anthracis. The clear correlation between wet density and volume of the B. anthracis spores suggest that mass differences among spore strains may be because of different amounts of water contained within wet dormant spores. Significance and Impact of the Study: Spores of nonvirulent Bacillus species are often used as simulants in the development and testing of countermeasures for biodefense against B. anthracis. The similarities and difference in density and volume that we found should assist in the selection of simulants that better resemble properties of B. anthracis and, thus more accurately represent the performance of countermeasures against this threat agent where spore density, size, volume, mass or related properties are relevant.     

Abundance and viability of fungal spores along a forestry gradient – responses to habitat loss and isolation?

Regional variation in spore deposition and viability was studied for two fungi, Fomitopsis rosea (Alb. & Schwein.: Fr.) P. Karst. and Phlebia centrifuga P. Karst., both confined to old‐growth spruce forests in the boreal zone. Seven regions in Sweden were studied along a north‐south transect in which the historical impact from forestry increases and the amount old forests decreases towards the south. The two southernmost regions were located outside the distribution border of the species. Spore deposition was measured species specifically as heterokaryotisation of homokaryotic mycelia growing on wood discs. There was a significant decline in spore deposition towards the south for both species. F. rosea deposited an average amount of 111 spores m^?2 24 h^?1 in the northernmost region compared to less than 1 spore in the four southernmost regions. The corresponding values for P. centrifuga were 27 spores m^?2 24 h^?1 in the north compared to less than 2 spores in the 4 southernmost regions. No deposition was found south of the distribution borders. The viability of spores from local populations within each region was measured as germination success on nutrient media. Individual fruiting bodies from large populations in the north generally produced spores with higher germinability than fruiting bodies from geographically isolated populations in the central and southern regions. However, there was a high variation among the southern populations. Our data suggest that some populations in mid‐ and south Sweden may suffer from negative genetic effects, possibly associated with fragmentation and loss of habitat. Thus, the combination of low spore deposition and low germinability of spores may be a threat to the long‐term persistence of F. rosea and P. centrifuga in southern Sweden. Several other species may experience the same situation, especially when considering the severe decline of dead wood in Swedish forests.     

PHOTOSYNTHETIC PHYSIOLOGY AND CHEMICAL COMPOSITION OF SPORES OF THE KELPS MACROCYSTIS PYRIFERA,NEREOCYSTIS LUETKEANA,LAMINARIA FARLOWII,AND PTERYGOPHORA CALIFORNICA (PHAEOPHYCEAE)1

Recently released spores of the kelps Macrocystis pyrifera (L.) C. Ag., Nereocystis luetkeana (Mert.) Post. and Rupr., Laminaria farlowii Setch., and Pterygophora californica Rupr. had different levels of net photosynthesis. Spore-specific photosynthesis–irradiance relationships were similar in many respects for M. pyrifera, N. luetkeana, and L. farlowii spores. All three species had low rates of net light-saturated photosynthesis. In contrast, spores of P. californica had higher photosynthetic potential and overall net photosynthesis than the other three species. On a cell carbon basis, however, photosynthetic rates in N. luetkeana spores were similar to those of P. californica spores and higher than those of M. pyrifera spores. Chlorophyll a content of spores varied 10-fold among species. The rank order of significant differences in chlorophyll a content was P. californica > L. farlowii > N. luetkeana > M. pyrifera. As a result, chlorophyll-specific measurements suggest M. pyrifera and N. luetkeana spores had much higher quantum efficiency and photosynthetic potential than either P. californica or L. farlowii spores. Maternal carbon and nitrogen investment significantly differed in spores of M. pyrifera, N. luetkeana, and P. californica with P. californica > M. pyrifera > N. luetkeana. Carbon content in spores of each of these three species increased by about 30% during 12 h of saturating irradiance. We suggest that the photosynthetic capabilities of and maternal investment in spores may be related to the spore as a unit of dispersal, to the reproductive ecology of the parental sporophytic stages, and to the growth and physiology of the germling gametophyte stages.     

Intradiurnal periodicity of fungal spore concentrations (Alternaria, Botrytis, Cladosporium, Didymella, Ganoderma) in Cracow, Poland

Aerobiological monitoring enables the definition of seasonal fungal spore concentrations and also intradiurnal time when the highest concentrations of spores could cause or increase allergy symptoms. These data are useful to estimate symptoms of disease, duration of infection and how advanced the illness is in people suffering from fungal allergens. The aim of the study was to compare the concentrations of fungal spores (Alternaria, Botrytis, Cladosporium, Didymella, Ganoderma) during dry and rainy periods and to analyse their intradiurnal changes. Average daily spore concentrations in dry and rainy periods were compared, using z test, separately for each taxon, season and for a combined 3-year period. Intradiurnal periodicity of fungal spore concentrations was analysed on the basis of three complementary diagrams. These spore concentrations were presented using three curves for all, dry and rainy days in 1997–1999 (April–November). The spore percentage in particular hours was normalized in relation to the daily spore sum accepted as 100%. Two further diagrams enabled the more precise analysis of the highest concentrations in dry days. Daily Botrytis and Cladosporium spore concentrations did not show significant differences between dry and rainy periods. In the case of Didymella and Ganoderma spore concentrations, there were no significant differences between both weather types in the single years, although there was a significant difference when a 3-year period was considered. The differences between daily concentrations of Alternaria spores in dry and rainy periods occurred in 1997 and in a 3-year period. Intradiurnal periodicity of spore concentrations was different for ‘dry’ and ‘wet’ fungal spores. Dry spores are released from the spore-producing parts of the fungus under conditions of decreasing humidity and increasing airflow. Examples of dry spores are those from Alternaria, Cladosporium and Botrytis. Wet spores, such as those from many Ascomycetes (Didymella) and Basidiomycetes (Ganoderma), are released into the atmosphere by processes related to humidity conditions or rain. The highest concentrations of ‘dry’ spores were observed early in the afternoon, while highest values of ‘wet’ spore concentrations occurred in the predawn hours. Statistically non-significant differences between daily spore concentrations in dry and rainy periods of single seasons were found except for Alternaria. Statistically significant differences could occur when the studied period was longer than one season (Alternaria, Didymella, Ganoderma). The highest concentrations of Alternaria, Botrytis and Cladosporium spores were recorded at noon and early in the afternoon. Concentrations of Didymella and Ganoderma spores were highest in the predawn hours.     

The explosive diversification of the diatom genus Chaetoceros across the Eocene/Oligocene and Oligocene/Miocene boundaries in the Norwegian Sea

Eocene to middle Miocene stratigraphic changes in species richness, abundance and valve size of Chaetoceros resting spores in the Norwegian Sea (DSDP Site 338) were investigated in order to understand past productivity and paleoenvironmental changes in upwelling regions. As a result, drastic resting spore events were recognized in a 6 myr interval across the Eocene/Oligocene boundary (EO Event), the Oligocene/Miocene boundary (OM Event) and in the early middle Miocene (emM Event). The EO Event was characterized by explosive diversification at both the morpho-generic and specific levels, an increase in abundance, and a decrease in valve size from the upper Eocene through the lowest Oligocene. The OM Event was defined by a two-fold increase in species richness. During the emM Event spore abundance decreased rapidly, and species richness and valve size decreased gradually. These changes may indicate changes in the nutrient supply, especially in upwelling regions. The increased species richness suggests a change from a stable water column with a constant nutrient supply in the Eocene to an unstable one with a sporadic nutrient supply by increased vertical mixing in the Oligocene, based on evaluation of the ecologic differences between dinoflagellate cysts and Chaetoceros resting spores. The role of main primary producer might have switched from dinoflagellates and/or nannoplankton in the Eocene to diatoms, especially Chaetoceros, in the Oligocene in the Norwegian Sea. Increased resting spore species richness during the OM Event may show that environmental changes such as global cooling and nutrient mixing led to a diversification of the spore producing genus Chaetoceros. The emM Event might have been affected by changes in paleoceanographic conditions, perhaps a decrease in nutrient supply. This study presents the first paleoceanographic analysis using not only the total resting spore abundance but also the abundances of individual species, and establishes the value of spore taxonomy and diatom analysis including spores.     

Size matters for violent discharge height and settling speed of Sphagnum spores: important attributes for dispersal potential

Background and Aims

Initial release height and settling speed of diaspores are biologically controlled components which are key to modelling wind dispersal. Most Sphagnum (peat moss) species have explosive spore liberation. In this study, how capsule and spore sizes affect the height to which spores are propelled were measured, and how spore size and spore number of discharged particles relate to settling speed in the aspherical Sphagnum spores.

Methods

Spore discharge and spore cloud development were filmed in a closed chamber (nine species). Measurements were taken from snapshots at three stages of cloud development. Settling speed of spores (14 species) and clusters were timed in a glass tube.

Key Results

The maximum discharge speed measured was 3·6 m s⁻¹. Spores reached a maximum height of 20 cm (average: 15 cm) above the capsule. The cloud dimensions at all stages were related positively to capsule size (R² = 0·58–0·65). Thus species with large shoots (because they have large capsules) have a dispersal advantage. Half of the spores were released as singles and the rest as clusters (usually two to four spores). Single spores settled at 0·84–1·86 cm s⁻¹, about 52 % slower than expected for spherical spores with the same diameters. Settling speed displayed a positive curvilinear relationship with spore size, close to predictions by Stokes'' law for spherical spores with 68 % of the actual diameters. Light-coloured spores settled slower than dark spores. Settling speed of spore clusters agrees with earlier studies. Effective spore discharge and small, slowly settling spores appear particularly important for species in forested habitats.

Conclusions

The spore discharge heights in Sphagnum are among the greatest for small, wind-dispersed propagules. The discharge heights and the slow settling of spores affect dispersal distances positively and may help to explain the wide distribution of most boreal Sphagnum species.     

Morphological and molecular comparison of <Emphasis Type="Italic">Myxobolus procerus</Emphasis> (Kudo, 1934) and <Emphasis Type="Italic">M. intramusculi</Emphasis> n. sp. (Myxozoa) parasitising muscles of the trout-perch <Emphasis Type="Italic">Percopsis omiscomaycus</Emphasis>

Inter- and intracellular cysts from a presumed single species, Myxobolus procerus (Kudo, 1934), parasitising the trout-perch Percopsis omiscomaycus were compared with respect to their spore morphology and small subunit 18S ribosomal DNA. The two cyst types have similar pyriform spores and comparable ranges in spore length and width, size of the polar capsules and number of filament coils. However, fixed spores of the intracellular cysts are significantly shorter (p < 0.05) in mean length than those from intercellular cysts, giving them a broader appearance. Comparing 991 base pairs of 18S rDNA revealed a 20 bp difference between the two cyst types (97.9% similarity), 10 due to base substitutions and 10 due to the presence or absence of a base. The described morphological differences and consistent 2.1% difference in the 18S rDNA reveal that the intracellular form is a separate species, which is described herein as Myxobolus intramusculi n. sp. M. intramusculi n. sp. and M. procerus may be sister species and it is suggested that the two species arose through sympatric speciation involving a switch in site specificity within a single host species.     

Effect of tree species on the coverage and field persistence of Bacillus thuringiensis spores

The effect of leaves of Quercus agrifolia, Cercis occidentalis, Eucalyptus globulus, and Juglans regia on the initial deposit and subsequent rate of decay of viable spores of Bacillus thuringiensis was investigated. Significant differences in the size of the initial viable spore deposit were found between the various species, with E. globulus markedly lower than the others. The viable spore decay patterns of the various species were analyzed using a segmented linear model and significant differences in initial rates of decay were also measured. Thus it appears that the characteristics of the leaves of the treated plants may have an effect on the coverage and field persistence of viable B. thuringiensis spores. Some possible mechanisms for these effects are discussed.     

First report of fern (Culcita macrocarpa) spore consumption by a small mammal (Apodemus sylvaticus)

Few vertebrates are known to consume ferns regularly. Several species of mammals consume leaves to some extent but the consumption of fern spores is much rare. In Galicia (Northwest Spain) we studied the seasonal variation in the consumption of Culcita macrocarpa fertile leaves (i.e. with spores) in two populations (Capelada and Eume), assessed whether consumption rate increased with fern population size, and evaluated whether the consumer was a spore predator or a spore disperser. Consumption began in December and finished by mid February, and occurred before spore release, which happened later in Capelada than in Eume, probably influenced by differences in altitude. The consumer was identified as Apodemus sylvaticus by DNA analysis of its droppings and by capture of live animals. Throughout Galicia there was a significant increase in fern consumption rate as the population size of C. macrocarpa increased. Germination tests from droppings were carried out in 14 dishes but only in two dishes 1% and 0.3% of the spores germinated. Our results suggest that woodmouse can disperse spores of C. macrocarpa, although most of the spores were digested.     

Observations of spore morphology of some Pottiaceae Schimp. species (Bryophyta) in Turkey

The spores of Syntrichia ruralis (Hedw.) F. Weber and D. Mohr., S. princeps (De Not.) Mitt., S. subulata (Hedw.) F.Weber and D.Mohr var. subulata, S. subulata (Hedw.) F.Weber and D.Mohr var. angustata (Schimp.) J.J. Amann and S. subulata (Hedw.) F.Weber and D.Mohr var. graeffii Warnst. were studied by light and scanning electron microscopy for the first time. The apertural region consists of a leptoma in all spores. All taxa of the family are uniform in their spore morphology. The spores of the five taxa are of granuloid type. The spore wall of the Pottiaceae family includes sclerine (the distinction between exine and perine may be difficult to define) and intine. The taxonomy of the genus Syntrichia is discussed on the basis of its spore morphology.     

A habitat colonisation model for spore-dispersed organisms—Does it work with eumycetozoans?

Spore productivities and establishment probabilities of eumycetozoans were estimated and compared with quantitative data obtained from field surveys, using series of cultures of a given substrate. Spore numbers per spore case were found to increase from one to four in protostelids to up to 10⁵–10⁶ in myxomycetes, whereas average spore size decreased slightly from 14.8 μm for protostelids to 10.3 μm in myxomycetes. Spore numbers of fructifications calculated from dimensions of spores and fruit bodies were in good agreement with direct counts carried out for six species of myxomycetes. A colonisation model is presented that estimates frequencies (as a percent of successfully colonized habitat islands), which is independent of a given density of spore rain and the sexual system of the species being considered. Whereas asexual species need a minimum spore rain of ca 0.7 spores per habitat island to reach a frequency of 50 %, this figure is at least 2.4-fold higher for sexual species, depending from the incompatibility system assumed. Data from cultures indicate that the maximum potential spore rain is usually three orders of magnitude higher than the minimum figure required to create the observed frequencies. Eumycetozoans seem to follow the evolutionary trends predicted by the model. Species with sexual reproductive systems produce often more spores than asexual ones; many morphospecies have sexual and asexual strains; and back-conversion from sexual to asexual reproduction occurs occasionally.     

Analysis of broth-cultured Bacillus atrophaeus and Bacillus cereus spores

Aims: To compare physical properties of spores that were produced in broth sporulation media at greater than 10⁸ spores ml⁻¹. Methods and Results: Bacillus atrophaeus reproducibly sporulated in nutrient broth (NB) and sporulation salts. Microscopy measurements showed that the spores were 0·68 ± 0·11 μm wide and 1·21 ± 0·18 μm long. Coulter Multisizer (CM3) measurements revealed the spore volumes and volume-equivalent spherical diameters, which were 0·48 ± 0·38 μm³ and 0·97 ± 0·07 μm, respectively. Bacillus cereus reproducibly sporulated in NB, sporulation salts, 200 mmol l⁻¹ glutamate and antifoam. Spores were 0·95 ± 0·11 μm wide and 1·31 ± 0·17 μm long. Spore volumes were 0·78 ± 0·61 μm³ and volume-equivalent spherical diameters were 1·14 ± 0·11 μm. Bacillus atrophaeus spores were hydrophilic and B. cereus spores were hydrophobic. However, spore hydrophobicity was significantly altered after treatment with pH-adjusted bleach. Conclusions: The utility of a CM3 for both quantifying Bacillus spores and measuring spore sizes was demonstrated, although the volume between spore exosporium and spore coat was not measured. This study showed fundamental differences between spores from a Bacillus subtilis- and B. cereus-group species. Significance and Impact of the Study: This is useful for developing standard methods for broth spore production and physical characterization of both living and decontaminated spores.     

Fine Structure of Five Species of Myxomycetes with Clustered Spores*

SYNOPSIS. Electronmicroscopic studies were made of 5 species of Myxomycetes with clustered spores to determine the exact mechanism holding the spores together. In Dianema corticatum the neighboring spore walls were closely appressed and appeared to be fused at numerous points. In Trichia synsporum there was a substance between the outer spore walls of adjacent spores which seemed to cement them together. In Badhamia nitens the spore walls were completely fused along their closely appressed surfaces. The adjacent spores of Badhamia versicolor appeared to be free, or if any fusion point was present it was only at the tips of the ornamentation. In Physarum bogoriense such tips were in contact with those of neighboring spores and were occasionally fused at these points.     

Spore production and mycorrhizal development in various tropical crop hosts infected with Glomus clarum

Plant growth, mycorrhizal development and vesicular arbuscular spore production were examined in five tropical crop host species inoculated with Glomus clarum and grown in a glasshouse. In one of the two experiments, sequential harvests of maize, sorghum and chickpea were made in order to study spore production in relation to plant growth and mycorrhizal development. Spore numbers in each of these hosts increased at a fairly constant rate until maximum plant dry weight, when spore production ceased. Sorghum and maize produced considerably more spores than chickpea, with spore numbers being closely correlated with mycorrhizal root length. In the second experiment, Glomus clarum was cultured on each of maize, millet, sorghum, groundnut and chickpea for three consecutive generations before cross-inoculation of the spores from each host onto all five hosts. Sporulation with respect to host size was generally greatest when the inoculum used to infect a host had been produced on that host. The growth-promoting effects of the fungus were not influenced by the source of the inoculum. More spores were produced on the cereals than the legumes. Differences in spore numbers amongst hosts and plant generations were apparently influenced mainly by infected root length and by the growth period.     

Growth and nitrogen fixation ofAzolla pinnata andAzolla caroliniana as affected by urea fertilizer and their influence on rice yield

Soils from avocado (Persea americana Mill.) orchards in Israel (IS) and California (CA), both sites with a Mediterranean climate, were sampled and analyzed for the species and quantities of vesicular-arbuscular mycorrhizal fungus (VAMF) spores in them, and for soil physical and chemical characteristics.Numbers of spores were similar in soil from IS and CA but the dominant VAMF species were very different. In IS the most common fungi were Sclerocystis sinuosa and Glomus macrocarpum. In CA, Gl. constrictum was present in every orchard examined and Gl. fasciculatum was nearly as widespread. Acaulospora spp. and other Glomus spp. also were found, including A. elegans which has never before been reported from CA.The differences in VAMF populations and species constituents found on two continents but in areas with similar climates and soil types may be due to host or edaphic factors. Different avocado rootstocks are used in the two countries and lower pH and higher soil fertility levels were present in CA soils.The total VAMF spore populations in each orchard was about 275 per 100 mL soil. The population level was not correlated with any of the soil physical or chemical characteristics examined nor with avocado cultivar or age. In IS no fungus spores were found in three orchards; available P, Ca, Mg and Cu levels were high in these soils.     

Spore morphology of the north Asian members of Cystopteridaceae

Spores of Cystopteridaceae from northern Asia were examined using scanning electron microscopy. To evaluate the utility of spore morphology in the taxonomy of each genus, we examined spores of 14 species: seven species each of Gymnocarpium and Cystopteris. Among these are 12 species occurring in northern Asia and two species from other regions for comparative studies. The study focused particularly on perispore characters and spore size. Spores of all species examined are monolete, bean-shaped, with a range in spore size of 26–56 × 18–37 μm for Cystopteris and 25–48 × 16–34 μm for Gymnocarpium. The perispore is morphologically diverse within Cystopteris, but less so within Gymnocarpium. The perispore of the Cystopteris spores is characterised by folds and spines that are separate or form complex sculptural elements. Sacci, ridges and flanges, sometimes on the same spore, are characteristic of the perispore of Gymnocarpium. Spores have straight laesura over which the perispore forms a crest. The crest represents a high and flat fold, which is entire, foveolate or reticulate.     

Release of elicitors from rice blast spores under the action of reactive oxygen species

Effects of reactive oxygen species (ROS) on the release of putative elicitors from spores of rice blast causal fungus Magnaporthe grisea (Hebert) Barr were studied. While studying the influence of exogenous ROS, the spores were germinated for 5 h in the presence of 50 μM H₂O₂ and then treated with catalase to decompose hydrogen peroxide. The spore germination fluid was then boiled to inactivate catalase. When the resulting diffusate was applied onto rice (Oryza sativa L.) leaves, it caused necroses and stimulated superoxide (O₂⁻) production. Both effects were observed with the resistant rice cultivar but not with the cultivar susceptible to the fungal strain. The susceptible cultivar did not acquire resistance to challenge with fungal spores, which were applied one day after the treatment. The fractionation of the spore diffusate showed that both low- and high-molecular compounds (mol wt < 3 kD and >3 kD, respectively) should be present in combination to induce O₂⁻ production by leaves. The diffusates from spores germinated in water also caused necroses and stimulated O₂⁻ generation, though to a weaker extent than diffusates from spores germinated in H₂O₂. The effect of diffusates from spores germinated in water was abolished by catalase or superoxide dismutase added initially to the spore suspension. The results suggest that germinating spores of M. grisea are able to release elicitors and this ability depends on ROS formation by spores. Presumably, the yield of elicitors is increased additionally if fungus M. grisea is stressed or subjected to exogenous ROS. The described phenomena may be involved in incompatibility mechanisms.     

Diversity of arbuscular mycorrhizal fungi across a fragmented forest in Panama: insular spore communities differ from mainland communities

It is now understood that alterations in the species composition of soil organisms can lead to changes in aboveground communities. In this study, we assessed the importance of spatial scale and forest size on changes in arbuscular mycorrhizal fungal (AMF) spore communities by sampling AMF spores in soils of forested mainland and island sites in the vicinity of Gatun Lake, Republic of Panama. We encountered a total of 27 AMF species or morphospecies, with 17, 8, 1 and 1 from the genera Glomus, Acaulospora, Sclerosystis, and Scutellospora, respectively. At small scales (<100 m²), we found little evidence for spatial structuring of AMF communities (decay of Morisita-Horn community similarity with distance). However, at large spatial scales, we found that the AMF spore community of a mainland plot was more similar to other mainland plots several kilometers (>5) away than to nearby island plots (within 0.7 km). Likewise, most island plots were more similar to other island plots regardless of geographic separation. There was no decay in AMF species richness (number of species), or Shannon diversity (number of species and their spore numbers) either with decreasing forest-fragment size, or with decreasing plant species richness. Of the six most common species that composed almost 70% of the total spore volume, spores of Glomus tsh and G. clavisporum were more common in soils of mainland plots, while spores of Glomus small brown and Acaulospora mellea were more abundant in soils of island plots. None of these common AMF species showed significant associations with soil chemistry or plant diversity. We suggest that the convergence of common species found in AMF spore communities in soils of similar forest sizes was a result of forest fragmentation. Habitat-dependent convergence of AMF spore communities may result in differential survival of tree seedlings regenerating on islands versus mainland.     

Population dynamics of an ice-associated diatom, Thalassiosira australis Peragallo, under fast ice near Syowa Station, East Antarctica, during austral summer

A clear shift from vegetative cells to auxospores and resting spores in Thalassiosira australis was observed in the water column and sinking fluxes under the fast ice near Syowa Station in the austral summer of 2005/2006. This is the first report of the auxosporulation by T. australis in situ. Resting spores were also observed in the sediment even before new spore formation, suggesting that T. australis can overwinter in the sediment. Heterotrophic dinoflagellates ingested and digested vegetative cells and auxospores but did not digest resting spores, suggesting a high tolerance of resting spores to grazing by heterotrophic dinoflagellates. We discuss the possible life history and overwintering strategy that T. australis uses in an Antarctic coastal area to cope with the unpredictable timing of sea ice growth and decay.