In situ evidence for microdomains in the polymer matrix of bacterial microcolonies

Confocal laser scanning microscopy and fluorescent lectin-binding analyses (FLBA) were used to study the form, arrangement, and composition of exopolymeric substances (EPS) surrounding naturally occurring microcolonies in biofilms. FLBA, using multiple lectin staining and multichannel imaging, indicated that the EPS of many microcolonies exhibit distinct multiple binding regions. A common pattern in the microcolonies is a three zone arrangement with cell-associated, intercellular, and an outer layer of EPS covering the exterior of the colony. Differential binding of lectins suggests that there are differences in the glycoconjugate composition or their arrangement in the EPS of microcolonies. The combination of FLBA with fluorescent in situ hybridization (FISH) indicates that the colonies consist of the major groups, alpha- and beta-Proteobacteria. It is suggested that the EPS arrangement observed provides a physical structuring mechanism that can segregate extracellular activities at the microscale.     

Use of lectins to in situ visualize glycoconjugates of extracellular polymeric substances in acidophilic archaeal biofilms

Biofilm formation and the production of extracellular polymeric substances (EPS) by meso‐ and thermoacidophilic metal‐oxidizing archaea on relevant substrates have been studied to a limited extent. In order to investigate glycoconjugates, a major part of the EPS, during biofilm formation/bioleaching by archaea on pyrite, a screening with 75 commercially available lectins by fluorescence lectin‐binding analysis (FLBA) has been performed. Three representative archaeal species, Ferroplasma acidiphilum DSM 28986, Sulfolobus metallicus DSM 6482^T and a novel isolate Acidianus sp. DSM 29099 were used. In addition, Acidianus sp. DSM 29099 biofilms on elemental sulfur were studied. The results of FLBA indicate (i) 22 lectins bound to archaeal biofilms on pyrite and 21 lectins were binding to Acidianus sp. DSM 29099 biofilms on elemental sulfur; (ii) major binding patterns, e.g. tightly bound EPS and loosely bound EPS, were detected on both substrates; (iii) the three archaeal species produced various EPS glycoconjugates on pyrite surfaces. Additionally, the substratum induced different EPS glycoconjugates and biofilm structures of cells of Acidianus sp. DSM 29099. Our data provide new insights into interactions between acidophilic archaea on relevant surfaces and also indicate that FLBA is a valuable tool for in situ investigations on archaeal biofilms.     

Fluvival tufa formation in a hard-water creek (Deinschwanger Bach, Franconian Alb, Germany)

Summary Cyanobacteria-dominated biofilms involved in tufa deposition in the hardwater creek Deinschwanger Bach, Bavaria, were investigated with regard to their effect on the carbonate equilibrium and fabric formation. Current tufa deposition is evident by up to 1.5 mm thick crusts that have formed on substrate plates placed in the creek for 10 months. Hydrochemistry data indicate that carbonate precipitation along the creek is physicochemically driven by CO₂ degassing, whereas photosynthetic carbon assimilation is without detectable effect on the macroscale carbonate equilibrium. However, stable isotope data indicate a minor photosynthetic effect, but only for the lower creek section where the pCO₂ already drops to the two-fold of the atmospheric level. Though the initial process of external nucleation on cyanobacterial sheaths in the lower creck section might be promoted of by a photosynthetically-induced microscale pH gradient, the effect is not strong enough to cause a CaCO₃ impregnation of the sheaths. The fabric of the laminated tufa crusts in the creek reflects the temporal alternation of porous microspariticPhormidium incrustatum-Phormidium foveolarum-diatom biofilms in spring, micrite-impregnatedPhormidium incrustatum-Phormidium foveolarum-diatom biofilms in summerautumn, and detritus-rich non-calcified diatom-biofilms in winter. By contrast, exopolymer-poor surfaces of cascade tufa mosses show large, euhedral spar crystals. Non-phototrophic bacteria, which occur in large numbers inPhormidium incrustatum-Phormidium foveolarums-diatom-communities, thrive on extracellular polymeric substances (EPS) and dead cells of the cyanobacteria and are unlikely to promote CaCO₃ precipitation.     

Cyanobacterial formation of intracellular Ca‐carbonates in undersaturated solutions

Cyanobacteria have long been thought to induce the formation of Ca‐carbonates as secondary by‐products of their metabolic activity, by shifting the chemical composition of their extracellular environment to conditions favoring mineral precipitation. Some cyanobacterial species forming Ca‐carbonates intracellularly were recently discovered. However, the environmental conditions under which this intracellular biomineralization process can occur and the impact of cyanobacterial species forming Ca‐carbonates intracellularly on extracellular carbonatogenesis are not known. Here, we show that these cyanobacteria can form Ca‐carbonates intracellularly while growing in extracellular solutions undersaturated with respect to all Ca‐carbonate phases, that is, conditions thermodynamically unfavorable to mineral precipitation. This shows that intracellular Ca‐carbonate biomineralization is an active process; that is, it costs energy provided by the cells. The cost of energy may be due to the active accumulation of Ca intracellularly. Moreover, unlike cyanobacterial strains that have been usually considered before by studies on Ca‐carbonate biomineralization, cyanobacteria forming intracellular carbonates may slow down or hamper extracellular carbonatogenesis, by decreasing the saturation index of their extracellular solution following the buffering of the concentration of extracellular calcium to low levels.     

A freshwater analog for the production of Epiphyton‐like microfossils

Calcified microbial microfossils—often interpreted as cyanobacteria—were important components of Precambrian and Paleozoic limestones, but their paucity in modern marine environments complicates our ability to make conclusive interpretations about their taxonomic affinity and geologic significance. Freshwater spring‐associated limestones (e.g., travertine and tufa) serve as terrestrial analogs to investigate mineralization in and around aquatic biofilms on observable timescales. We document the diagenesis of calcite fabrics associated with the freshwater algae Oocardium stratum, an epiphytic colonial green algae (desmid) known for producing stalks of extracellular polymeric substances (EPS) and passively producing a bifurcating tubular calcite monocrystal. Bifurcating EPS stalks produced by Oocardium colonies can become calcified and preserved in ancient carbonate deposits. Calcified micritic EPS stalks have a filamentous morphology, show evidence of branching, and maintain uniformity in diameter thickness throughout the mm‐scale colony, much like the enigmatic calcimicrobe Epiphyton. We provide a mechanism by which calcification associated with a colonial semispherical micro‐organism produces microfossils that deceptively resemble filamentous forms. These findings have implications for the use of morphological traits when assigning taxonomic affinities to extinct microfossil groups and highlight the utility of calcifying freshwater modern environments to investigate microbial taphonomy.     

Microbial geochemical calcium cycle

The participation of microorganisms in the geochemical calcium cycle is the most important factor maintaining neutral conditions on the Earth. This cycle has profound influence on the fate of inorganic carbon, and, thereby, on the removal of CO2 from the atmosphere. The major part of calcium deposits was formed in the Precambrian, when prokaryotic biosphere predominated. After that, calcium recycling based on biogenic deposition by skeletal organisms became the main process. Among prokaryotes, only a few representatives, e.g., cyanobacteria, exhibit a special calcium function. The geochemical calcium cycle is made possible by the universal features of bacteria involved in biologically mediated reactions and is determined by the activities of microbial communities. In the prokaryotic system, the calcium cycle begins with the leaching of igneous rock predominantly through the action of the community of organotrophic organisms. The release of carbon dioxide to the soil air by organotrophic aerobes leads to leaching with carbonic acid and soda salinization. Under anoxic conditions, of major importance is the organic acid production by primary anaerobes (fermentative microorganisms). Calcium carbonate is precipitated by secondary anaerobes (sulfate reducers) and to a smaller degree by methanogens. The role of the cyanobacterial community in carbonate deposition is exposed by stromatolites, which are the most common organo-sedimentary Precambrian structures. Deposition of carbonates in cyanobacterial mats as a consequence of photoassimilation of CO2 does not appear to be a significant process. It is argued that carbonates were deposited at the boundary between the "soda continent", which emerged as a result of subaerial leaching with carbonic acid, and the ocean containing Ca2+. Such ecotones provided favorable conditions for the development of the benthic cyanobacterial community, which was a precursor of stromatolites.     

Mapping glycoconjugate-mediated interactions of marine Bacteroidetes with diatoms

The degradation of diatoms is mainly catalyzed by Bacteroidetes and this process is of global relevance for the carbon cycle. In this study, a combination of catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH) and fluorescent lectin binding analysis (FLBA) was used to identify and map glycoconjugates involved in the specific interactions of Bacteroidetes and diatoms, as well as detritus, at the coastal marine site Helgoland Roads (German Bight, North Sea). The study probed both the presence of lectin-specific extracellular polymeric substances (EPS) of Bacteroidetes for cell attachment and that of glycoconjugates on diatoms with respect to binding sites for Bacteroidetes. Members of the clades Polaribacter and Ulvibacter were shown to form microcolonies within aggregates for which FLBA indicated the presence of galactose containing slime. Polaribacter spp. was shown to bind specifically to the setae of the abundant diatom Chaetoceros spp., and the setae were stained with fucose-specific lectins. In contrast, Ulvibacter spp. attached to diatoms of the genus Asterionella which bound, among others, the mannose-specific lectin PSA. The newly developed CARD-FISH/FLBA protocol was limited to the glycoconjugates that persisted after the initial CARD-FISH procedure. The differential attachment of bacteroidetal clades to diatoms and their discrete staining by FLBA provided evidence for the essential role that formation and recognition of glycoconjugates play in the interaction of bacteria with phytoplankton.     

Interaction between Calcite and Phosphorus in Biomineralization Processes in Tufa Carbonates

Using electron microscopy techniques (SEM, LTSEM) coupled with analytical methods (XRD and EDS) the role of phosphorus has been assessed in the formation of freshwater calcite deposits (tufa) in a small pond of the Ruidera Lakes (Spain). Differences between the cell walls and sheaths of bacteria and eukaryotic algae as well as the existence of additional layers of extracellular polymeric substances (EPS) were features that lead to differences in the process of induced calcite biomineralization. Phosphorus has influence in the biomineralization of the EPS, sheaths and cell walls of cyanobacteria allowing for fossil preservation whereas does not participate in the calcite precipitation around algae and mosses. This variability may explain the different positive or negative roles played by natural or artificial inputs of phosphorus in hard water lakes and the different morphological features of calcite precipitates associated with eukaryotic and cyanobacteria picoplankton found in natural environments. The biomineralization observed is in agreement with the isotopic composition of the tufa layers that reflect the variations in environmental conditions around biological communities. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dissolution of calcite in the twilight zone: bacterial control of dissolution of sinking planktonic carbonates is unlikely

We investigated the ability of bacterial communities to colonize and dissolve two biogenic carbonates (Foraminifera and oyster shells). Bacterial carbonate dissolution in the upper water column is postulated to be driven by metabolic activity of bacteria directly colonising carbonate surfaces and the subsequent development of acidic microenvironments. We employed a combination of microsensor measurements, scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM) and image analysis and molecular documentation of colonising bacteria to monitor microbial processes and document changes in shell surface topography. Bacterial communities rapidly colonised shell surfaces, forming dense biofilms with extracellular polymeric substance (EPS) deposits. Despite this, we found no evidence of bacterially mediated carbonate dissolution. Dissolution was not indicated by Ca²⁺ microprofiles, nor was changes in shell surface structure related to the presence of colonizing bacteria. Given the short time (days) settling carbonate material is actually in the twilight zone (500–1000 m), it is highly unlikely that microbial metabolic activity on directly colonised shells plays a significant role in dissolving settling carbonates in the shallow ocean.     

Potential role for microbial ureolysis in the rapid formation of carbonate tufa mounds

Modern carbonate tufa towers in the alkaline (~pH 9.5) Big Soda Lake (BSL), Nevada, exhibit rapid precipitation rates (exceeding 3 cm/year) and host diverse microbial communities. Geochemical indicators reveal that carbonate precipitation is, in part, promoted by the mixing of calcium-rich groundwater and carbonate-rich lake water, such that a microbial role for carbonate precipitation is unknown. Here, we characterize the BSL microbial communities and evaluate their potential effects on carbonate precipitation that may influence fast carbonate precipitation rates of the active tufa mounds of BSL. Small subunit rRNA gene surveys indicate a diverse microbial community living endolithically, in interior voids, and on tufa surfaces. Metagenomic DNA sequencing shows that genes associated with metabolisms that are capable of increasing carbonate saturation (e.g., photosynthesis, ureolysis, and bicarbonate transport) are abundant. Enzyme activity assays revealed that urease and carbonic anhydrase, two microbial enzymes that promote carbonate precipitation, are active in situ in BSL tufa biofilms, and urease also increased calcium carbonate precipitation rates in laboratory incubation analyses. We propose that, although BSL tufas form partially as a result of water mixing, tufa-inhabiting microbiota promote rapid carbonate authigenesis via ureolysis, and potentially via bicarbonate dehydration and CO₂ outgassing by carbonic anhydrase. Microbially induced calcium carbonate precipitation in BSL tufas may generate signatures preserved in the carbonate microfabric, such as stromatolitic layers, which could serve as models for developing potential biosignatures on Earth and elsewhere.     

Ultrastructural and genetic characteristics of endolithic cyanobacterial biofilms colonizing Antarctic granite rocks

The precise identification of the cyanobacteria that comprise an endolithic biofilm is hindered by difficulties in culturing the organisms found in these biofilms and a lack of previous molecular and ultrastructural data. This study characterizes, both at the ultrastructural and molecular level, two different cyanobacterial biofilms found in fissures of granite from continental Antarctica. Electron microscopy revealed structural differences between the two biofilms. One was only loosely adhered to the substrate, while the other biofilm showed a closer association between cells and rock minerals and was tightly attached to the substrate. Cells from both biofilms where ultrastructurally distinct, displaying, for instance, clear differences in their sheaths. The amounts of EPS and their organization associated with the cyanobacteria may determine the differences in adhesion and effects on the lithic substrate observed in the biofilms. By sequencing part of the 16S rRNA gene, the two cyanobacteria were also genetically characterized. The gene sequence of the cells comprising the biofilm that was tightly attached to the lithic substrate showed most homology with that of an endolithic cyanobacterium from Switzerland (AY153458), and the cyanobacterial type loosely adhered to the rock, clustered with Acaryochloris marina, the only organism unequivocally known to contain chlorophyll d. This study reveals the presence of at least two different types of endolithic biofilm, dominated each by a single type of cyanobacterium, able to withstand the harsh conditions of the Antarctic climate.     

Microbial Geochemical Calcium Cycle

The participation of microorganisms in the geochemical calcium cycle is the most important factor maintaining neutral conditions on the Earth. This cycle has profound influence on the fate of inorganic carbon, and, thereby, on the removal of CO₂ from the primitive atmosphere. Most calcium deposits were formed in the Precambrian, when the prokaryotic biosphere predominated. After that, calcium recycling based on biogenic deposition by skeletal organisms became the main process. Among prokaryotes, only a few representatives, e.g. cyanobacteria, exhibit a special calcium function. The geochemical calcium cycle is made possible by the universal features of bacteria involved in biologically mediated reactions and is determined by the activities of microbial communities. In the prokaryotic system, the calcium cycle begins with the leaching of igneous rocks, predominantly through the action of the community of organotrophic organisms. The release of carbon dioxide to the soil air by organotrophic aerobes leads to leaching with carbonic acid and soda salinization. Under anoxic conditions, of major importance is the organic acid production by primary anaerobes (fermentative microorganisms). Calcium carbonate is precipitated by secondary anaerobes (sulfate reducers) and to a smaller degree by methanogens. The role of the cyanobacterial community in carbonate deposition is recorded by stromatolites, which are the most common organo–sedimentary Precambrian structures. Deposition of carbonates in cyanobacterial mats as a consequence of photoassimilation of CO₂ does not appear to be a significant process. It is argued that carbonates were deposited at the boundary between the soda continent, which emerged as a result of subaerial leaching with carbonic acid, and the ocean containing Ca²⁺. Such ecotones provided favorable conditions for the development of the benthic cyanobacterial communities, which were the precursors of stromatolites.     

Phototrophic microbes form endolithic biofilms in ikaite tufa columns (SW Greenland)

Marine tufa‐columns, formed by the hydrated carbonate mineral ikaite, present a unique alkaline microbial habitat only found in Ikka Fjord (SW‐Greenland). The outermost parts of the ikaite columns exhibit a multitude of physico‐chemical gradients, and the porous ikaite is colonized by endolithic phototrophic biofilms serving as a substrate for grazing epifauna, where scraping by sea urchins affects overall column‐topography. We present a detailed study of the optical microenvironment, spatial organization, and photosynthetic activity of endolithic phototrophs within the porous ikaite crystal matrix. Cyanobacteria and diatoms formed distinctly coloured zones and were closely associated with ikaite‐crystals via excretion of exopolymers. Scalar‐irradiance measurements showed strong attenuation of visible light (400–700 nm), where only ～1% of incident irradiance remained at 20 mm depth. Transmission spectra showed in vivo absorption signatures of diatom and cyanobacterial photopigments, which were confirmed by HPLC‐analysis. Variable‐chlorophyll‐fluorescence‐imaging showed active photosynthesis with high‐light acclimation in the outer diatom layer, and low‐light acclimation in the underlying cyanobacterial part. Phototrophs in ikaite thus thrive in polymer‐bound endolithic biofilms in a complex gradient microhabitat experiencing constant slow percolation of highly alkaline phosphate‐enriched spring water mixing with cold seawater at the tufa‐column‐apex. We discuss the potential role of these biofilms in ikaite column formation.     

A microbial role in the construction of Mono Lake carbonate chimneys?

Lacustrine carbonate chimneys are striking, metre‐scale constructions. If these were bioinfluenced constructions, they could be priority targets in the search for early and extraterrestrial microbial life. However, there are questions over whether such chimneys are built on a geobiological framework or are solely abiotic geomorphological features produced by mixing of lake and spring waters. Here, we use correlative microscopy to show that microbes were living around Pleistocene Mono Lake carbonate chimneys during their growth. A plausible interpretation, in line with some recent works by others on other lacustrine carbonates, is that benthic cyanobacteria and their associated extracellular organic material (EOM) formed tubular biofilms around rising sublacustrine spring vent waters, binding calcium ions and trapping and binding detrital silicate sediment. Decay of these biofilms would locally have increased calcium and carbonate ion activity, inducing calcite precipitation on and around the biofilms. Early manganese carbonate mineralisation was directly associated with cell walls, potentially related to microbial activity though the precise mechanism remains to be elucidated. Much of the calcite crystal growth was likely abiotic, and no strong evidence for either authigenic silicate growth or a clay mineral precursor framework was observed. Nevertheless, it seems likely that the biofilms provided initial sites for calcite nucleation and encouraged the primary organised crystal growth. We suggest that the nano‐, micro‐ and macroscale fabrics of these Pleistocene Mono Lake chimneys were affected by the presence of centimetre‐thick tubular and vertically stacked calcifying microbial mats. Such carbonate chimneys represent a promising macroscale target in the exploration for ancient or extraterrestrial life.     

Carbonic anhydrase: Enzyme that has transformed the biosphere

The bases of modern type biosphere were laid down about two billion years ago during the predominance of prokaryotes on the Earth. Cyanobacteria changed radically the composition of the Proterozoic atmosphere by saturating it with photosynthetic oxygen. At the same time, large quantities of atmospheric CO₂ became sequestered in carbonates owing to mineralization of ancient cyano-bacterial communities; the latter have reached us in the form of laminated limestone deposits, termed stromatolites. The mechanism of carbonate depositing by cyanobacteria is still poorly understood. It is not yet clear whether physiological processes are involved in cell mineralization or if the outer membranes of cyanobacteria serve as a kind of crystallization center and arrange the structure for natural accumulation of sediments. We proposed that a key role in the mechanism of biomineralization belongs to the enzyme carbonic anhydrase (CA), which regulates the equilibrium between the inorganic carbon forms (C_i), including bicarbonate that participates in natural sedimentation of calcium. Since the deposition of calcium carbonate by prokaryotes occurs in the pericellular space and this deposition is controlled by pH, it seems likely that CA, localized on the periphery of cyanobacterial cells, is involved in stabilizing the external pH and in promoting cell mineralization. This review summarizes information concerning possible mechanisms of biogenic calcification (CaCO₃ deposition). The function of CA in the living cell and the role of this enzyme in biological processes are considered, and the data on localization of CA in cyano-bacterial cells are presented. Based on available evidence, a scheme is suggested to describe the role of extracellular CA in photosynthetic carbon assimilation and to relate this process with CaCO₃ deposition during mineralization of cyanobacteria.     

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     

Generation of reactive oxygen species upon red light exposure of cyanobacteria from Roman hypogea

Subterranean archaeological sites in Rome (Italy) are threatened by phototrophic biofilms predominated by cyanobacteria and associated microorganisms. They damage the frescoes, mortar, marble, and tufa rock wherever artificial lighting is installed. During the past two decades, the conservation strategies have evolved gradually; rather than restricting the illumination time and intensity, the latest approach is to use strong light to reduce their growth. Since cyanobacterial cells are abundant in phycobilisomes and chlorophyll a, which produce reactive oxygen species (ROS) upon irradiation, strong red light (620–650 nm) was applied to generate high amount of ROS in a rate beyond the quenching capacity of the organism. After 25 h of irradiation, the photosystem II quantum yields of seven cyanobacterial isolates in culture were reduced by 65–94%. Conversely, blue light (460–480 nm) promoted photosystem II activity by up to 35%. δ-Aminolevulinic acid (D-ALA) was introduced to enhance the treatment, as it can be transformed into protochlorophyllide by cyanobacteria and then excited by red light to generate ROS inside the cells. Since the natural photosynthetic pigments as well as the endogenous protochlorophyllide exist only within the cyanobacterial cells, they are unlikely to contaminate or damage the underlying stone substrata. Electron spin resonance spectroscopy confirmed that D-ALA treatment caused the formation of ROS; spin trap experiments indicated that radicals were produced in the system.     

Characterization of Exopolysaccharides Produced by Cyanobacteria Isolated from Polynesian Microbial Mats

Six cyanobacterial isolates recovered from Polynesian microbial mats, called “kopara,” were cultured using laboratory-closed photobioreactors and were shown to produce exopolymers as released and capsular exopolysaccharides (EPS). These polymers have been chemically characterized using colorimetric and elemental assays, infrared spectrometry, and gas chromatography. Both capsular and released EPS consisted of 7 to 10 different monosaccharides with neutral sugars predominating. Interestingly, four isolates exhibited sulfate contents ranging from 6% to 19%. On the basis of preliminary data, cyanobacteria from this unusual ecosystem appear to be an important source of novel EPS of a great interest in terms of their biological activities.     

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.     

Growth and release of extracellular organic compounds by benthic diatoms depend on interactions with bacteria

Phototrophic epilithic biofilms harbour a distinct assemblage of heterotrophic bacteria, cyanobacteria and photoautotrophic algae. Secretion of extracellular polymeric substances (EPS) by these organisms and the physicochemical properties of the EPS are important factors for the development of the biofilms. We have isolated representative diatom and bacteria strains from epilithic biofilms of Lake Constance. By pairwise co-cultivating these strains we found that diatom growth and EPS secretion by diatoms may depend on the presence of individual bacteria. Similar results were obtained after addition of spent bacterial medium to diatom cultures, suggesting that soluble substances from bacteria have an impact on diatom physiology. While searching for putative bacterial signal substances, we found that concentrations of various dissolved free amino acids (DFAA) within the diatom cultures changed drastically during co-cultivation with bacteria. Further, the secretion of extracellular carbohydrates and proteins can be influenced by bacteria or their extracellular substances. We have performed mass spectrometric peptide mapping to identify proteins which are secreted when co-cultivating the diatom Phaeodactylum tricornutum Bohlin and Escherichia coli. The identified proteins are possibly involved in signalling, extracellular carbohydrate modification and uptake, protein and amino acid modification, and cell/cell aggregation of diatom and bacteria strains. Our data indicate that diatom-bacteria biofilms might be regulated by a complex network of chemical factors involving EPS, amino acid monomers and other substances. Thus interactions with bacteria can be considered as one of the main factors driving biofilm formation by benthic diatoms.