Gudrun Radtke

Experimental studies on microbial bioerosion at Lee Stocking Island, Bahamas and One Tree Island, Great Barrier Reef, Australia: implications for paleoecological reconstructions

Different kinds of experimental calcareous substrates were exposed at Lee Stocking Island (Bahamas) and One Tree Island (Great Barrier Reef, Australia) to study which endolithic bacteria, algae and fungi contribute to bioerosion and what their bioerosion rates are. The sites at Lee Stocking Island were several leeward shallow water and several windward shallow and deep-water positions (from the Acropora palmata reef at 2 m down to 275 m depth). At One Tree Island, the experiments were conducted in patch reefs treated with P and N to study the influence of mineral nutrients on bioerosion. The exposure periods ranged from 1 week to 2 years. The micritic carbonate substrates exposed on Lee Stocking Island contained 6 genera with 15 species of cyanobacteria, green and red algae, and different kinds of microendolithic heterotrophs. The mean values of bioerosion rates measured between 1 to 2 g/m 2 /y at 275 m and 520 g/m 2 /y at one of the leeward sites. The composition of the endolithic community and the bioerosion rates changed over time. At One Tree Island, shell pieces of Tridacna were used as substrate exposed for 5 months to endolith activity. Five genera and 6 species of cyanobacteria, green and red algae and different kinds of heterotrophic microendoliths were found with bioerosion rates of 20–30 g/m 2 /y. There are differences in abundance of taxa between Lee Stocking Island and One Tree Island. The introduction of nutrients had no apparent impact on the microborer community. Controlling factors for the distribution and abundance of microborers are mainly light, but also the kind of substrate and, possibly, the biogeographic position. The results support the paleoecological importance of microendoliths.

The distribution of microborings in molluscan shells from recent reef environments at Lee Stocking Island, Bahamas

SummaryThe distribution of microbial borings in bivalve shells was assessed between five research sites in windward and leeward environments at the Lee Stocking Island, Bahamas. The research sites are on windward coral reefs (sites B at 2 m, Fat 12 m, and C at 30 m), a tidal channel stromatolite reef (site A at 5 m), and a leeward hard ground reef (site D at 3 m). A total of 22 ichnotaxa have been recognized within 100 samples. Each site contained between 14 and 17 ichnotaxa. Although the diversity of microborings, expressed both as number of taxa and number of individuals per taxon, is similar between sites, the following ranking (in descending order) was found: B-F-A-C-D.Rhopalia catenata was found to dominate at site D, whileReticulina elegans dominated at site C, andFasciculus dactylus dominated at site F. The results are in accordance with the known bathymetric distribution of the boring microorganisms.

FACTORS CONTROLLING HOLOCENE REEF GROWTH - AN INTERDISCIPLINARY APPROACH

SummaryThis interim report deals with investigations on key factors controlling reef growth by zoophysiologists, ecologists, paleontologists and geologists. The different levels of emphasis are the coral animal and the reef community. The main study area is the Red Sea which reaches over 20°C latitude up to the northernmost margin of the global coral reef belt. Supplementary results on microborer ecology are provided from the Bahamas.The desert enclosed Red Sea, not influenced by land runoff and only minimally by anthropogenic (urban and touristic) nutrient inputs, is predestined for a study on the principal influence of light on calcification within bathymetrical and latitudinal gradients. Hence, on the level of the zooxanthellate scleractinian animal phototrophic and heterotrophic energy supply and its bearing on calcification are being measured in different coral species—in particular inPorites sp., one of the most important reef builders.The growth of 15 zooxanthellate scleractinians in the Gulf of Aqaba correlates with the annual light cycle. This correlation is observable down to 40 m depth. Other growth promoting factors seem to have less influence on coral extension. The availability of organically enriched sediments in shallow water probably yields nutritional value, in particular for filter feeding species, thus restricting their distribution to those areas. Zooxanthellae, when isolated fromMycedium elephantotus, are different in their dependence on depth in maximum rates of photosynthesis and photosynthetic efficiency (-slope). Increasing concentrations of pigments as a function of depth could be determined. Maximum rates of photosynthesis of zooxanthellae in vivo, collected at corresponding depth, have been 4 times higher. Structural and physiological adaptations improving heterotrophic and phototrophic energy intake are highlighted. Porites sp. was the subject of annual growth studies at locations extending from Aqaba in the North over the northern and southern Egyptian coast and islands, Sanganeb Atoll and Wingate reef offshore Sudan to the Gulf of Tadjoura in the Gulf of Aden (Djibouti). Mean growth rates in the shallow water zone increase with decreasing latitude and are highest at the southernmost studied reefs in the Gulf of Tadjoura. However, the observed latitutdinal growth reduction is restricted to the upper ca. 15 m of the water column. The upper limit of growth potential decreases with depth parallel to the decrease of light availability. Highest growth rates are recorded in shallow depth (10–2.9 mm yr−1). This zone reaches at Aqaba (29°30′N) to a depth of ca. 10 m. At the southern Egyptian reefs (24°30′N) this zone extends to ca. 15 m water depth. This effect is probably a result of the stronger reduction of winter light levels and water temperature in the northern regions. Compared to other oceans the decrease of growth with increasing latitude of Red SeaPorites corals is far less, and growth rates at Aqaba are the highest observed at these latttudes.On the level of the community of reef inhabitants four principal topics are addressed:The first one is the dynamics of the proportions of hermatypic and ahermatypic organisms and open space. The occurrence of stony and soft corals and the sharing of empty space in different reef sections at Aqaba and on Sanganeb Atoll were quantified. Soft corals, mainlySinularia- and xeniid species, occupy decreasing shares with depth. Among theXenia species a bathymetrical zonation pattern was detected.The next issue is the growth impeding role of soft corals and gastropod parasites and predators on scleractinians. Experimental and field observations showed xeniid soft corals to be opportunistic i.e. occupying rapidly open space rather than to attacking and outcompeting stony corals. An increasingly specialized behaviour was detected among corallivorous gastropods of the family Coralliophilidae to exploit their coral hosts. Whereas these snails are more or less sessile and depend for a long time on the surrounding host polyps the mobileDrupella cornus (Thaididae) forms feeding aggregations which denude mainly branching corals on shallow reef parts.Furthermore, the role counteracting reef growth of macro- and microbioeroders is investigated.Diadema setosum is a major destructive agent on reefs at Aqaba (not in the central Red Sea). The grazing sea urchins do not only keep potential colonization area free but also erode carbonate material (e. g. 1468 g/m2/year, 10 m depth). Demographic and bathymetric patterns in the sea urchin population are analyzed including their bearing on bioerosion of the reef. Investigations on microboring organisms in carbonate material have started in the Red Sea; initial results, however, are only available from similar studies near Lee Stocking Island, Bahamas.Three major environments have been identified based on the distribution of the different microborers. These are1)the intertidal environment dominated by boring cyanobacteria.,2)reef sites from 2 to 30 m water depth dominated by a diverse assemblage of boring cyanobacteria and chlorophytes, and3)the deep reef slope from 100 to 300 m dominated by boring green algae and heterotrophs. The boring chlorophyte genusPhaeophila appears rapidly and dominates at sites from 2 to 30 m, but it leaves vacated borings and is replaced byOstreobium quekettii after 1 year. Different substrate types show very different rates of colonization by microborers. The greatest excavation rates (100 g/m2/3 months) occur in fine-grained limestone, while the slowest rates (0.5 g/m2/3 months) occur in calcite crystals. Molluscan shell material shows intermediate rates of excavation. Light conditions appear very important in determining the growth rate and distribution of different microborers between the sites, however, the interaction of light with other factors, such as substrate, time period of exposure, and water quality conditions may be involved.

Microborings and Microbial Endoliths: Geological Implications

SUMMARY: The most important ichnotaxa (and some biotaxa) of microendolithic cyanobacteria, algae and fungi are presented. Their relations to environmental factors such as light, temperature, salinity, inorganic nutrients and their bioerosion potential are discussed. From the Silurian on, they form characteristic ichnocoenoses that are bathymetrically controlled and can be used for the reconstruction of water depths of fossil marine basins. Some ichnotaxa appeared as far back as the Ordovician, and some biotaxa in the Precambrian.

The trace Rhopalia clavigera isp. n. reflects the development of its maker Eugomontia sacculata Kornmann, 1960

Complex boring patterns often reflect the complexity of life cycle of the euendoliths that produce them. They are illustrated here by different stages in the development of the euendolithic ulotrichacean chlorophyte Eugomontia sacculata reconstructed on the basis of its complex trace in the shells of Mya arenariain brackish waters of the Baltic Sea at Gdansk, Poland. The trace consists of different types of boring morphologies as distinctive from one another as many traces specific to separate organisms. Because they occur associated, they may be misinterpreted as separate members of an ichnocoenosis. We propose to describe them as parts of a complex trace instead, because they are based on the genetic program of a single organism, but expressed in different proportions at different stages of its development. A new ichnospecies, Rhopalia clavigeraisp. n., is described to characterize these traces.

Microbial Euendolithic Assemblages and Microborings in Intertidal and Shallow Marine Habitats: Insight in Cyanobacterial Speciation

The term “stromatolite” was coined by Kalkowsky (1908) while describing coastal paleoenvironments of a Lower Triassic (Buntsandstein) epicontinental sea. Stable, finely laminated structures appeared together with massive deposits of shoaling ooid sands, and later consolidated into rocks (“Stromatolith und Oolith”).

Modern Marine Stromatolites of Little Darby Island, Exuma Archipelago, Bahamas: Environmental Setting, Accretion Mechanisms and Role of Euendoliths

The search for modern stromatolites was initiated by geologists in an attempt to understand the processes that governed the Earth for about five sixths of the history of Life. Entire land- and seascapes dominated by stromatolites are rare in the modern world of plants and animals.

A New Dichotomous Microboring: Abeliella bellafurca isp. nov., Distribution, Variability and Biological Origin

Cluster-forming boring traces in carbonate substrates, characterized by regular dichotomous ramifications, are described as a new ichnospecies of Abeliella Mägdefrau 1937: A. bellafurca isp. nov. The new trace differs from A. riccioides Mägdefrau by a three- rather than two-dimensional display of tunnels and branchings and by penetrating carbonatic rather than phosphatic substrates. These boring traces occur in bivalve shells from the Oligocene to the Recent in shallow, illuminated marine settings. Two distinct morphotypes of A. bellafurca were observed, both originating from a single point of entry: one produces prostrate boring system in early developmental stages followed by deeper penetration, and the other, which forms only deep penetrating three-dimensional boring systems. Tunnels with dichotomous ramifications in modern marine environments are produced by the cyanobacterium Hyella stella Al-Thukair et Golubic. The organism penetrates the shells and shell fragments, whereby the apical cell divides alternately parallel and perpendicular to the direction of penetration. Bifurcation follows each longitudinal division of the apical cell. This type of cell division and bifurcations was described in Neoproterozoic endolithic cyanobacterium Eohyella dichotoma Green, Knoll et Swett. Both Proterozoic and modern organisms penetrated carbonatic ooid sand grains in shallow marine settings. Other species of Hyella and Solentia are known to form traces described as Fascichnus (Radtke). Dichotomously branched microborings in phosphatic substrates described as Abeliella riccioides Mägdefrau were attributed to boring fungi. Distinctions between borings produced by phototrophs vs. heterotrophs are discussed because they have bearing on paleobathymetric interpretations.

The Complex Fungal Microboring Trace Saccomorpha stereodiktyon isp. nov. Reveals Growth Strategy of its Maker

A complex microboring trace of fungal affinity is described in shells as a new ichnotaxon Saccomorpha stereodiktyon isp. nov. and compared with the earlier established ichnotaxon Saccomorpha terminalis Radtke, 1991. The new trace is characterized by a three-dimensional network of tunnels composed of a bifurcate horizontal (parallel to substrate surface) network with an upright (perpendicular to surface) system of tunnels and by the formation of cylindrical to multilobate terminal sporangial swellings. The new trace shares with Saccomorpha terminalis Radtke, 1991 the terminal position of sporangial swellings but differs from this ichnotaxon by its complexity in spatial arrangement, segmented construction, and ramification of tunnels. The horizontal parts of the network in the new taxon adhere to the substrate surface and regularly produce thinner tunnels that explore the interior of the substrate, allowing the producer to participate in digestion of organic lamellae incorporated in the shell. Microborings similar to the new trace fossil have been observed in modern bivalve shells of the Atlantic Ocean, North Sea, Adriatic Sea and Red Sea at depths ranging from the intertidal down to 1,550 m. The fossil record of the trace reaches back to the Jurassic and the type material stems from a Lower Oligocene oyster shell. The study shows that complex microboring traces reflect both behaviour and developmental strategy of their makers.

Conchocelichnus seilacheri igen. et isp. nov., a Complex Microboring Trace of Bangialean Rhodophytes

ABSTRACT Traces of microboring microbial endoliths (euendoliths) conform closely to the outlines of their makers as they are made. This habit evolved among prokaryotic cyanobacteria and eukaryotic rhodophytes, chlorophytes, and fungi. Among eukaryotic microborers, the endolithic mode of life is often limited to a phase in the course of their development, which alternates with an epilithic leafy phase. These endolithic phases are sometimes independently described as separate genera and species. This was the case with Conchocelis rosea Batters, 1891, later identified as the endolithic phase of Porphyra and Bangia that helped in identification of fossil bangialean rhodophytes of Silurian age: Palaeoconchocelis starmachii Campbell, Kazmierczak et Golubic, 1979. However, the traces of Conchocelis phases, although conspicuous and used to characterize the resident body fossil, were never formally described. We present here the formal ichnotaxonomic description of the complex and variable Conchocelis trace as Conchocelichnus seilacheri igen. et isp. nov., based primarily on Oligocene (Tertiary) borings in molluscan shells of Mainz Basin, Germany, while consulting similar fossil borings of the Silurian and Jurassic findings and compareing them with modern occurrences in nature and culture. With the new trace name Conchocelichnus seilacheri igen. et isp. nov., we honor the work of the renowned paleontologist and ichnologist, Prof. Dr. Dolf Seilacher.