Salinity, depth and the structure and composition of microbial mats in continental Antarctic lakes

1. Lakes and ponds in the Larsemann Hills and Bølingen Islands (East‐Antarctica) were characterised by cyanobacteria‐dominated, benthic microbial mats. A 56‐lake dataset representing the limnological diversity among the more than 150 lakes and ponds in the region was developed to identify and quantify the abiotic conditions associated with cyanobacterial and diatom communities. 2. Limnological diversity in the lakes of the Larsemann Hills and Bølingen Islands was associated primarily with conductivity and conductivity‐related variables (concentrations of major ions and alkalinity), and variation in lake morphometry (depth, catchment and lake area). Low concentrations of pigments, phosphate, nitrogen, DOC and TOC in the water column of most lakes suggest extremely low water column productivity and hence high water clarity, and may thus contribute to the ecological success of benthic microbial mats in this region. 3. Benthic communities consisted of prostrate and sometimes finely laminated mats, flake mats, epilithic and interstitial microbial mats. Mat physiognomy and carotenoid/chlorophyll ratios were strongly related to lake depth, but not to conductivity. 4. Morphological‐taxonomic analyses revealed the presence of 26 diatom morphospecies and 33 cyanobacterial morphotypes. Mats of shallow lakes (interstitial and flake mats) and those of deeper lakes (prostrate mats) were characterised by different dominant cyanobacterial morphotypes. No relationship was found between the distribution of these morphotypes and conductivity. In contrast, variation in diatom species composition was strongly related to both lake depth and conductivity. Shallow ponds were mainly characterised by aerial diatoms (e.g. Diadesmis cf. perpusilla and Hantzschia spp.). In deep lakes, communities were dominated by Psammothidium abundans and Stauroforma inermis. Lakes with conductivities higher than ±1.5 mS cm^?1 became susceptible to freezing out of salts and hence pronounced conductivity fluctuations. In these lakes P. abundans and S. inermis were replaced by Amphora veneta. Stomatocysts were important only in shallow freshwater lakes. 5. Ice cover influenced microbial mat structure and composition both directly by physical disturbance in shallow lakes and by influencing light availability in deeper lakes, as well as indirectly by generating conductivity increases and promoting the development of seasonal anoxia. 6. The relationships between diatom species composition and conductivity, and diatom species composition and depth, were statistically significant. Transfer functions based on these data can therefore be used in paleolimnological reconstruction to infer changes in the precipitation–evaporation balance in continental Antarctic lakes.     

Community structure and physiological characterization of microbial mats in Byers Peninsula, Livingston Island (South Shetland Islands, Antarctica)

The community structure and physiological characteristics of three microbial mat communities in Byers Peninsula (Livingston Island, South Shetland Islands, Antarctica) were compared. One of the mats was located at the edge of a stream and was dominated by diatoms (with a thin basal layer of oscillatorian cyanobacteria), whereas the other two mats, located over moist soil and the bottom of a pond, respectively, were dominated by cyanobacteria throughout their vertical profiles. The predominant xanthophyll was fucoxanthin in the stream mat and myxoxanthophyll in the cyanobacteria-dominated mats. The sheath pigment scytonemin was absent in the stream mat but present in the soil and pond mats. The stream mat showed significantly lower delta13C and higher delta15N values than the other two mats. Consistent with the delta15N values, N2 fixation was negligible in the stream mat. The soil mat was the physiologically most active community. It showed rates of photosynthesis three times higher than in the other mats, and had the highest rates of ammonium uptake, nitrate uptake and N2 fixation. These observations underscore the taxonomic and physiological diversity of microbial mat communities in the maritime Antarctic region.     

Wigwamma scenozonion sp.nov. (Prymnesiophyceae) from West Greenland

Wigwamma scenozonion sp. nov. (Prymnesiophyceae) is described on the basis of electron microscopy of shadowcast whole mounts prepared from water samples collected in the vicinity of Godhavn (West Greenland) in July and August 1977. This nanoplanktonic coccolithophorid possesses two smooth flagella and a shorter coiling haptonema. Coccoliths of one type cover the whole cell. Each coccolith is composed of a ring of rod-like crystallites joined end to end and arranged parallel to the edge of the oval coccolith base-plates. A single enlarged crystallite is found on most coccoliths. W. scenozonion is distinguished from the two previously described Wigwamma species by the lack of coccolith superstructures and by having one, rather than two rings of crystallites along the base-plate edge. In addition to the West Greenland specimens a single W. scenozonion cell has been encountered in a water sample from Denmark.     

Annual growth layers as proxies of past growth conditions for benthic microbial mats in a perennially ice-covered Antarctic lake

Perennial microbial mats can be the dominant autotrophic community in Antarctic lakes. Their seasonal growth results in clearly discernible annual growth layering. We examined features of live microbial mats from a range of depths in Lake Hoare, Antarctica, that are likely to be preserved in these layers to determine their potential as proxies of past growth performance. Cyanobacteria dominated the mat for all but the deepest depth sampled. Changes in areal concentrations of phycobilin pigments, organic matter and extracellular polysaccharide and in species composition did not correspond to changes in various water column properties, but showed a linear relationship with irradiance. Carbonate accumulation in the mats correlated with biomass markers and may be inferred as an index of mat performance. We examined the carbonate content of annual layers laid down from 1958–1959 to 1994–1995 in sediment cores from 12 m depth. The carbonate content in the layer showed a significant correlation with the mean summer air temperature. These data suggest a link between air temperature and microbial mat growth performance, and suggest that it is mediated via irradiance. Laminated microbial mats in Antarctic lakes have the potential to act as fine-resolution records of environmental conditions in the recent past, although interpretation is complex.     

Diurnal and seasonal variations of nitrogen fixation and photosynthesis in cyanobacterial mats

In situ measurements of nitrogenase activity and photosynthesis were performed simultaneously in cyanobacterial mats of intertidal sand flats of the Southern North Sea. Two types of cyanobacterial mats, which differed in species composition and biomass content, were investigated. The measurements were done monthly during 3 years to detect seasonal variations of nitrogen fixation and photosynthesis. Diurnal variations were investigated as well. The results showed that (i) freshly colonized sediment with the cyanobacteriumOscillatoria limosa as the dominant organism revealed the highest specific nitrogenase activities (ii) nitrogenase activities were highest in spring and summer, when mat development was initiated and (iii) diurnal fluctuations of nitrogenase activity indicated that it occurred temporally separated from oxygenic photosynthesis.     

The microfauna of algal mats and artificial substrates in Southern Victoria Land lakes of Antarctica

The first microfaunal colonization study of continental lakes in Antarctica is reported. In addition to the general taxonomic composition of six Antarctic lakes, detailed community composition analyses of two adjacent lakes, Lake Fryxell and Lake Hoare, using polyurethane foam artificial substrates suspended in the open water and placed directly over benthic algal mats are also presented. Protozoans, rotifers, tardigrades and nematodes constituted the microfaunal community in all lakes. Higher metazoans do not occur in lakes that we studied. Protozoans were the most diverse of the four groups, and the first dinoflagellates reported from southern Victoria Land, Gymnodinium and Glenodinium, were collected from Lake Fryxell. The greatest number of protozoan taxa (55) were associated with the algal mats in the more productive Lake Fryxell. Both the physical and chemical properties and taxonomic diversity showed marked variability between all lakes that were studied, especially the adjacent lakes, Fryxell and Hoare. Ecological variables which regulate taxonomic diversity in other lakes of the world also appear to exert similar influences in remote Antarctic lakes.     

Nutrient status,algal and cyanobacterial flora of six fresh water streams of Schirmacher Oasis,Antarctica

The algal and cyanobacterial flora and the chemical environment of six freshwater streams of Schirmacher Oasis, Antarctica were investigated. Over 30 species of algae, predominantly cyanobacteria (Cyanophyceae), were recorded. N₂-fixing species, both heterocystous and unicellular diazotrophs, contributed more than 50% to the counts and their dominance was greatest in the middle of the stream where nitrogen and other nutrients were low. Green algae and diatoms also contributed to the flora. The species composition varied between streams. Glacial and snow drift meltwater streams contained a distinctive community. Based on diversity indices, these streams could be classified into two clusters.     

Community structure and spatial distribution of benthic fauna in the Bellingshausen Sea (West Antarctica)

The structure and spatial distribution of the macrofauna community of the Bellingshausen Sea in the western sector of Antarctica was studied during the ‘BENTART–06’ oceanographic expedition. This is one of the least explored Antarctic seas. A total of 20 box cores were sampled at 11 stations ranging from 157 to 3,304 m depth, using an USNEL-type box corer (BC) dredge. Representatives of 25 higher taxa of invertebrates were collected. Deeper sampling sites were less rich in taxa (4–7 taxa), whereas the figures were higher at shallower sites (up to 17 taxa). Faunal density on the sea bottom revealed a horizontal spatial gradient from the western sites with extremely low figures (90 indiv./m²) towards the eastern ones with the highest figures (1,360 indiv./m²) close to the Antarctic Peninsula. Several abiotic factors (depth, redox, organic matter, carbonates and particle size of surficial sediments) were measured simultaneously on the sea floor to characterise the substrate preferences of the fauna. Positive correlations were found between the faunal distribution and a combination of depth, redox values, and organic matter content of sediments. This indicates decreasing availability of food in the deeper bottoms of the Bellingshausen Sea with a prevalence of depauperated bottoms dominated almost exclusively by a foraminiferans community.     

Stratification and dynamics of microbial loop communities in Lake Fryxell, Antarctica

1. Lake Fryxell, situated in the McMurdo Dry Valleys, Antarctica, offers the opportunity to study microbial loop processes in the absence of crustacean zooplankton and other higher organisms. This is the first study of Lake Fryxell to provide detailed temporal and vertical variations of microbial loop organisms.
2. Protozoan communities are concentrated around the chemocline (9–10 m) in Lake Fryxell. Phototrophic nanoflagellates (PNAN), heterotrophic nanoflagellates (HNAN) and ciliates formed deep maxima of 14 580, 694 and 58 cells mL−1 respectively. Although abundance and biomass at the chemocline was high, diversity of protozoa was low, Plagiocampa accounting for> 80% of the total ciliate biomass.
3. In the mixolimnion (4.5–8 m), protozoa were less abundant, but more diverse, with 24 ciliate morphotypes being identified within this region of the water column. Inter-annual variability of protozoan biomass and abundance was greater in the mixolimnion than at the chemocline due to more variable nutrient and prey concentrations.
4. Physicochemical gradients in Lake Fryxell were very stable because the perennial ice cover reduced wind driven currents. As a consequence, ciliate species occurred in distinct depth strata, Monodinium being most abundant directly beneath the ice cover, Askenasia having maximum abundance at 8 m and Plagiocampa dominating ciliate biomass at the chemocline. The lack of vertical mixing reduced seasonal successions of PNAN and ciliate species. Three cryptophyte species dominated the PNAN community at all times (>79% of total biomass).     

Distribution of inorganic species in two Antarctic cryptoendolithic microbial communities

Chemical differences were noted between two Antarctic cryptoendolithic (hidden within rock) microenvironments colonized by different microbial communities. Microenvironments dominated by cyanobacteria (BPC) had a higher pH (pH 7–8) than those dominated by lichen (LTL) (pH 4.5–5.5). In order to understand the interactions between the microbiota and the inorganic environment, the inorganic environment was characterized. Water‐soluble, carbonate‐bound, metal‐oxide, organically bound, and residual inorganic species were sequentially extracted from rock samples by chemical means. Each fraction was then quantified using inductively coupled plasma atomic emission spectrometry. BPC contained much more water‐soluble and carbonate‐bound Ca and Mg than LTL. Metal‐oxide species of Al, Fe, and Mn were more abundant in LTL than BPC. Metal oxides appeared to be mobilized (in the order Mn > Fe > Al) from the LTL lichen zone but remained immobile in BPC sandstone. The distribution of K and P bound to metal oxide reflected the distribution of iron oxide in LTL, an indication of the importance of iron in controlling the availability of nutrients in this ecosystem. Metal oxides in turn were likely controlled or influenced by organic matter associated with the lichen community. Despite overall depletion of Fe, Al, and K in the lichen zone, SEM X‐ray analysis showed that they were enriched in fungal hyphae. Water‐soluble P was present despite the presence of metal oxides, which sequester phosphate. This has biological relevance since P is an essential nutrient.     

Eyespot fine structure in Eudorina illinoiensis

Electron microscopy of the eyespot of the coenobic yolvocacean Eudorina illinoiensis (Kofoid) Pascher shows that besides the well-documented anterior-posterior decrease in size, congruity of structure is involved in eyespot differentiation.

In the anterior cells the eyespot lies adjacent to the plasmalemma with the thylakoid underlayers parallel to it, while in the posterior cells eyespot placement and orientation may vary considerably. Deep-sited eyespots of congruent organisation probably represent the mother-cell eyespot; discongruent ones are typical of posterior cells, but the smallest ones may be random associations of osmiophilic globules.

Eyespot structure is discussed in relation to photoreception and phototaxis.     

Community ecology of hot spring cyanobacterial mats: predominant populations and their functional potential

Phototrophic microbial mat communities from 60 °C and 65 °C regions in the effluent channels of Mushroom and Octopus Springs (Yellowstone National Park, WY, USA) were investigated by shotgun metagenomic sequencing. Analyses of assembled metagenomic sequences resolved six dominant chlorophototrophic populations and permitted the discovery and characterization of undescribed but predominant community members and their physiological potential. Linkage of phylogenetic marker genes and functional genes showed novel chlorophototrophic bacteria belonging to uncharacterized lineages within the order Chlorobiales and within the Kingdom Chloroflexi. The latter is the first chlorophototrophic member of Kingdom Chloroflexi that lies outside the monophyletic group of chlorophototrophs of the Order Chloroflexales. Direct comparison of unassembled metagenomic sequences to genomes of representative isolates showed extensive genetic diversity, genomic rearrangements and novel physiological potential in native populations as compared with genomic references. Synechococcus spp. metagenomic sequences showed a high degree of synteny with the reference genomes of Synechococcus spp. strains A and B′, but synteny declined with decreasing sequence relatedness to these references. There was evidence of horizontal gene transfer among native populations, but the frequency of these events was inversely proportional to phylogenetic relatedness.     

Microbial mats of twenty-six lakes from Potter Peninsula,King George Island,Antarctica

The taxonomic composition and ecology of microbial mats were investigated in 26 lentic environments of Potter Peninsula (King George Island, Antarctica) during the summer of 1995/96. These have different limnological characteristics according to the location of the basins. Among the 139 algal taxa registered, 10 are new records for Antarctica. Bacillariophyceae was the dominant class in terms of species richness, followed by the Cyanobacteria and Chlorophyceae. TWINSPAN classification was used to describe six epilithic algal assemblages; Canonical Correspondence Analysis ordination showed that soluble reactive phosphorus, phytoplanktonic chlorophyll a concentration and conductivity were the main sources for the variation in the data.     

Biodiversity in extreme aquatic environments: Lakes, ponds and streams of the Ross Sea sector, Antarctica

The Ross Sea Sector (RSS) of Antarcticallies between the lines of longitude 150°E and 150°W and contains diverse landscapes with a variety of lakes, ponds and streams. Neither insects nor crustacean species have been recorded in these ecosystems but most contain planktonic and/or benthic communities that are composed exclusively of microscopic organisms. Microbial brodiversity is low with a small number of species (e.g. filamentous cyanobacteria of the family Oscillatoriaceae) occurring under a broad range of environmental conditions throughout the region. There is no evidence to date of microbial endemism in the RSS; however, there is a need to apply molecular and cellular techniques to compare biodiversity and genetic characteristics with assemblages elsewhere in Antarctica and with comparable communities in the north polar zone. A series of hypotheses are advanced to help guide further work. These derive from the conclusion that environmental extremes plus biogeographical isolation control the biodiversity of RSS communities, and that biological interactions (competition, grazing, predation, parasitism) are weak and play a minor role by comparison with temperate latitude ecosystems.     

Phylogeography of microbial phototrophs in the dry valleys of the high Himalayas and Antarctica

High-elevation valleys in dry areas of the Himalayas are among the most extreme, yet least explored environments on Earth. These barren, rocky valleys are subjected to year-round temperature fluctuations across the freezing point and very low availability of water and nutrients, causing previous workers to hypothesize that no photoautotrophic life (primary producers) exists in these locations. However, there has been no work using modern biogeochemical or culture-independent molecular methods to test the hypothesis that photoautotrophs are absent from high Himalayan soil systems. Here, we show that although microbial biomass levels are as low as those of the Dry Valleys of Antarctica, there are abundant microbial photoautotrophs, displaying unexpected phylogenetic diversity, in barren soils from just below the permanent ice line of the central Himalayas. Furthermore, we discovered that one of the dominant algal clades from the high Himalayas also contains the dominant algae in culture-independent surveys of both soil and ice samples from the Dry Valleys of Antarctica, revealing an unexpected link between these environmentally similar but geographically very distant systems. Phylogenetic and biogeographic analyses demonstrated that although this algal clade is globally distributed to other high-altitude and high-latitude soils, it shows significant genetic isolation by geographical distance patterns, indicating local adaptation and perhaps speciation in each region. Our results are the first to demonstrate the remarkable similarities of microbial life of arid soils of Antarctica and the high Himalayas. Our findings are a starting point for future comparative studies of the dry valleys of the Himalayas and Antarctica that will yield new insights into the cold and dry limits to life on Earth.     

Diversity of 746 heterotrophic bacteria isolated from microbial mats from ten Antarctic lakes

Microbial mats, growing in Antarctic lakes constitute unique and very diverse habitats. In these mats microorganisms are confronted with extreme life conditions. We isolated 746 bacterial strains from mats collected from ten lakes in the Dry Valleys (lakes Hoare and Fryxell), the Vestfold Hills (lakes Ace, Druzhby, Grace, Highway, Pendant, Organic and Watts) and the Larsemann Hills (lake Reid), using heterotrophic growth conditions. These strains were investigated by fatty acid analysis, and by numerical analysis, 41 clusters, containing 2 to 77 strains, could be delineated, whereas 31 strains formed single branches. Several fatty acid groups consisted of strains from different lakes from the same region, or from different regions. The 16S rRNA genes from 40 strains, representing 35 different fatty acid groups were sequenced. The strains belonged to the alpha, beta and gamma subclasses of the Proteobacteria, the high and low percent G+C Gram-positives, and to the Cytophaga-Flavobacterium-Bacteroides branch. For strains representing 16 fatty acid clusters, validly named nearest phylogenetic neighbours showed pairwise sequence similarities of less than 97%. This indicates that the clusters they represent, belong to taxa that have not been sequenced yet or as yet unnamed new taxa, related to Alteromonas, Bacillus, Clavibacter, Cyclobacterium, Flavobacterium, Marinobacter, Mesorhizobium, Microbacterium, Pseudomonas, Saligentibacter, Sphingomonas and Sulfitobacter.     

Cyanobacterial composition of microbial mats from an Australian thermal spring: a polyphasic evaluation

Cyanobacterial composition of microbial mats from an alkaline thermal spring issuing at 43-71 degrees C from tropical north-eastern Australia are described using a polyphasic approach. Eight genera and 10 species from three cyanobacterial orders were identified based on morphological characters. These represented taxa previously known as thermophilic from other continents. Ultrastructural analysis of the tower mats revealed two filamentous morphotypes contributed the majority of the biomass. Both types had ultrastructural characteristics of the family Pseudanabaenaceae. DNA extracts were made from sections of the tentaculiform towers and the microbial community analysed by 16S cyanobacteria-specific PCR and denaturing-gradient gel electrophoresis. Five significant bands were identified and sequenced. Two bands clustered closely with Oscillatoria amphigranulata isolated from New Zealand hot springs; one unique phylotype had only moderate similarity to a range of Leptolyngbya species; and one phylotype was closely related to a number of Geitlerinema species. Generally the approaches yielded complementary information, however the results suggest that species designation based on morphological and ultrastructural criteria alone often fails to recognize their true phylogenetic position. Conversely some molecular techniques may fail to detect rare taxa suggesting that the widest possible suite of techniques be applied when conducting analyses of cyanobacterial diversity of natural populations. This is the first polyphasic evaluation of thermophilic cyanobacterial communities from the Australian continent.     

Characterization and comparison of potential denitrifiers in microbial mats from King George Island,Maritime Antarctica

Cyanobacterial microbial mats are highly structured communities commonly found in Antarctic inland waters including melt streams. These benthic microbial associations comprise a large number of microorganisms with different metabolic capacities, impacting nutrient dynamics where established. The denitrification process is a feasible nitrogen loss pathway and a biological source of nitrous oxide, a potent greenhouse gas that also promotes ozone depletion. Potential denitrifiers from five microbial mats were characterized using a PCR-DGGE (denaturing gradient gel electrophoresis) approach. Molecular markers encoding for key enzymes in the denitrification process (nirK, nirS and nosZ) were used. Fingerprints were obtained for the five sampled mats and compared for two successive years. Distance analysis showed that despite the sampled year, the denitrifying genetic potential was similar between most of the sites when represented in Euclidean space. The number of dominant denitrifiers detected for each sample ranged between 6 and 18 for nirK, 4–10 for nirS and 6–17 for nosZ. The seventy-two sequenced phylotypes showed 80–98 % identity to previously reported environmental sequences from water column, sediments and soil samples. These results suggest that Antarctic microbial mats have a large denitrification potential, previously uncharacterized and composed by both site-specific and common phylotypes belonging mainly to Alpha-, Beta- and Gammaproteobacteria.