R. Pamela Reid

Microscale observations of sulfate reduction: Correlation of microbial activity with lithified micritic laminae in modern marine stromatolites

We report for the first time micrometer-scale correlation of geologic and microbial processes in modern marine stromatolites. Precipitation of micritic laminae in these stromatolites was studied by comparing microstructure, as observed in petrographic thin sections, with microbial sulfatereduction activity. Two-dimensional mapping of sulfate-reduction rates was implemented by incubating a vertical section of a stromatolite face on silver foil coated with 35 SO 4 2– . Our results show that sulfate-reduction activity is high in zones of CaCO 3 precipitation and indicate that microbial activity produces lithified micritic laminae near the surface of the stromatolites. Similarities with micritic laminae in ancient stromatolites suggest that sulfate reduction may also have been an important mechanism of carbonate precipitation in these fossilized structures.

Biodiversity and biogeography of phages in modern stromatolites and thrombolites

Viruses, and more particularly phages (viruses that infect bacteria), represent one of the most abundant living entities in aquatic and terrestrial environments. The biogeography of phages has only recently been investigated and so far reveals a cosmopolitan distribution of phage genetic material (or genotypes). Here we address this cosmopolitan distribution through the analysis of phage communities in modern microbialites, the living representatives of one of the most ancient life forms on Earth. On the basis of a comparative metagenomic analysis of viral communities associated with marine (Highborne Cay, Bahamas) and freshwater (Pozas Azules II and Rio Mesquites, Mexico) microbialites, we show that some phage genotypes are geographically restricted. The high percentage of unknown sequences recovered from the three metagenomes (>97%), the low percentage similarities with sequences from other environmental viral (n = 42) and microbial (n = 36) metagenomes, and the absence of viral genotypes shared among microbialites indicate that viruses are genetically unique in these environments. Identifiable sequences in the Highborne Cay metagenome were dominated by single-stranded DNA microphages that were not detected in any other samples examined, including sea water, fresh water, sediment, terrestrial, extreme, metazoan-associated and marine microbial mats. Finally, a marine signature was present in the phage community of the Pozas Azules II microbialites, even though this environment has not been in contact with the ocean for tens of millions of years. Taken together, these results prove that viruses in modern microbialites display biogeographical variability and suggest that they may be derived from an ancient community.

Modern marine stromatolites in the Exuma Cays, Bahamas: Uncommonly common

SummaryModern stromatolites in open marine environments, unknown until recently, are common throughout the Exuma Cays, Bahamas. They occur in three distinct settings: subtidal tidal passes, subtidal sandy embayments and intertidal beaches. These stromatolites have a relief of up to 2.5 m and occur in water depths ranging from intertidal to 10 m. Surfaces near the sediment-water interface are typically colonized by cyanobacterial mats, whereas high relief surfaces are commonly colonized by algal turf and other macroalgae such asBatophora, Acetabularia, andSargassum. The internal structure of the stromatolites is characterized by millimeter-scale lamination defined by differential lithification of agglutinated sediment. In thin section, the lithified laminae appear as micritic horizons with distinct microstructures: they consist of thin micritic crusts (20–40 μm thick) overlying layers of micritized sediment grains (200–1000 μm thick); the micritized grains are cemented at point-contacts and are trucated along a surface of intense microboring. The Exuma stromatolites are built by cyanobacterial-dominated communities. These laminated prokaryotic structures grade to unlayered thrombolites built by eukaryotic algae. The variety of sites, settings and shapes of stromatolites in the Exuma Cays present excellent opportunities for future studies of stromatolite morphogenesis.

Autoinducers extracted from microbial mats reveal a surprising diversity of N-acylhomoserine lactones (AHLs) and abundance changes that may relate to diel pH.

Microbial mats are highly structured and diverse communities, and one of the earliest-known life assemblages. Mat bacteria interact within an environment marked by strong geochemical gradients and fluctuations. We examined natural mat systems for the presence of autoinducers involved in quorum sensing, a form of cell-cell communication. Our results revealed that a diverse array of N-acylhomoserine lactones (AHLs) including C(4)- to C(14)-AHLs, were identified from mat extracts using mass spectrometry (MS), with further confirmation by MS/MS-collision-induced dissociation (CID), and additions of external standards. Microelectrode measurements showed that mats exhibited diel pH fluctuations, ranging from alkaline (pH 9.4) during daytime (net photosynthesis) to acidic (pH 6.8) during darkness (net respiration/fermentation). Under laboratory conditions, AHLs having shorter acyl-chains were degraded within the time frame that daily alkaline pH (> 8.2) conditions exist in mats. Intensive sampling of mats after full day- or night-time incubations revealed that accumulations of extractable shorter-chain AHLs (e.g. C(8)- and C(10)-AHLs) were significantly (P < 0.001) diminished during daytime. Our study offers evidence that stabilities of AHLs under natural conditions may be influenced by the proximal extracellular environment. We further propose that the ancient periodicity of photosynthesis/respiration in mats may potentially drive a mechanism for diel differences in activities of certain autoinducers, and hence bacterial activities mediated through quorum sensing.

Molecular and morphological characterization of cyanobacterial diversity in the stromatolites of Highborne Cay, Bahamas

Stromatolites are sedimentary deposits that are the direct result of interactions between microbes and their surrounding environment. Once dominant on ancient Earth, actively forming stromatolites now occur in just a few remote locations around the globe, such as the island of Highborne Cay, Bahamas. Although the stromatolites of Highborne Cay contain a wide range of metabolically diverse organisms, photosynthetic cyanobacteria are the driving force for stromatolite development. In this study, we complement previous morphological data by examining the cyanobacterial phylogenetic and physiological diversity of Highborne Cay stromatolites. Molecular analysis of both clone and culture libraries identified 33 distinct phylotypes within the stromatolites. Culture libraries exhibited several morphologically similar but genetically distinct ecotypes, which may contribute to ecosystem stability within the stromatolites. Several of the cultured isolates exhibited both a positive phototactic response and light-dependent extracellular polymeric secretions production, both of which are critical phenotypes for stromatolite accretion and development. The results of this study reveal that the genetic diversity of the cyanobacterial populations within the Highborne Cay stromatolites is far greater than previous estimates, indicating that the mechanisms of stromatolite formation and accretion may be more complex than had been previously assumed.

Isotopic fingerprints of microbial respiration in aragonite from Bahamian stromatolites

Authigenic aragonite preserves a carbon isotopic record of heterotrophic microbial influences on dissolved inorganic carbon (DIC) in microenvironments within shallow subtidal stromatolites from Highborne Cay, Bahamas. A greater amount of aragonite precipitates when and where respiration, rather than photosynthesis, influences local DIC, which is consistent with sulfate reduction promoting carbonate precipitation and calcium release during decay of exopolymeric substances. Thus, heterotrophs play a more direct role than phototrophs in stromatolite lithification. Cyanobacteria are spatially associated with aragonite containing heterotrophic isotopic signatures. Hence, the absence of an autotrophic isotopic signature in the rock record does not imply the absence of photosynthetic organisms.

Microboring Versus Recrystallization: Further Insight into the Micritization Process

SEM observations of lightly etched thin sections of Bahamian sediments reveal an unusual process of micritization that involves carbonate precipitation in microborings concurrent with endolithic activity. A coccoid cyanobacterium, tentatively identified as Solentia sp., bores tunnels, which initially penetrate just beneath grain surfaces and eventually extend throughout the entire grain. These tunnels are filled by radial fibrous aragonite, which is precipitated as the microorganism advances. Extensive multicyclic repetitions of this process result in obliteration of original grain textures with almost complete preservation of grain margins and rare empty bore holes. The rapidly filled tunnels cannot be detected by resin cast embedding techniques that are commonly used to study microboring. This type of multicyclic boring and concurrent filling of bore holes forms micritized grains that can be difficult or impossible to distinguish from micritized grains formed by recrystallization.

Intertidal stromatolites in a fringing Holocene reef complex, Bahamas

Intertidal stromatolites are major components of a fringing Holocene reef complex at Stocking Island, Exuma Cays, Bahamas. Trapping and binding of fine-grained carbonate sand by microorganisms, primarily filamentous cyanobacteria and diatoms, and differential lithification of sediment have produced layered structures with up to 0.5 m of relief. Formation of these stromatolites in an environment of normal marine salinity is attributed to reduced competition from crustose red algae, availability of a stable substrate, and rapid cementation. Destruction of lamination in Stocking Island stromatolites by molluscs, sponges, and worms suggests that boring invertebrates play a key role in stromatolite preservation.

Foraminiferal-algal nodules from the eastern Caribbean; growth history and implications on the value of nodules as paleoenvironmental indicators

Foraminiferal-algal nodules, 2 to 15 cm in size, are actively growing in depths of 30 to 60 m on insular shelves in the eastern Caribbean. These nodules are characterized by two-part internal structures: an outer envelope, 1 to 3 cm thick, of well-preserved, concentrically laminated crusts of Gypsina andlor coralline algae, and an inner core of coral, or, more commonly, an altered nodule that has been extensively bored, infilled, and lithified. The outer envelopes are less than 600 years old; the cores are variable in age-some are modern, others may be relict. The internal structure and ages of shelf nodules in the eastern Caribbean are similar to those of deep-water nodules in a number of other locations. Abrupt transitions from well-preserved outer envelopes to altered cores in all of these nodules suggest hiatuses in growth, possibly related to burial; the transitions are not as previously interpreted, related to a change from shallow to deep water associated with the Holocene transgression. Re-evaluation of aspects of nodules that have been proposed as paleoecologic indicators (shape, coralline-algal growth form, biotic assemblages, and diagenetic alteration) shows that, in the eastern Caribbean, these parameters have little predictable relationship with specific environmental conditions.

Microbial diversity in modern marine stromatolites, Highborne Cay, Bahamas

Living marine stromatolites at Highborne Cay, Bahamas, are formed by microbial mat communities that facilitate precipitation of calcium carbonate and bind and trap small carbonate sand grains. This process results in a laminated structure similar to the layering observed in ancient stromatolites. In the modern marine system at Highborne Cay, lamination, lithification and stromatolite formation are associated with cycling between three types of microbial communities at the stromatolite surface (Types 1, 2 and 3, which range from a leathery microbial mat to microbially fused sediment). Examination of 923 universal small-subunit rRNA gene sequences from these communities reveals that taxonomic richness increases during transition from Type 1 to Type 3 communities, supporting a previous model that proposed that the three communities represent different stages of mat development. The phylogenetic composition also changes significantly between these community types and these community changes occur in concert with variation in biogeochemical rates. The dominant bacterial groups detected in the stromatolites include Alphaproteobacteria, Planctomycetes, Cyanobacteria and Bacteroidetes. In addition, the stromatolite communities were found to contain novel cyanobacteria that may be uniquely associated with modern marine stromatolites. The implications of these findings are discussed in the context of current models for stromatolite formation.