Mercury in a stream-lake network of Andean Patagonia (Southern Volcanic Zone): Partitioning and interaction with dissolved organic matter.

Lake Nahuel Huapi (NH) is a large, ultraoligotrophic deep system located in Nahuel Huapi National Park (NHNP) and collecting a major headwater network of Northwestern Patagonia (Argentina). Brazo Rincón (BR), the westernmost branch of NH, is close to the active volcanic formation Puyehue-Cordón Caulle. In BR, aquatic biota and sediments display high levels of total Hg (THg), ranging in contamination levels although it is an unpolluted region. In this survey, Hg species and fractionation were assessed in association with dissolved organic matter (DOM) in several aquatic systems draining to BR. THg varied between 16.8 and 363 ng L-1, with inorganic Hg (Hg2+) contributing up to 99.8% and methyl mercury (MeHg) up to 2.10%. DOC levels were low (0.31-1.02 mg L-1) resulting in high THg:DOC and reflecting in high Hg2+ availability for binding particles (partitioning coefficient log Kd up to 6.03). In streams, Hg fractionation and speciation related directly with DOM terrestrial prints, indicating coupled Hg-DOM inputs from the catchment. In the lake, DOM quality and photochemical and biological processing drive Hg fractionation, speciation and vertical levels. Dissolved gaseous Hg (Hg0) reached higher values in BR (up to 3.8%), particularly in upper lake layers where solar radiation enhances the photoreduction of Hg2+ and Hg-DOM complexes. The environmental conditions in BR catchment promote Hg2+ binding to abiotic particles and bioaccumulation and the production of Hg0, features enhancing Hg mobilization among ecosystem compartments. Overall, the aquatic network studied can be considered a "natural Hg hotspot" within NHNP.

Influence of precipitation, landscape and hydrogeomorphic lake features on pelagic allochthonous indicators in two connected ultraoligotrophic lakes of North Patagonia.

We investigated the terrestrial influence on two chained deep ultraoligotrophic lakes of North Patagonia (Argentina) through the seasonal analysis of two pelagic allochthonous indicators: i) water color, as a proxy of allochthonous dissolved organic matter in lakes; and ii) the color to chlorophyll a ratio (Color:Chla), as an indicator of the relationship between allochthonous and autochthonous carbon pools. We also evaluated the potential transfer pathways of the allochthonous dissolved organic matter into the pelagic food webs of these deep lakes, including the natural zooplankton δ(13)C in the analysis. The dynamics of the allochthonous indicators were related to the precipitation regime, water level fluctuations, and hydrogeomorphic and catchment features of lakes Moreno East and Moreno West. The water color (absorbance at 440 nm) was extremely low (<0.28 m(-1)) in both lakes regardless of the season. However, precipitation and snowmelt regimes drove the increase and decrease of water color, respectively. A significant positive relationship between the zooplankton bulk δ(13)C with the water color would suggest an input of allochthonous organic carbon into the pelagic consumers. The incorporation of the dissolved allochthonous material into higher trophic levels is likely favored by the bacterivorous behavior of planktonic organisms, mixotrophic flagellates and ciliates, which dominate the pelagic food webs of these Patagonian lakes. Morphometric aspects, mainly the higher water residence time, led to lower values of allochthony in Moreno East compared to Moreno West, probably accentuated by its upper position in the lake chain. Overall, our results suggest that these allochthonous signals can bring insight into the magnitude of the interaction between terrestrial environments and lake ecosystems, even in extremely clear and ultraoligotrophic systems, such as the Andean Patagonian lakes.

Photoprotection by carotenoid pigments in the yeast Rhodotorula mucilaginosa: the role of torularhodin.

In this paper we report the relationship between carotenoids and ergosterol and cell UV-B resistance in different strains of the yeast Rhodotorula mucilaginosa. Cell survival was studied using a set of 13 strains; additionally, two mutants (a hyper-producing one and a colourless one) in combination with diphenylamine (DPA), a carotenogenesis inhibitor, were used. A positive correlation between total carotenoids and survival to UV-B radiation was found. However, when individual carotenoid concentrations were tested, only torularhodin was found to be significantly related to UV-B survival. On the contrary, ergosterol did not affect survival. The hyper-pigmented strain showed an enhanced survival (up to 250%) compared to the parental strain, while the survival of the albino mutant was similar to that experienced by the parental strain; however, observed changes in survival were dose dependent. The cyclobutane pyrimidine dimers (CPDs), one of the major forms of DNA damage caused by UV exposure, appears as unrelated to the accumulation of carotenoids and cell survival. These results indicate that bearing higher torularhodin concentrations enhances UV-B survival in yeasts and, thus, the accumulation of this pigment constitutes an important mechanism that improves the resistance of yeasts to UV-B.

Photoprotective role of carotenoids in yeasts: Response to UV-B of pigmented and naturally-occurring albino strains.

In this work, the photoprotective role of carotenoids in yeasts was analysed by contrasting the responses to UV-B of pigmented and naturally occurring albino strains of Sporobolomyces ruberrimus and Cystofilobasidium capitatum in different conditions. Albino and pigmented strains were confirmed to be conspecific by PCR fingerprinting and rDNA sequencing. Experimental exposure to UV-B conducted with both yeast species showed that the pigmented strains were more tolerant to UV-B than the albino strains and that the increment in carotenoid contents during the stationary growth phase enhance survivorship. These results indicated that carotenoid pigments afford UV-B protection in yeasts.

Methylmercury production in the water column of an ultraoligotrophic lake of Northern Patagonia, Argentina.

Methyl-mercury (CH3Hg+) production was studied in freshwaters from lake Moreno, an ultraoligotrophic system belonging to Northern Patagonia. Hg2+ labelled with high specific activity 197Hg was spiked to water samples in concentrations of 10 ng l(-1), and incubated in laboratory for 3d time trends under different conditions. Experimental water was sampled daily to evaluate CH3(197)Hg+ production. Lake water used in the experiments was sampled just below the upper limit of the metalimnion ( approximately 30 m depth), where maximum values of chlorophyll a have been measured previously. Sampling was performed in late autumn, when the plankton fraction <50 microm exhibited mercury concentrations up to 260 microg g(-1) dry weight. The experiments analysed lake water filtered through 50, 20, and 0.2 microm (filter-sterilized) mesh nets. ASTM grade 1 water was also incubated for control. All the experiments were run in an environmental chamber under controlled temperature and light regime. High Hg2+ conversion to CH3Hg+, up to 50%, was measured in lake water, in a process stimulated by light. CH3Hg+ production was two-fold higher after 3d of incubation with illumination compared to total darkness. Sterile lake water showed conversions up to 30%, while the planktonic components seem to enhance the CH3Hg+ production. Overall, our results provide evidence that lake Moreno waters favour CH3Hg+ production in processes stimulated by light. Although biotic components certainly contribute to enhance mercury methylation, water chemistry plays a key role in this process. We hypothesize that dissolved organic matter, particularly its quality, could be decisive.

Effect of UV-B exposure on eggs and embryos of patagonian anurans and evidence of photoprotection.

In Patagonia, enhanced levels of ultraviolet radiation (UVR) are affecting not only the southernmost territories but the northern as well, and there is a growing body of evidence indicating effects on terrestrial and aquatic communities. Anurans occur along a wide geographical gradient in Patagonia and are common in shallow waters, usually breeding during spring when seasonal UV-B radiation is at its highest level. In this paper we analyse the effective irradiances (I) experienced in nature by eggs and embryos of the patagonian anurans Pleurodema bufoninum, Pleurodema thaul and Bufo spinulosus papillosus, considering water and egg jelly optical properties. Besides, we determine photoprotective pigments in eggs and embryos of the different species, and ontogenetic shifts in pigment concentration. Finally, we studied the effects of natural and enhanced levels of UV-B on eggs of P. bufoninum aiming to evaluate the role of the jelly envelope in screening the damaging radiation. The evidence gathered in this work shows that natural and enhanced UV-B levels did not induce significant changes on the survivorship but incremented the occurrence of malformations. Also, the species experiencing high levels of exposure presented high concentrations of melanin suggesting that exposure to sunlight triggers photoprotection by pigments. Collectively our results suggest that the studied species bear certain levels of adaptation to cope with high ambient UVR conferred by environmental and biological factors.

UV radiation as a potential driving force for zooplankton community structure in Patagonian lakes.

This article explores the potential role of UV radiation (UVR) as an influence on zooplankton communities. In the first section we provide a general overview of UVR effects on freshwater zooplankton, with an emphasis on Argentine and Chilean environments. In the second section we present the results of a survey involving 53 temperate lakes across a gradient of UVR exposure to determine patterns of species richness and specific diversity. These community characteristics decreased at high potential UVR exposure (i.e. high mean water column irradiance or low lake optical density). A threshold value of mean water column irradiance of approximately 10% of the surface level seems to limit both richness and diversity to minimum values. On the basis of the collected evidence it is not possible to definitely conclude that UVR rather than another covarying factor is responsible for the decrease in specific diversity observed at the lowest end of lake optical depth. However, lakes with values above the previous threshold are likely to exhibit highly depauperate zooplankton communities regardless of the mechanism. As a cautionary note we suggest that changes in the optical characteristics (i.e. changes due to atmospheric conditions, precipitation patterns or vertical displacement of the tree line) may result in sudden shifts in zooplankton community structure.

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.

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.