Microbial eukaryote plankton communities of high-mountain lakes from three continents exhibit strong biogeographic patterns.

Microbial eukaryotes hold a key role in aquatic ecosystem functioning. Yet, their diversity in freshwater lakes, particularly in high-mountain lakes, is relatively unknown compared with the marine environment. Low nutrient availability, low water temperature and high ultraviolet radiation make most high-mountain lakes extremely challenging habitats for life and require specific molecular and physiological adaptations. We therefore expected that these ecosystems support a plankton diversity that differs notably from other freshwater lakes. In addition, we hypothesized that the communities under study exhibit geographic structuring. Our rationale was that geographic dispersal of small-sized eukaryotes in high-mountain lakes over continental distances seems difficult. We analysed hypervariable V4 fragments of the SSU rRNA gene to compare the genetic microbial eukaryote diversity in high-mountain lakes located in the European Alps, the Chilean Altiplano and the Ethiopian Bale Mountains. Microbial eukaryotes were not globally distributed corroborating patterns found for bacteria, multicellular animals and plants. Instead, the plankton community composition emerged as a highly specific fingerprint of a geographic region even on higher taxonomic levels. The intraregional heterogeneity of the investigated lakes was mirrored in shifts in microbial eukaryote community structure, which, however, was much less pronounced compared with interregional beta-diversity. Statistical analyses revealed that on a regional scale, environmental factors are strong predictors for plankton community structures in high-mountain lakes. While on long-distance scales (>10 000 km), isolation by distance is the most plausible scenario, on intermediate scales (up to 6000 km), both contemporary environmental factors and historical contingencies interact to shift plankton community structures.

Phylogenetic distribution of fungal mycosporines within the Pucciniomycotina (Basidiomycota).

The synthesis of the UV Absorbing compounds named mycosporines (MYCs) has been detected in a few basidiomycetous yeast groups. Conspicuous accumulation of mycosporine-glutaminol-glucoside (MGG) in yeasts requires photo-induction and its photoprotective function has been postulated. The distribution of the ability to produce MYCs appeared to be related to the yeast taxonomic affiliation. In view of the potential significance of MYCs in yeast taxonomy, we here studied the distribution of this trait among dimorphic basidiomycetes of the Pucciniomycotina. Of the 94 fungal species (377 strains and 33 genera) tested, almost half were MYC-positive and MGG was the main compound produced. MGG synthesis was observed for representatives of five of seven Pucciniomycotina classes, indicating that this trait is widely distributed in this group. MGG detection proved useful for the differentiation of species of the polyphyletic genera, such as Rhodotorula and Sporobolomyces, that are phylogenetically separated. MGG quantification and UV tolerance studies in Cystobasidiomycetes supported the idea that the habitat of origin of each strains is important in the level of MGG synthesis and that MYCs have a photoprotective function in yeasts. The taxonomic value of this trait in fungal systematics is discussed.

Alteration of chromophoric dissolved organic matter by solar UV radiation causes rapid changes in bacterial community composition.

We evaluated the effect of photochemical alterations of chromophoric dissolved organic matter (CDOM) on bacterial abundance, activity and community composition in a coastal lagoon of the Atlantic Ocean with high dissolved organic carbon concentration. On two occasions during the austral summer, bacteria-free water of the lagoon was exposed to different regions of the solar spectrum (full solar radiation, UV-A+PAR, PAR) or kept in the dark. Subsequently, dilution cultures were established with bacterioplankton from the lagoon that were incubated in the pre-exposed water for 5 h in the dark. Cell abundance, activity, and community composition of bacterioplankton were assessed before and after incubation in the different treatments. Changes in absorption, fluorescence, and DOC concentration were used as proxies for CDOM photoalteration. We found a significant CDOM photobleaching signal, DOC loss, as well as a stimulation of bacterial activity in the treatments pre-exposed to UV radiation, suggesting increased bioavailability of DOM. Bacterial community analysis by fluorescence in situ hybridization revealed that this stimulation was mainly accompanied by the specific enrichment of Alpha- and Betaproteobacteria. Thus, our results suggest that CDOM photoalteration not only stimulates bacterioplankton growth, but also induces rapid changes in bacterioplankton composition, which can be of relevance for ecosystem functioning, particularly considering present and future changes in the input of terrestrial CDOM to aquatic systems.

Blooms of single bacterial species in a coastal lagoon of the southwestern Atlantic Ocean.

We investigated seasonal differences in community structure and activity (leucine incorporation) of the planktonic bacterial assemblage in the freshwater and brackish-water zones of a shallow coastal lagoon of the southwestern Atlantic Ocean. Alphaproteobacteria formed the dominant microbial group in both zones throughout the sampling period. After an intrusion of marine water, members of the SAR11 lineage became abundant in the brackish-water zone. These bacteria were apparently distributed over the lagoon during the following months until they constituted almost 30% of all prokaryotic cells at both sampling sites. At the first sampling date (March 2003) a single alphaproteobacterial species unrelated to SAR11, Sphingomonas echinoides, dominated the microbial assemblages in both zones of the lagoon concomitantly with a bloom of filamentous cyanobacteria. Pronounced maxima of leucine incorporation were observed once in each zone of the lagoon. In the freshwater zone, this highly active microbial assemblage was a mix of the typical bacteria lineages expected in aquatic systems. By contrast, a single bacterial genotype with >99% similarity to the facultative pathogen gammaproteobacterial species Stenotrophomonas maltophilia formed >90% of the bacterial assemblage (>10(7) cell ml(-1)) in the brackish-water zone at the time point of highest bacterial leucine incorporation. Moreover, these bacteria were equally dominant, albeit less active, in the freshwater zone. Thus, the pelagic zone of the studied lagoon harbored repeated short-term blooms of single bacterial species. This finding may have consequences for environmental protection.

Mycosporines from freshwater yeasts: a trophic cul-de-sac?

Mycosporine-like amino-acids (MAAs) are found in aquatic bacteria, algae, and animals. A related compound, the mycosporine-glutaminol-glucoside (myc-glu-glu), has recently been reported in freshwater yeasts. Although animals depend on other organisms as their source of MAAs, they can efficiently accumulate them in their tissues. In this work we assessed the potential transfer of the yeast mycosporine myc-glu-glu from the diet into the copepod Boeckella antiqua and the ciliate Paramecium bursaria. For this purpose, we performed experiments to study the feeding of B. antiqua and P. bursaria on the yeast Rhodotorula minuta and their ability to bioaccumulate myc-glu-glu. Bioaccumulation of myc-glu-glu in B. antiqua was assessed through long-term factorial experiments manipulating the diet (Chlamydomonas reinhardii and C. reinhardii + yeasts) and radiation exposure (PAR and PAR + UVR). Shorter term experiments were designed in the case of P. bursaria. The composition and concentration of MAAs in the diet and in the consumers were determined by HPLC analyses. Our results showed that even though both consumers ingested yeast cells, they were unable to accumulate myc-glu-glu. Moreover, when exposed to conditions that stimulated the accumulation of photoprotective compounds (i.e. UVR exposure), an increase in MAAs concentration occurred in copepods fed C. reinhardii plus yeasts as well as in those fed only C. reinhardii. This suggests that the copepods were able to modify their tissue concentrations of MAAs in response to environmental clues but also that the contribution of yeast mycosporines to total MAAs concentration was negligible.

Mycosporines in carotenogenic yeasts.

The ability to produce mycosporines (MYCs) in 157 pigmented yeast strains (eight genera, 25 species) isolated from natural environments of Patagonia (Argentina) was assessed. The strains belong to four taxonomic groups: the Sporidiobolales and Erythrobasidium clade of the class Urediniomycetes, and Cystofilobasidiales and Tremellales of the class Hymenomycetes. Induction of MYCs did not occur in all yeast strains tested and appeared to be an exclusive trait of members of the Erythrobasidium clade and Tremellales. This is the first report on the production of MYCs by pigmented species from the latter group, as well as the first extensive screening of mycosporinogenic yeasts. The consistent occurrence of MYCs in some specific phylogenetic groups suggests this trait bears evolutionary significance and that the presence/absence of MYCs may have potential applications in yeast systematics.

Constitutive and UV-inducible synthesis of photoprotective compounds (carotenoids and mycosporines) by freshwater yeasts.

Twelve yeasts isolated from lakes of Northwestern Patagonia, Argentina, belonging to eight genera (Sporobolomyces, Sporidiobolus, Rhodotorula, Rhodosporidium, Cystofilobasidium, Cryptococcus, Torulaspora, and Candida) were analysed for their ability to produce photoprotective compounds. For this purpose, three laboratory experiments were performed to study the effect of photosynthetically active radiation (PAR) and PAR in combination with UV radiation (PAR + UVR) on the production of carotenoids and mycosporines. The synthesis of carotenoid compounds was clearly stimulated in six out of nine red yeast strains tested upon exposure to PAR or PAR + UVR; however, the latter conditions produced a stronger response than PAR alone. The increase in carotenoids in the red strains under PAR + UVR irradiation showed a negative exponential relationship with their basal carotenoid content, suggesting that cells with higher constitutive levels of carotenoids are less responsive to induction by PAR + UVR. Three red yeasts, Rhodotorula minuta, Rh. pinicola, and Rhodotorula sp., and the colourless Cryptococcus laurentii produced a UV-absorbing compound when exposed to PAR or PAR + UVR. This compound showed an absorption maximum at 309-310 nm and was identified as mycosporine-glutaminol-glucoside (myc-glu-glu). In these strains, exposure to PAR or PAR + UVR resulted in elevated concentrations of both carotenoids and myc-glu-glu. This is the first report on the production of mycosporines by yeasts. All strains that developed under PAR + UVR were able to synthesise carotenoids either constitutively or in response to PAR exposure, and a few of them also produced myc-glu-glu when exposed to PAR. Collectively, our results suggest that the presence of carotenoids, either alone or in combination with mycosporines, are required for sustaining growth under exposure to PAR + UVR in the freshwater yeast strains studied.