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Fig. 4.2.
The Lizard Island model
(Reitner 1993, Reitner et al. 1995). Lizard Island is located in the northern part of the Great
Barrier Reef. The granitic island is fringed by coral reefs. The microbialites growing in cryptic habitats of dark reef caves
offer a model for describing the biological controls on the formation of automicrites. The microbialites are products of
matrix-mediated calcification via specific Ca-binding acid organic macromolecules and baffled detritus. The model shown in
the figure underlines the interaction between biofilm activity, sponge development, and the development of microbialites.
Microbes and sponges played a central role in the formation of ancient automicrites from the beginning of the Cambrian
(Pl. 8/7, Pl. 82). The interaction between sponges and microbial biofilms is not restricted to cryptic niches in tropical reefs,
but also occurs in cool-water and polar sponge mounds.
A
: The surface of the microbialites is covered by microbial biofilms (1) which control the input of Dissolved Organic
Matter and of Dissolved Organic Carbon. Biofilms are complex structures consisting of variably diverse microbial single- or
multilayered sheets (see part B). The biofilms control the distribution of sponges (2) and, via biochemical signals, the settlement
of sponge larvae (3). The sponge profits from the supply of microbial nutrients and DOM from the water column (4) and
DOC from the biofilm. Metabolic waste (5) is recycled. The products of symbiotic heterotrophic bacteria living within the
sponges are important sources of nutrients; they also control metabolic processes and enhance calcification. Microbial biofilms
are responsible for the degradation of organic matter into Ca-binding macromolecules, and therefore for the production of
free calcifying mucilages. Calcified microbes and partly mineralized fungal hyphae have been observed in some biofilms
(6). Fe/Mn bacteria contribute to the formation of Fe/Mn crusts (7) which are also common in many ancient automicrites.
The sediment-trapping property of biofilms is important for the growth of microbialites consisting of thrombolitic crusts
forming irregular pillar-like structures at the top of the microbialite (Pl. 8/1). These crusts, dome-shaped micritic and irregular
peloidal textures are major constituents of ancient mud mounds (Pl. 8/2, 6). Calcium crystal growth requires increase in
alkalinity. In the Lizard Island reef caves increase in alkalinity is triggered by the weathering of granitic silicates which form
the island.
Microbialites grow at the surface (8) or in erosional pockets (9) forming 'accretionary organomicrites'. 'Container
organomicrites' are represented by the carbonates precipitated within the decaying sponge. The vertical sequence (10) starts
with a coralgal facies followed by a coralline algal crust and the microbialite facies. This succession corresponds to the light-
dependent horizontal succession seen in the caves.
B
: Biofilm model after Wilderer and Charaklis (1989) adapt to microbialite formation. The biofilms are about 100
m
thick. They act as a reactor to take up dissolved organic carbon and organic particles via EPS channel systems. The reactor
filters and produces DOM and DOC, e.g. as waste products, and release the organic matter back to the seawater or into the
open system of microcavities and irregular pocket structures of the upper microbialite. Some of the products are acidic
macromolecules which enhance mineralization and the formation of automicrite.
DOC - Dissolved Organic Carbon, DOM - Dissolved Organic Matter, EPS - Extracellular Polymeric Substances, IPS -
Intracellular Polymeric Substances. Modified from Reitner (1993).
formation has been discussed for some time, espe-
cially in regard to micrites formed in peritidal 'algal
mats'.
open water column. The macromolecules become en-
riched within immature microbialites or at the inter-
space water/mature sediments. They are produced and/
or fixed by organisms (e.g. in biofilms, Fig. 4.2B) or
trapped in microcavities and pockets. Differences in
the composition of these compounds may result in dif-
ferent carbonate fabrics: Enrichment in acidic amino
acids (Asp + Glu) presumably is linked to the forma-
tion of stromatolite and thrombolite fabrics; protein-
rich substances within confined spaces (e.g. micro-
cavities) result in the formation of peloidal pockets,
peloidal coatings and peloidal stromatolites; and the
decay of soft sponge tissue, enriched in bacteria, leads
to micropeloidal structures mirroring parts of the former
sponge bodies.
The characteristic criteria of automicrites are bio-
laminated structures, clotted peloidal microfabrics or
extremely fine-grained cryptocrystalline textures. A
common automicrite microfacies of automicrites
formed in slope settings are boundstones composed of
small peloids that are associated with cement layers
The formation of organomicrites is controlled by or-
ganic macromolecules (Fig. 4.2). The organic matter is
characterized by a surplus of specific compounds de-
rived from microbes, free organic matter, and decay-
ing organisms. Evidence of Ca-binding Acidic Organic
Macromolecules (AOM) have been found in modern
and ancient automicrites by chromatography and fluo-
rescence microscopy (Neuweiler and Reitner 1995,
Russo et al. 1997). Fluorescence microcopy is of par-
ticular value in the determination of residual organic
matter (Dravies and Yurewicz 1985, Machel et al. 1991).
Two sources of Acidic Organic Macromolecules can
be distinguished: (1) Internal production of AOM by
eucaryotic and procaryotic organisms via metabolic
enzymatic and decaying processes, (2) External origin
of AOM as a part of dissolved organic matter in the
