Ingunn H. Thorseth

The importance of microbiological activity in the alteration of natural basaltic glass

Abstract The textural development of palagonite may differ profoundly depending on whether alteration occurred in the outermost 6–7 mm thick light-exposed surface zone of deposits, or elsewhere. In the former case, a pit-textured development of the parent basaltic glass develops as a consequence of local establishment of a highly alkaline micro-environment (pH > 9) for which the light-dependent cryptoendolithic cyanobacteria are considered most likely to be responsible. A highly porous, sponge-textured variety of palagonite, frequently defining zoned layers, contains abundant examples of bacteria. The shape and size of the pores combined with the geochemical development strongly suggest that bacteria have played an important role in the development of this texture.

Microbes play an important role in the alteration of oceanic crust

Microbes have been identified within altered parts of the glass rims of pillow lavas in the upper oceanic crust, 237 m below the top of the volcanic basement of ODP Hole 896A at the Costa Rica Rift. Their presence is verified by spherical and vermicular microbodies containing DNA. The elemental composition of the microbially processed areas differ from the parent glass. Further, the microbially processed parts, showing different morphological forms also show different elemental composition. Extreme K-enrichment indicate that microbes are presently active in the alteration process. The carbon isotopic composition of disseminated carbonates within the basaltic section of the Hole 504B (1 km distance from Hole 896A) also give evidence for microbial activity during rock alteration. The σ13C values of most of these trace carbonates are very low, reflecting metabolic control of the carbon cycle in these rocks. Microbial alteration of basaltic glass, comprising a substantial volume and surface area of the upper oceanic crust, may thus play an important role in the element exchange between oceanic crust and seawater.

Textural and chemical effects of bacterial activity on basaltic glass: an experimental approach

Abstract Naturally altered basaltic glass may show features such as pitted textures and variable degree of element mobilization relative to the fresh parent. The alteration process has generally been considered from only a chemical/physical point of view, but recent observations of bacteria in altered glass have, however, led to questions about the importance of microbial activity. In order to examine this, an experiment has been performed in which basaltic glass samples were immersed in growth media at room temperature for up to 394 days, inoculated with bacteria derived from a naturally altered pyroclastic deposit (Surtsey tuff). During the experiment it was observed that bacteria had a great affinity for attachment to the glass surface, which is in most cases connected to the production of extracellular polymers. Further, different species of bacteria were dominant at different time intervals. The bacteria activity caused a general decrease in pH from 8.0 to 5.8 during the time of the experiment. After 46 days of incubation, SEM studies of samples show rare examples of clear etching marks on the surface corresponding in size and shape of a minor group of bacteria. A local corrosion in a more irregular manner was observed after 181 days. Chemical analyses of the glass surface show no difference in composition compared to the fresh glass at this stage, i.e. any dissolution is congruent. Bacteria and biofilms attached to the glass surface show accumulation of elements, of which Al and Si could only have been derived by dissolution of the glass. However, the extent of accumulation of various elements may differ pronouncedly within and between the runs at 44, 77 and 181 days. This scatter probably reflects the diversity of the community and the ability of the different species of bacteria to accumulate elements. After 394 days the outermost glass rim, 1 μm in thickness, is highly depleted in all cations, except Si, which is relatively enriched. This incongruent dissolution of the glass, has only been active during the last 7 months of the experiment. The alteration rate is increased, at least, by a factor of 10 compared to that of the first 6 months. This is thought to be caused by the activity of a new, dominant bacterium group during this period. Microanalyses of the bacteria attached to the residual, leached glass rim, show more frequent accumulation of Si, and generally their chemistries are more homogenous than that observed in the other, shorter-termed runs. Bacterial activity may hence have a great influence on the textural and chemical developments commonly observed in naturally altered basaltic glass deposits.

Evidence for microbial activity at the glass-alteration interface in oceanic basalts

A detailed microbiological and geochemical study related to the alteration of basaltic glass of pillow lavas from the oceanic crust recovered from Hole 896A on the Costa Rica Rift (penetrating 290 m into the volcanic basement) has been carried out. A number of independent observations, pointing to the influence of microbes, may be summarized as follows: (1) Alteration textures are reminiscent of microbes in terms of form and shape. (2) Altered material contains appreciable amounts of C, N and K, and the N=C ratios are comparable to those of nitrogen-starved bacteria. (3) Samples stained with a dye (DAPI) that binds specifically to nucleic acids show the presence of DNA in the altered glass. Further, staining with fluorescent labeled oligonucleotide probes that hybridize specifically to 16S-ribosomal RNA of bacteria and archaea demonstrate their presence in the altered part of the glass. (4) Disseminated carbonate in the glassy margin of the majority of pillows shows d 13 C values, significantly lower than that of fresh basalt, also suggests biological activity. The majority of the samples have d 18 O values indicating temperatures of 20‐100oC, which is in the range of mesophilic and thermophilic micro-organisms.

Diversity of life in ocean floor basalt

Abstract Electron microscopy and biomolecular methods have been used to describe and identify microbial communities inhabiting the glassy margins of ocean floor basalts. The investigated samples were collected from a neovolcanic ridge and from older, sediment-covered lava flows in the rift valley of the Knipovich Ridge at a water depth around 3500 m and an ambient seawater temperature of −0.7°C. Successive stages from incipient microbial colonisation, to well-developed biofilms occur on fracture surfaces in the glassy margins. Observed microbial morphologies are various filamentous, coccoidal, oval, rod-shaped and stalked forms. Etch marks in the fresh glass, with form and size resembling the attached microbes, are common. Precipitation of alteration products around microbes has developed hollow subspherical and filamentous structures. These precipitates are often enriched in Fe and Mn. The presence of branching and twisted stalks that resemble those of the iron-oxidising Gallionella , indicate that reduced iron may be utilised in an energy metabolic process. Analysis of 16S-rRNA gene sequences from microbes present in the rock samples, show that the bacterial population inhabiting these samples cluster within the γ- and ϵ-Proteobacteria and the Cytophaga/Flexibacter/Bacteroides subdivision of the Bacteria, while the Archaea all belong to the Crenarchaeota kingdom. This microbial population appears to be characteristic for the rock and their closest relatives have previously been reported from cold marine waters in the Arctic and Antarctic, deep-sea sediments and hydrothermal environments.

Correlating microbial community profiles with geochemical data in highly stratified sediments from the Arctic Mid-Ocean Ridge

Microbial communities and their associated metabolic activity in marine sediments have a profound impact on global biogeochemical cycles. Their composition and structure are attributed to geochemical and physical factors, but finding direct correlations has remained a challenge. Here we show a significant statistical relationship between variation in geochemical composition and prokaryotic community structure within deep-sea sediments. We obtained comprehensive geochemical data from two gravity cores near the hydrothermal vent field Loki’s Castle at the Arctic Mid-Ocean Ridge, in the Norwegian-Greenland Sea. Geochemical properties in the rift valley sediments exhibited strong centimeter-scale stratigraphic variability. Microbial populations were profiled by pyrosequencing from 15 sediment horizons (59,364 16S rRNA gene tags), quantitatively assessed by qPCR, and phylogenetically analyzed. Although the same taxa were generally present in all samples, their relative abundances varied substantially among horizons and fluctuated between Bacteria- and Archaea-dominated communities. By independently summarizing covariance structures of the relative abundance data and geochemical data, using principal components analysis, we found a significant correlation between changes in geochemical composition and changes in community structure. Differences in organic carbon and mineralogy shaped the relative abundance of microbial taxa. We used correlations to build hypotheses about energy metabolisms, particularly of the Deep Sea Archaeal Group, specific Deltaproteobacteria, and sediment lineages of potentially anaerobic Marine Group I Archaea. We demonstrate that total prokaryotic community structure can be directly correlated to geochemistry within these sediments, thus enhancing our understanding of biogeochemical cycling and our ability to predict metabolisms of uncultured microbes in deep-sea sediments.

Microbial community diversity in seafloor basalt from the Arctic spreading ridges

Microbial communities inhabiting recent (< or =1 million years old; Ma) seafloor basalts from the Arctic spreading ridges were analyzed using traditional enrichment culturing methods in combination with culture-independent molecular phylogenetic techniques. Fragments of 16S rDNA were amplified from the basalt samples by polymerase chain reaction, and fingerprints of the bacterial and archaeal communities were generated using denaturing gradient gel electrophoresis. This analysis indicates a substantial degree of complexity in the samples studied, showing 20-40 dominating bands per profile for the bacterial assemblages. For the archaeal assemblages, a much lower number of bands (6-12) were detected. The phylogenetic affiliations of the predominant electrophoretic bands were inferred by performing a comparative 16S rRNA gene sequence analysis. Sequences obtained from basalts affiliated with eight main phylogenetic groups of Bacteria, but were limited to only one group of the Archaea. The most frequently retrieved bacterial sequences affiliated with the gamma-proteobacteria, alpha-proteobacteria, Chloroflexi, Firmicutes, and Actinobacteria. The archaeal sequences were restricted to the marine Group 1: Crenarchaeota. Our results indicate that the basalt harbors a distinctive microbial community, as the majority of the sequences differed from those retrieved from the surrounding seawater as well as from sequences previously reported from seawater and deep-sea sediments. Most of the sequences did not match precisely any sequences in the database, indicating that the indigenous Arctic ridge basalt microbial community is yet uncharacterized. Results from enrichment cultures showed that autolithotrophic methanogens and iron reducing bacteria were present in the seafloor basalts. We suggest that microbial catalyzed cycling of iron may be important in low-temperature alteration of ocean crust basalt. The phylogenetic and physiological diversity of the seafloor basalt microorganisms differed from those previously reported from deep-sea hydrothermal systems.

Enumeration of Archaea and Bacteria in seafloor basalt using real-time quantitative PCR and fluorescence microscopy

A SYBR Green real-time quantitative PCR (Q-PCR) assay for the detection and quantification of Bacteria and Archaea present in the glassy rind of seafloor basalts of different ages and water depths is presented. Two sets of domain-specific primers were designed and validated for specific detection and quantification of bacterial and archaeal 16S rRNA genes in DNA extracted from basaltic glass. Total cell numbers were also estimated by fluorescence microscopy analysis of SYBR Gold-stained samples. The results from the two different approaches were concurrent, and Q-PCR results showed that the total number of cells present in basalts was in the range from 6 x 10(5) to 4 x 10(6) cells g(-1) basaltic glass. Further, it was demonstrated that these cells were almost exclusively from the domain Bacteria. When applying the same methods on samples of different ages (22 years-0.1 Ma) and water depths (139-3390 mbsl), no significant differences in cell concentrations or in the relative abundance of Archaea and Bacteria were detected.

A textural and chemical study of Icelandic palagonite of varied composition and its bearing on the mechanism of the glass-palagonite transformation

Palagonite of basalt and basaltic andesite parentages from hyaloclastite deposits in Iceland has been investigated. SEM studies indicate a sharp to diffuse alteration front which may propagate along microfractures in the glass, resulting in a progressive partial dissolution yielding palagonite of variable, but generally increasing porosity towards grain surfaces. The palagonite has a granular texture. Incipient alteration is indicated by the development of individual globules (ca. 0.01 μm in diameter), whereas at an advanced stage chains of globules, defining a sponge-like texture, characterize the palagonite. In the basaltic andesite, the precursor to brown Ti- and Fe-rich palagonite is a white variety, for which an evolutionary model is presented. EDS line-scans across the fresh glass-palagonite boundary show the existence of a 2–4 μm thick zone of glass-like material in which all elements have been depleted, except Si, and in some cases Al, which have been relatively enriched. The white palagonite is characterized by strong depletion of Ti, Fe, Na, Mg, and to lesser extent, Ca, Al (in order of decreasing loss). Regardless of the degree of porosity development (ca. 1–43 vol%), the extent of element depletion relative to SiO2 is constant. This gives evidence for selective element mobility prior to a variable degree of congruent network dissolution of the Si-rich residue, yielding zoned palagonite with a different porosity. In order for Fe, Ti, and Al to dissolve, a pH 3, Fe, Ti, and Al will precipitate in the pores of the white palagonite as oxides/hydroxides, thus creating the brown variety, which is characterized by highly variable contents of the above-mentioned elements. The applicability of this model to palagonite derived from basalt parentages at different pH conditions is discussed.

Discovery of a black smoker vent field and vent fauna at the Arctic Mid-Ocean Ridge

The Arctic Mid-Ocean Ridge (AMOR) represents one of the most slow-spreading ridge systems on Earth. Previous attempts to locate hydrothermal vent fields and unravel the nature of venting, as well as the provenance of vent fauna at this northern and insular termination of the global ridge system, have been unsuccessful. Here, we report the first discovery of a black smoker vent field at the AMOR. The field is located on the crest of an axial volcanic ridge (AVR) and is associated with an unusually large hydrothermal deposit, which documents that extensive venting and long-lived hydrothermal systems exist at ultraslow-spreading ridges, despite their strongly reduced volcanic activity. The vent field hosts a distinct vent fauna that differs from the fauna to the south along the Mid-Atlantic Ridge. The novel vent fauna seems to have developed by local specialization and by migration of fauna from cold seeps and the Pacific.