A hypersaline microbial mat from the Pacific Atoll Kiritimati: insights into composition and carbon fixation using biomarker analyses and a 13C-labeling approach

Modern microbial mats are widely recognized as useful analogs for the study of biogeochemical processes relevant to paleoenvironmental reconstruction in the Precambrian. We combined microscopic observations and investigations of biomarker composition to investigate community structure and function in the upper layers of a thick phototrophic microbial mat system from a hypersaline lake on Kiritimati (Christmas Island) in the Northern Line Islands, Republic of Kiribati. In particular, an exploratory incubation experiment with ¹³C-labeled bicarbonate was conducted to pinpoint biomarkers from organisms actively fixing carbon. A high relative abundance of the cyanobacterial taxa Aphanocapsa and Aphanothece was revealed by microscopic observation, and cyanobacterial fatty acids and hydrocarbons showed ¹³C-uptake in the labeling experiment. Microscopic observations also revealed purple sulfur bacteria (PSB) in the deeper layers. A cyclic C_19:0 fatty acid and farnesol were attributed to this group that was also actively fixing carbon. Background isotopic values indicate Calvin–Benson cycle-based autotrophy for cyc C_19:0 and farnesol-producing PSBs. Biomarkers from sulfate-reducing bacteria (SRB) in the top layer of the mat and their ¹³C-uptake patterns indicated a close coupling between SRBs and cyanobacteria. Archaeol, possibly from methanogens, was detected in all layers and was especially abundant near the surface where it contained substantial amounts of ¹³C-label. Intact glycosidic tetraether lipids detected in the deepest layer indicated other archaea. Large amounts of ornithine and betaine bearing intact polar lipids could be an indicator of a phosphate-limited ecosystem, where organisms that are able to substitute these for phospholipids may have a competitive advantage.     

Distribution, Taxonomy, and Boring Patterns of Marine Endolithic Algae

Penetration of calcareous substrates by algae involves differentmicroenvironments. Endolithic algae include two different lifeforms: (1) algae colonizing existing spaces within the rock,and (2) algae actively boring within the carbonate substrate. Two species of boring Cyanophyta were studied, and their boringswere compared in exposed calcite (Island spar). The size andpattern of borings reflect the corresponding properties of thealgae and show taxonomic distinctions. The direction of thetunnels and their wall sculptureare determined by the planesof crystal cleavage and twinning. Algal boring is a dissolution process performed by the terminalcells of endolithic filaments. The space dissolved away by analga has the shape of a miniature calcite crystal. Dissolutionproceeds along the crystal twinning lines diagonally to theprevious microcrystal space. Thetunnel represents a sequenceof such microcrystal-shaped spaces. Both biological and mineralogicaldeterminants should be considered in the interpretation of algalboring patterns.     

Are environmental conditions recorded by the organic matrices associated with precipitated calcium carbonate in cyanobacterial microbialites?

The amino acid composition of organic matrices associated with calcium carbonate precipitates in microbialites built by different Phormidium species (cyanobacteria) has been compared for samples recovered in lagoonal settings from two regions of the Southern Tropical Pacific separated by more than 4000 km: New Caledonia (Nouméa lagoon) and French Polynesia (Tikehau atoll). Calcium carbonate precipitation in these microbial structures was observed mainly in the interior of the domes and clearly separated from the photosynthetically active surface layer. This study focuses on the hydrolysable amino acid composition of the associated organic matrices that are typically rich in cysteine, leucine, alanine and arginine in New Caledonia, whereas they are particularly rich in dicarboxylic amino acids in French Polynesia. This striking difference is seemingly related to different environmental conditions that characterize the two reef settings. The high cysteine content suggests an origin from metallothioneins produced by the cyanobacteria and/or by epiphytic diatoms that were observed on the top layer, as the result of the input of metals from terrestrial origin in the Nouméa lagoon. In addition, we analysed the bulk organic matter of the photosynthetically active surface layer and of the interior of the domes. The former showed remarkable variations of amino acid composition throughout the year 2001, which may potentially reflect the impact of climatological events (e.g. cyclones) and/or a much stronger seasonality in New Caledonia than in French Polynesia. Although the mechanisms behind the differences remain elusive, our study clearly shows that environmental conditions can be reflected by amino acid compositions, particularly for the organic matrices associated with carbonate precipitates.     

Mesoproterozoic Archaeoellipsoidès: akinetes of heterocystous cyanobacteria

The genus Archaeoellipsoides Horodyski & Donaldson comprises large (up to 135 μ long) ellipsoidal and rod-shaped microfossils commonly found in silicified peritidal carbonates of Mesoproterozoic age. Based on morphometric and sedimentary comparisons with the akinetes of modern bloom-forming Anabaena species, Archaeoellipsoides is interpreted as the fossilized remains of akinetes produced by planktic heterocystous cyanobacteria. These fossils set a minimum date for the evolution of derived cyanobacteria capable of marked cell differentiation, and they corroborate geochemical evidence indicating that atmospheric oxygen levels were well above 1% of present day levels 1,500 million years ago. Akinetes, atmospheric oxygen, cyanobacteria, heterocyst, microbial fossils, nitrogen fixation, peritidal, Proterozoic.     

A complex marine shallow-water boring trace: Dendrorete balani n. ichnogen. et ichnospec

Tavernier, A., Campbell. S. E. & Golubic, S. 1992 07 15: A complex marine shallow-water boring trace: Dendrorete balani n. ichnogen. et ichnospec.
Exceptionally well-preserved reticulate boring patterns in Pliocene barnacle shells are described as a new ichnogenus and species. The organism that formed these borings is a presumed phototroph, possibly belonging to green algae. The principles and needs of a taxonomic treatment of microborings as trace fossils in relation to that of their makers are discussed. It is suggested that ichno-taxonomy be extended to include modern traces, when the taxonomic identity of the agent cannot be ascertained. Bio-erosion, endoliths, ichnofossils, marine, microborings, shallow marine, shells .     

Early Cretaceous record of microboring organisms in skeletons of growing corals

Ko?odziej, B., Golubic, S., Bucur, I.I., Radtke, G. & Tribollet, A. 2011: Early Cretaceous record of microboring organisms in skeletons of growing corals. Lethaia, Vol. 45, pp. 34–45. A spectacularly preserved assemblage of microbial euendoliths, penetrating into skeletons of growing scleractinian corals, has been recognized in Early Aptian (Early Cretaceous) reef limestone of the Rar?u Mountains (East Carpathians, NE Romania). Microboring euendolithic filaments were found in five coral colonies of the suborder Microsolenina. They remained in part well‐preserved, often impregnated with iron oxides, which made them visible even in strongly recrystallized parts of coral skeletons. Filaments of a wide range of sizes (2–40 μm in diameter) were concentrated within medium parts of coral septa, oriented along the septa in the direction of the coral growth. The larger filaments were tubular, occurring in bundles and branched into finer, often tapering branches. Their behaviour and organization were quite similar to the modern euendolithic siphonalean chlorophyte Ostreobium. Filament diameters exceeded those reported for the modern species, but covered a similarly wide size range. Narrower frequently branching filaments, 4–8 μm in diameter, resemble distal branching patterns of modern Ostreobium quekettii. Some very thin filaments (ca. 1–2 μm) observed within skeleton or inside the large tubular filaments, sometimes associated with globular swellings, may represent euendolithic fungi. The recrystallization of coral skeleton had limited effect on preservation of euendoliths due to their impregnation with iron oxides; microbial euendoliths were subjected to different taphonomic changes. □Chlorophytes, Early Cretaceous, fungi, microbial euendoliths, Romania, scleractinian corals.     

Endolith associations and their relation to facies distribution in the Middle Devonian of New York State, U.S.A.

Vogel, Klaus, Golubic, Stjepko & Brett, Carlton E. 1987 07 15: Endolith associations and their relation to facies distribution in the Middle Devonian of New York State, U.S.A.
Microborings of endolithic organisms (e.g. algae, fungi) provide valuable indicators of paleoenvironments. The present study documents the distribution of microborings in fossil skeletal substrates from a spectrum of shelf to basinal facies in the Middle Devonian of New York. Endolithic boring in brachiopod shells and corals were prepared using special embedding-casting method and were examined and photographed with SEM. About 13 distinct types of microborings were discovered, presumably produced by microorganisms (pro- and eukaryotic), sponges, bryozoans and worms. Brachiopod holdfast etchings are reported for the first time from Paleozoic rocks. Microborings are non-randomly distributed among Middle Devonian facies. The highest diversity and abundance of borings occurs in diverse brachiopod and coral-bearing calcareous mudstones; which appear to represent shallow, well-aerated and low turbidity environments. Microboring diversity and frequency decreases markedly into mudstones and siltstones recording higher turbidity as well as into dark gray to black basinal shale facies. 'Leiorhynchus' brachiopods from probable deeper water, dysaerobic environments display very low diversity of micro-boring assemblages. They include brachiopod pedicle borings and one endolith which shows similarities to a modem cyanobacterium. If the latter is correctly identified, it brackets all of the studied facies within the photic zone.     

Multi-trichomous cyanobacterial microfossils from the Mesoproterozoic Gaoyuzhuang Formation, China: Paleoecological and taxonomic implications

Populations of the multi-trichomous microbial fossil Eoschizothrix composita n.gen. et sp. are preserved in growth position in silicified stratiform stromatolites of the Gaoyuzhuang Formation, Hebei Province, northern China. The microbial fossils consist predominantly of preserved sheaths, although several specimens retain shriveled remains of trichomes within sheaths. Comparisons with modern morphological counterparts, including shape, growth habit and orientation, degradational sequences, and habitat, support the interpretation of the multi-trichomous microfossils as cyanobacteria, which acted as frame-builders of ancient stromatolites. The distribution and orientation of multi-trichomous microfossils within a synsedimentary context reveal their behavioral responses to sedimentation regime. Horizontally spread, interwoven mats formed during periods of sedimentary stasis. During periods of rapid sediment influx, the filaments assumed an upright orientation, possibly to avoid accumulating particles. This is the first record of fossil stromatolite-building multi-trichomous cyanobacterial which underscores early morphological and functional diversification in cyanobacterial evolution.     

SCANNING ELECTRON MICROSCOPY OF ENDOLITHIC ALGAE AND FUNGI USING A MULTIPURPOSE CASTING-EMBEDDING TECHNIQUE

An embedding-casting method was developed to preserve endolithic algal and fungal structures in situ and to make them optically accessible by removal of carbonate matrix. A slow penetration of Epon-812 resin was applied to fill fine boring tunnels and to embed the organisms within the rock. Fixed, dehydrated, embedded organisms were protected from structural damage during the dissolution of the matrix. Remaining Epon-cast reflects boring patterns and surface details. Casts were studied by a scanning electron-microscope and interpreted by an incident light microscope. After being reembedded in Epon-812, the preparations were sectioned, thin-ground and mounted for study by regular light and electron microscopy.