Exposed limestones of the Northern Province of the Great Barrier Reef

The exposed reef limestones occur principally on the inner-shelf reefs and can be separated into two groups — organically cemented (reef-rock) and inorganically cemented (beach-rock, rampart-rock, boulder-rock and phosphate-rock). No examples were found of exposed sub tidal reef framework; the reef-rock exposed is entirely of intertidal origin resulting from incipient encrustration by intertidal corals and coralline algae. Most of the beach-rock, rampart-rock and boulder-rock exposures are intertidal and many show vadose cement fabrics. The cements, chiefly aragonite needles in beach-rock and cryptocrystalline high Mg calcite in ram part and boulderrocks, are thought to be derived from seawater, though the environments of precipitation on windward sides of reefs where rampart-rocks form are quite different from those on the leeward sides where beach-rocks form. Phosphate-rock develops supratidally on the surface of some sand cays. Solutions derived from guano precipitate thin layers of phosphatic cement which bring about the centripetal replacement of carbonate grains.

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Coralline algae indicate Pleistocene evolution from deep, open platform to outer barrier reef environments in the northern Great Barrier Reef margin

Coral Reefs ◽

10.1007/s00338-004-0414-x ◽

2004 ◽

Cited By ~ 4

Author(s):

Juan C. Braga ◽

Julio Aguirre

Keyword(s):

Great Barrier Reef ◽

Coralline Algae ◽

Barrier Reef ◽

Open Platform ◽

Reef Environments

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Epilithic Cyanobacterial Communities of a Marine Tropical Beach Rock (Heron Island, Great Barrier Reef): Diversity and Diazotrophy

Applied and Environmental Microbiology ◽

10.1128/aem.02067-06 ◽

2007 ◽

Vol 73 (11) ◽

pp. 3656-3668 ◽

Cited By ~ 50

Author(s):

Beatriz Díez ◽

Karolina Bauer ◽

Birgitta Bergman

Keyword(s):

16S Rrna ◽

16S Rrna Gene ◽

Great Barrier Reef ◽

Rrna Gene ◽

Barrier Reef ◽

Gene Sequences ◽

16S Rrna Gene Sequences ◽

Nitrogen Fixers ◽

Gene Optimization ◽

Beach Rock

ABSTRACT 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.

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