Sub-aerial biofilms as blockers of solar radiation: spectral properties as tools to characterise material-relevant microbial growth

Sub-aerial biofilms (SABs) are ubiquitous microbial communities that develop at the interface between hard surfaces and the atmosphere. Inherent SAB “core-settlers” include phototrophic algae, cyanobacteria, heterotrophic bacteria and microcolonial fungi (MCF). SABs do not simply cover hard surfaces; they interact with them in myriads of ways and bind to the underlying substrate. Secretion of extracellular mucilage aids adhesion, while organic acids and acidic polysaccharides weather the surface. As protection against solar radiation, many members of the SAB consortia produce shielding pigments while the phototrophic inhabitants are laden with photosynthetic pigments. All absorb light of many wavelengths and in addition, the cells themselves scatter light. Both effects change the spectra of incoming radiation (including wavelengths that are converted to electricity by photovoltaic cells) and decrease its intensity. To quantify these effects on SABs as complex entities of organisms and pigments, we measured the spectral properties of model and natural biofilms transferred to glass. Here we show that SABs growing on solar panels and other substrates scatter incident radiation between 250 nm up to 1800 nm and block up to 70% of its transmission. Model biofilms have the advantage that their microbial components can be “tuned” to resemble natural ones of different compositions thus providing a novel materials-testing tool.     

Subaerial biofilms on granitic historic buildings: microbial diversity and development of phototrophic multi-species cultures

Microbial communities of natural subaerial biofilms developed on granitic historic buildings of a World Heritage Site (Santiago de Compostela, NW Spain) were characterized and cultured in liquid BG11 medium. Environmental barcoding through next-generation sequencing (Pacific Biosciences) revealed that the biofilms were mainly composed of species of Chlorophyta (green algae) and Ascomycota (fungi) commonly associated with rock substrata. Richness and diversity were higher for the fungal than for the algal assemblages and fungi showed higher heterogeneity among samples. Cultures derived from natural biofilms showed the establishment of stable microbial communities mainly composed of Chlorophyta and Cyanobacteria. Although most taxa found in these cultures were not common in the original biofilms, they are likely common pioneer colonizers of building stone surfaces, including granite. Stable phototrophic multi-species cultures of known microbial diversity were thus obtained and their reliability to emulate natural colonization on granite should be confirmed in further experiments.     

Characteristics and role of the exocellular polysaccharides produced by five cyanobacteria isolated from phototrophic biofilms growing on stone monuments

Three coccoid and two filamentous cyanobacterial strains were isolated from phototrophic biofilms exposed to intense solar radiation on lithic surfaces of the Parasurameswar Temple and Khandagiri caves, located in Orissa State, India. Based on to their morphological features, the three coccoid strains were assigned to the genera Gloeocapsosis and Gloeocapsa, while the two filamentous strains were assigned to the genera Leptolyngbya and Plectonema. Eleven to 12 neutral and acidic sugars were detected in the slime secreted by the five strains. The secretions showed a high affinity for bivalent metal cations, suggesting their ability to actively contribute to weakening the mineral substrata. The secretion of protective pigments in the polysaccharide layers, namely mycosporine amino acid-like substances (MAAs) and scytonemins, under exposure to UV radiation showed how the acclimation response contributes to the persistence of cyanobacteria on exposed lithoid surfaces in tropical areas.     

Phototrophic Biofilms on Ancient Mayan Buildings in Yucatan, Mexico

Buildings at the important archaeological sites of Uxmal and Kabah, Mexico, are being degraded by microbial biofilms. Phospholipid fatty acid (PLFA) and chlorophyll a analyses indicated that phototrophs were the major epilithic microorganisms and were more prevalent on interior walls than exterior walls. Culture and microscopical techniques showed that Xenococcus formed the major biomass on interior surfaces, but the stone-degrading genera Gloeocapsa and Synechocystis were also present in high numbers. Relatively few filamentous algae and cyanobacteria were detected. The fatty acid analysis also showed that complex biofilms colonize these buildings. Circular depressions observed by scanning electron microscopy (SEM) on stone and stucco surfaces beneath the biofilm corresponded in shape and size to coccoid cyanobacteria. SEM images also demonstrated the presence of calcareous deposits on some coccoid cells in the biofilm. Phototrophic biofilms may contribute to biodegradation by (1) providing nutrients that support growth of acid-producing fungi and bacteria and (2) active “boring” behavior, the solubilized calcium being reprecipitated as calcium carbonate. Received: 15 March 1999 / Accepted: 24 June 1999     

Characteristics and role of the exocellular polysaccharides produced by five cyanobacteria isolated from phototrophic biofilms growing on stone monuments

Three coccoid and two filamentous cyanobacterial strains were isolated from phototrophic biofilms exposed to intense solar radiation on lithic surfaces of the Parasurameswar Temple and Khandagiri caves, located in Orissa State, India. Based on to their morphological features, the three coccoid strains were assigned to the genera Gloeocapsosis and Gloeocapsa, while the two filamentous strains were assigned to the genera Leptolyngbya and Plectonema. Eleven to 12 neutral and acidic sugars were detected in the slime secreted by the five strains. The secretions showed a high affinity for bivalent metal cations, suggesting their ability to actively contribute to weakening the mineral substrata. The secretion of protective pigments in the polysaccharide layers, namely mycosporine amino acid-like substances (MAAs) and scytonemins, under exposure to UV radiation showed how the acclimation response contributes to the persistence of cyanobacteria on exposed lithoid surfaces in tropical areas.     

Biodeterioration of Mayan buildings at uxmal and tulum,Mexico

Uxmal and Tulum are two important Mayan sites in the Yucatan peninsula. The buildings are mainly composed of limestone and grey/black discoloration is seen on exposed walls and copious greenish biofilms on inner walls. The principal microorganisms detected on interior walls at both Uxmal and Tulum were cyanobacteria; heterotrophic bacteria and filamentous fungi were also present. A dark‐pigmented mitosporic fungus and Bacillus cereus, both isolated from Uxmal, were shown to be acidogenic in laboratory cultures. Cyanobacteria belonging to rock‐degrading genera Synechocystis and Gloeocapsa were identified at both sites. Surface analysis previously showed that calcium ions were present in the biofilms on buildings at Uxmal and Tulum, suggesting the deposition of biosolubilized stone. Apart from their potential to degrade the substrate, the coccoid cyanobacteria supply organic nutrients for bacteria and fungi, which can produce organic acids, further increasing stone degradation.     

Hypolithic microbial communities: between a rock and a hard place

Drylands are the largest terrestrial biome on Earth and a ubiquitous feature is desert pavement terrain, comprising rocks embedded in the mineral soil surface. Quartz and other translucent rocks are common and microbial communities termed hypoliths develop as biofilms on their ventral surfaces. In extreme deserts these represent major concentrations of biomass, and are emerging as key to geobiological processes and soil stabilization. These highly specialized communities are dominated by cyanobacteria that support diverse heterotrophic assemblages. Here we identify global-scale trends in the ecology of hypoliths that are strongly related to climate, particularly with regard to shifts in cyanobacterial assemblages. A synthesis of available data revealed a linear trend for colonization with regard to climate, and we suggest potential application for hypoliths as 'biomarkers' of aridity on a landscape scale. The potential to exploit the soil-stabilizing properties of hypolithic colonization in environmental engineering on dryland soils is also discussed.     

Biogenic Forsterite and Opal as a Product of Biodeterioration and Lichen Stromatolite Formation in Table Mountain Systems (Tepuis) of Venezuela

Biogenic physical weathering and leaching of the quartzite dominated table mountain systems of South America is a slow but strongly biologically influenced process. Observations and analyses on the basis of sample materials collected during an expedition to the protected areas of the most conspicuous tepuis of Venezuela are reported. The rock material consists of more than 98% silica, and the waters collected reflect rainwater quality further deprived of some essential elements. Wear-down of these rocks is recognized as a biogeomorphogenetic process ruled by the microbiota surviving under harsh and nutrient-poor conditions. Poikilotrophic subaerial biofilms of cyanobacteria, fungi, and some lichens perforate quartz grains and idiomorphic quartz crystals, as well as the subcrystalline cement. The typical pattern of biopitting is regarded as proof of biogenic quartz destruction. Within the subaerial biofilms, which form a massive cover of the slowly biocorroded rock surface, lichens were found that mineralize to microstromatolitic structures in situ. The mineralization occurs exclusively in the lichen thalli and not in the associated massive biofilms of free-living cyanobacteria and fungi. The minerals deposited were identified as opal and considerable amounts of forsterite, the pure Mg end-member of the olivine mixing series (fayalite being the pure iron silicate). Forsterite, thus far, has been regarded as an igneous mineral phase typical for mantle-derived rocks, highly metamorphic dolomitic marbles, and as a planetary mineral found in meteorites. The biogenic dissolution/mineralization paragenesis is explained by the slow weathering and bioleaching processes dominant on these table mountains and by the exclusion of all other potential biomineralization products due to the peculiar geochemistry of the interstitial and run-off water on these plateaus, thus leading to the exceptional biogenesis of forsterite under surface conditions.     

Genetic Characterization of Epilithic Cyanobacteria and Their Associated Bacteria

Epilithic phototrophic biofilms develop inside Roman Necropolis and Catacombs on rock surfaces exposed to artificial light sources and are composed by a microbial consortium dominated by cyanobacteria. In this work, six non-axenic cultures of Leptolyngbya sp. strains isolated from biofilms from different Roman hypogea and maintained in cultures from 11 to 20 years were analysed along with their associated bacteria isolated in culture. The employment of PCR-fingerprinting techniques, using HIP1 and ERIC derived primers, allowed the clustering in three groups of the six Leptolyngbya strains and the typing of their isolated bacteria. The bacterial fingerprinting patterns were in agreement with the 16S rRNA gene sequencing and showed the presence in Leptolyngbya isolates of Pseudomonas, Stenotrophomonas, Agrobacterium and Bacillus representatives that were detected also in biofilms sampled from catacombs.     

Sunlight-Exposed Biofilm Microbial Communities Are Naturally Resistant to Chernobyl Ionizing-Radiation Levels

Background

The Chernobyl accident represents a long-term experiment on the effects of exposure to ionizing radiation at the ecosystem level. Though studies of these effects on plants and animals are abundant, the study of how Chernobyl radiation levels affect prokaryotic and eukaryotic microbial communities is practically non-existent, except for a few reports on human pathogens or soil microorganisms. Environments enduring extreme desiccation and UV radiation, such as sunlight exposed biofilms could in principle select for organisms highly resistant to ionizing radiation as well.

Methodology/Principal Findings

To test this hypothesis, we explored the diversity of microorganisms belonging to the three domains of life by cultivation-independent approaches in biofilms developing on concrete walls or pillars in the Chernobyl area exposed to different levels of radiation, and we compared them with a similar biofilm from a non-irradiated site in Northern Ireland. Actinobacteria, Alphaproteobacteria, Bacteroidetes, Acidobacteria and Deinococcales were the most consistently detected bacterial groups, whereas green algae (Chlorophyta) and ascomycete fungi (Ascomycota) dominated within the eukaryotes. Close relatives to the most radio-resistant organisms known, including Rubrobacter species, Deinococcales and melanized ascomycete fungi were always detected. The diversity of bacteria and eukaryotes found in the most highly irradiated samples was comparable to that of less irradiated Chernobyl sites and Northern Ireland. However, the study of mutation frequencies in non-coding ITS regions versus SSU rRNA genes in members of a same actinobacterial operational taxonomic unit (OTU) present in Chernobyl samples and Northern Ireland showed a positive correlation between increased radiation and mutation rates.

Conclusions/Significance

Our results show that biofilm microbial communities in the most irradiated samples are comparable to non-irradiated samples in terms of general diversity patterns, despite increased mutation levels at the single-OTU level. Therefore, biofilm communities growing in sunlight exposed substrates are capable of coping with increased mutation rates and appear pre-adapted to levels of ionizing radiation in Chernobyl due to their natural adaptation to periodical desiccation and ambient UV radiation.     

Tales from the tomb: the microbial ecology of exposed rock surfaces

Although a broad diversity of eukaryotic and bacterial taxa reside on rock surfaces where they can influence the weathering of rocks and minerals, these communities and their contributions to mineral weathering remain poorly resolved. To build a more comprehensive understanding of the diversity, ecology and potential functional attributes of microbial communities living on rock, we sampled 149 tombstones across three continents and analysed their bacterial and eukaryotic communities via marker gene and shotgun metagenomic sequencing. We found that geographic location and climate were important factors structuring the composition of these communities. Moreover, the tombstone‐associated microbial communities varied as a function of rock type, with granite and limestone tombstones from the same cemeteries harbouring taxonomically distinct microbial communities. The granite and limestone‐associated communities also had distinct functional attributes, with granite‐associated bacteria having more genes linked to acid tolerance and chemotaxis, while bacteria on limestone were more likely to be lichen associated and have genes involved in photosynthesis and radiation resistance. Together these results indicate that rock‐dwelling microbes exhibit adaptations to survive the stresses of the rock surface, differ based on location, climate and rock type, and seem pre‐disposed to different ecological strategies (symbiotic versus free‐living lifestyles) depending on the rock type.     

A comparative study of the major microbial biomass of biofilms on exteriors of buildings in Europe and Latin America

Microorganisms in biofilms on building surfaces include algae, bacteria and fungi and cause discolouration and degradation, but definitive information about preferences of microbial groups for given building substrates and how this is affected by environmental conditions is lacking. Major biomass in 230 biofilms from buildings in seven Latin American and six European countries was analysed. Substrates included composites (cement, mortar, concrete, brick), painted surfaces and dimensional stone. Cyanobacteria, mostly coccoid types, were most frequently present as major biomass in LA, followed by fungi, whereas in Europe algae were most common, followed by cyanobacteria. Algae were more frequent than other groups on all substrates in Europe. Fungi, particularly uncommon as major biomass on stone, were more frequent on paint than on all other substrates (40% cf. 12%). Actinomycetes (frequently streptomycetes) were detected occasionally as major biomass, mainly on stone; climatic differences may explain the relative prevalence of this group in Europe.     

Occurrence of Aspergillus allahabadii on sandstone at Bayon temple,Angkor Thom,Cambodia

Microbial biofilms on surface of sandstone is detrimental to the integrity of the substratum material and they are biodeteriogens responsible for the damage of sandstone over time. We observed that fungi formed extensive biofilms on areas previously colonized by autotrophic and heterotrophic microbial biofilms causing darkening of the stone surface. Appearance of fungi on these biofilms has resulted in removal of the preformed biofilm through extensive examination of sandstone surfaces in Angkor Thom, Cambodia. One fungus, isolated from the surface with capability of removing biofilms, was purified and identified as Aspergillus allahabadii during our survey and sampling of microbial biofilms at Bayon temple, Angkor Thom, Cambodia in 2008. Ribosomal RNA (ITS and 5.8S) and β-tubulin gene sequences were phylogenetically analyzed to confirm the taxonomy of this strain. In addition, its protein profile and enzyme assays were also carried out and β-galactosidase was the highest among 7 enzymes tested. Our results suggest that fungi may have an important role in removing microbial biofilms on surfaces of stone and potential mechanisms and applications are discussed.     

Morphogenesis of digitate structures in hot spring silica sinters of the El Tatio geothermal field,Chile

In silica-rich hot spring environments, internally laminated, digitate sinter deposits are often interpreted as bio-mediated structures. The organic components of microbial communities (cell surfaces, sheaths and extracellular polymeric substances) can act as templates for silica precipitation, therefore influencing digitate sinter morphogenesis. In addition to biologic surface-templating effects, various microenvironmental factors (hydrodynamics, local pH and fluctuating wind patterns) can also influence silica precipitation, and therefore the morphology of resulting digitate sinters. Digitate sinter morphology thus depends on the dynamic interplay between microenvironmentally driven silica precipitation and microbial growth, but the relative contributions of both factors are a topic of continuing research. Here we present a detailed study of digitate silica sinters in distal, low-temperature regimes of the El Tatio geothermal field, Chile. This high-altitude geothermal field is extremely arid and windy, and has one of the highest silica precipitation rates found in the world. We find that digitate silica sinters at El Tatio always accrete into the prevailing eastward wind direction and exhibit laminar growth patterns coinciding with day–night cycles of wind- and thermally driven evaporation and rewetting. Subaerial parts of digitate sinters lack preserved organics and sinter textures that would indicate past microbial colonization, while filamentous cyanobacteria with resistant, silicified sheaths only inhabit subaqueous cavities that crosscut the primary laminations. We conclude that, although fragile biofilms of extremophile micro-organisms may have initially been present and templated silica precipitation at the tips of these digitate sinters, the saltation of sand grains and precipitation of silica by recurrent wind- and thermally driven environmental forcing at El Tatio are important, if not dominant factors shaping the morphology of these digitate structures. Our study sheds light on the relative contributions of biogenic and abiogenic factors in sinter formation in geothermal systems, with geobiological implications for the cautious interpretation of stromatolite-like features in ancient silica deposits on Earth and Mars.     

Epilithic cyanobacterial communities of a marine tropical beach rock (Heron Island, Great Barrier Reef): diversity and diazotrophy

The diversity and nitrogenase activity of epilithic marine microbes in a Holocene beach rock (Heron Island, Great Barrier Reef, Australia) with a proposed biological calcification "microbialite" origin were examined. Partial 16S rRNA gene sequences from the dominant mat (a coherent and layered pink-pigmented community spread over the beach rock) and biofilms (nonstratified, differently pigmented microbial communities of small shallow depressions) were retrieved using denaturing gradient gel electrophoresis (DGGE), and a clone library was retrieved from the dominant mat. The 16S rRNA gene sequences and morphological analyses revealed heterogeneity in the cyanobacterial distribution patterns. The nonheterocystous filamentous genus Blennothrix sp., phylogenetically related to Lyngbya, dominated the mat together with unidentified nonheterocystous filaments of members of the Pseudanabaenaceae and the unicellular genus Chroococcidiopsis. The dominance and three-dimensional intertwined distribution of these organisms were confirmed by nonintrusive scanning microscopy. In contrast, the less pronounced biofilms were dominated by the heterocystous cyanobacterial genus Calothrix, two unicellular Entophysalis morphotypes, Lyngbya spp., and members of the Pseudanabaenaceae family. Cytophaga-Flavobacterium-Bacteroides and Alphaproteobacteria phylotypes were also retrieved from the beach rock. The microbial diversity of the dominant mat was accompanied by high nocturnal nitrogenase activities (as determined by in situ acetylene reduction assays). A new DGGE nifH gene optimization approach for cyanobacterial nitrogen fixers showed that the sequences retrieved from the dominant mat were related to nonheterocystous uncultured cyanobacterial phylotypes, only distantly related to sequences of nitrogen-fixing cultured cyanobacteria. These data stress the occurrence and importance of nonheterocystous epilithic cyanobacteria, and it is hypothesized that such epilithic cyanobacteria are the principal nitrogen fixers of the Heron Island beach rock.     

The effect of bacteria and fungi on chemical weathering and chemical denudation fluxes in pine growth experiments

Vascular plants and associated microbial communities affect the nutrient resources of terrestrial ecosystems by impacting chemical weathering that transfers elements from primary minerals to other ecosystem pools, and chemical denudation that transports weathered elements out of the system in solution. We performed a year-long replicated flow-through column growth experiment to isolate the effects of vascular plants, ectomycorrhiza-forming fungi and associated bacteria on chemical weathering and chemical denudation. The study focused on Ca²⁺, K⁺ and Mg²⁺, for which the sole sources were biotite and anorthite mixed into silica sand. Concentrations of the cations were measured in input and output solutions, and three times during the year in plant biomass and on exchangeable cation sites of the growth medium. Weathering and denudation fluxes were estimated by mass balance, and mineral surface changes, biofilm and microbial attachments to surfaces were investigated with scanning electron microscopy. Both bacteria and fungi increased weathering fluxes compared to abiotic controls. Without a host plant denudation rates were as large as weathering rates i.e. the weathering to denudation ratio was about one. Based on whole year fluxes, ectomycorrhizal seedlings produced the greatest weathering to denudation ratios (1.5). Non-ectomycorrhizal seedlings also showed a high ratio of 1.3. Both ectomycorrhizal hyphal networks and root hairs of non-ectomycorrhizal trees, embedded in biofilm (microorganisms surrounded by extracellular polymers), transferred nutrients to the host while drainage losses were minimized. These results suggest that biofilms localize both weathering and plant nutrient uptake, isolating the root-hypha-mineral interface from bulk soil solution.     

Monitoring Seasonal Changes in Winery-Resident Microbiota

During the transformation of grapes to wine, wine fermentations are exposed to a large area of specialized equipment surfaces within wineries, which may serve as important reservoirs for two-way transfer of microbes between fermentations. However, the role of winery environments in shaping the microbiota of wine fermentations and vectoring wine spoilage organisms is poorly understood at the systems level. Microbial communities inhabiting all major equipment and surfaces in a pilot-scale winery were surveyed over the course of a single harvest to track the appearance of equipment microbiota before, during, and after grape harvest. Results demonstrate that under normal cleaning conditions winery surfaces harbor seasonally fluctuating populations of bacteria and fungi. Surface microbial communities were dependent on the production context at each site, shaped by technological practices, processing stage, and season. During harvest, grape- and fermentation-associated organisms populated most winery surfaces, acting as potential reservoirs for microbial transfer between fermentations. These surfaces harbored large populations of Saccharomyces cerevisiae and other yeasts prior to harvest, potentially serving as an important vector of these yeasts in wine fermentations. However, the majority of the surface communities before and after harvest comprised organisms with no known link to wine fermentations and a near-absence of spoilage-related organisms, suggesting that winery surfaces do not overtly vector wine spoilage microbes under normal operating conditions.     

The Multifunctional Role of Ectomycorrhizal Associations in Forest Ecosystem Processes

Belowground biological interactions that occur among plant roots, microorganisms and animals are dynamic and substantially influence ecosystem processes. Among these interactions, the ectomycorrhizal (ECM) symbiosis is remarkable but unfortunately these associations have mainly been considered within the rather narrow perspective of their effects on the uptake of dissolved mineral nutrients by individual plants. More recent research has placed emphasis on a wider, multifunctional perspective, including the effects of ectomycorrhizal symbiosis on plant and microbial communities, and on ecosystem processes. This includes mobilization of N and P from organic polymers, release of nutrients from mineral particles or rock surfaces via weathering, effects on carbon cycling, interactions with mycoheterotrophic plants, mediation of plant responses to stress factors such as drought, soil acidification, toxic metals, and plant pathogens, rehabilitation and regeneration of degraded forest ecosystems, as well as a range of possible interactions with groups of other soil microorganisms. Ectomycorrhizas are almost invariably characterized by a Hartig net composed of highly branched hyphae which entirely surround the outer root cortical cells. The Hartig net is the place of massive bidirectional exchanges of nutrients between the host and the fungus. Through these branched hyphae ectomycorrhizal fungi connect their plant hosts to the heterogeneously distributed nutrients required for their growth, enabling the flow of energy-rich compounds required for nutrient mobilization whilst simultaneously providing conduits for the translocation of mobilized products back to their hosts. In addition to increasing the nutrient absorptive surface area of their host plant root systems, the extraradical mycelium of ectomycorrhizal fungi provides a direct pathway for translocation of photosynthetically derived carbon from their hosts to microsites in the soil and a large surface area for interaction with other soil micro-organisms. The detailed functioning and regulation of these mycorrhizosphere processes is still poorly understood and needs detailed molecular approach to study these mycorrhizosphere processes but recent progress in ectomycorrhizal associations is reviewed and potential benefits of improved understanding of mycorrhizosphere interactions are discussed.     

Microbial and chemical characterization of underwater fresh water springs in the Dead Sea

Due to its extreme salinity and high Mg concentration the Dead Sea is characterized by a very low density of cells most of which are Archaea. We discovered several underwater fresh to brackish water springs in the Dead Sea harboring dense microbial communities. We provide the first characterization of these communities, discuss their possible origin, hydrochemical environment, energetic resources and the putative biogeochemical pathways they are mediating. Pyrosequencing of the 16S rRNA gene and community fingerprinting methods showed that the spring community originates from the Dead Sea sediments and not from the aquifer. Furthermore, it suggested that there is a dense Archaeal community in the shoreline pore water of the lake. Sequences of bacterial sulfate reducers, nitrifiers iron oxidizers and iron reducers were identified as well. Analysis of white and green biofilms suggested that sulfide oxidation through chemolitotrophy and phototrophy is highly significant. Hyperspectral analysis showed a tight association between abundant green sulfur bacteria and cyanobacteria in the green biofilms. Together, our findings show that the Dead Sea floor harbors diverse microbial communities, part of which is not known from other hypersaline environments. Analysis of the water's chemistry shows evidence of microbial activity along the path and suggests that the springs supply nitrogen, phosphorus and organic matter to the microbial communities in the Dead Sea. The underwater springs are a newly recognized water source for the Dead Sea. Their input of microorganisms and nutrients needs to be considered in the assessment of possible impact of dilution events of the lake surface waters, such as those that will occur in the future due to the intended establishment of the Red Sea-Dead Sea water conduit.