Occurrence, distribution, and localisation of metals in cnidarians

Abstract

The Cnidaria are simple organisms that have remarkable physiological features susceptible to microscopic investigation. As a group they produce cnidae, the most complex intracellular organelles known, form symbioses with a range of unicellular algae, contain mucocytes that account for a very substantial fraction of their body mass, and form complex skeletal structures of calcium carbonate. This review summarises contributions dealing with the distribution and localisation of metals of physiological and pathological importance within soft tissues and skeletons. Whilst there have been detailed studies of microscale metal distribution, using X-ray microanalysis, in the stinging organelles or cnidocysts and in mucocytes, other cells such as symbiotic algae and the epithelial cells have received little attention. In the skeleton-producing scleractinian corals X-ray microanalysis has provided tenuous, but persistent, evidence of Ca associated with intracellular vesicles or granules in the skeletogenic epithelium, even though the investigations were technically limited. These observations may be germane to the intriguing and intransigent problem of the mechanism of coral calcification. Metal localisation in coral skeleton at the resolution of annual growth rings has been concerned with the validity of Sr/Ca and Mg/Ca ratios as thermometers for paleoclimatic studies. It is not clear whether these ratios are influenced primarily by environmental or biological parameters. Microscale analyses by X-ray microanalysis and ion microprobe indicate a much greater variability of metal ratios which suggests biological control of metal deposition. New data are provided on the elemental composition, measured by X-ray microanalysis, of cells and cell compartments in the coral Galaxea fascicularis and zooxanthellae in the anemone Aiptasia sp. New information is also presented on changing Ca/Sr ratios at the skeletal interface in Galaxea fascicularis.

Monitoring of heavy metal partitioning in reef corals of Lakshadweep Archipelago, Indian Ocean

“…The tissue enrichment of Fe in all these species may be attributed to the biological role of Fe in various enzymatic functions. Iron is an essential element required by certain enzymes and proteins that carry electron during photosynthesis and (Marshall, 2002). High concentrations of Fe observed in the skeletal phase of P. andrewsi, compared to relatively less concentrations within its tissue, may be pointed towards the usefulness of this species as an indicator organism to monitor anthropogenic loads of iron in coral ecosystems.…”

Microstructure of common reef-building coral genera Acropora, Pocillopora, Goniastrea and Porites: constraints on spatial resolution in geochemical sampling

“…Corrège (2006) calculated similar results in an up-to-date summary of published Sr/Ca vs SST calibrations for Porites. Some of the 'noise' in analyses may reflect vital effects wherein trace-element distributions correlate directly to microstructural patterns (e.g., Cohen et al 2001;Marshall 2002;Rollion-Bard et al 2003;Allison and Finch 2004;Meibom et al 2004;2006), but other noise could be produced by sampling non-sequential or inappropriately time-averaged data. Confidence in paleoclimate reconstruction requires a better understanding of all factors that may contribute to measured chemical signatures and to their spatial heterogeneity, and such understanding is particularly important as technology allows increasingly finer sampling resolutions.…”

Earliest diagenesis in scleractinian coral skeletons: implications for palaeoclimate-sensitive geochemical archives

“…Individual aragonite 'crystals' are precipitated in a 'hydro-organic gel' Cuif and Dauphin 2005) and are arranged into macroscopic skeletal elements, such as walls, septa, and dissepiments as controlled by the distribution of the ectoderm and organic molecules at various scales of interaction down to nanometer-scale organization within individual 'crystals' Cuif and Dauphin 2005;Stolarski and Mazur 2005;Przenioslo et al 2008). Trace elements within coral skeletons that are of interest as palaeoenvironmental proxies are thought to be derived primarily from ambient seawater, but with acknowledged vital eVects of the polyp and/or symbiotic algae (e.g., Marshall 2002). Regardless, the experimental correlation of stable isotopes and elemental ratios such as Sr/Ca, Mg/Ca, and U/Ca between coral skeletons and the temperature of ambient seawater forms the basis for sea surface temperature (SST) proxies (e.g., Beck et al 1992;de Villiers et al 1995;Mitsuguchi et al 1996;Grottoli and Eakin 2007).…”

“…Although evidence increasingly suggests that trace-element distributions in coral skeletons are heterogeneous at very Wne scales, possibly reXecting poorly understood vital eVects in part (Cohen et al 2002;Marshall 2002;Meibom et al 2003Meibom et al , 2004Meibom et al , 2006Meibom et al , 2008Allison et al 2005;Shirai et al 2005;Sinclair 2005; Stolarski and Mazur 2005;Gaetani and Cohen 2006;Sinclair and Risk 2006;, coral skeletons can provide useful palaeoenvironmental proxies provided: (1) original trace-element and stable-isotope inventories have not been altered by diagenesis; and (2) sample sizes and spatial resolution are adequately understood (e.g., . Hence, coral skeletons are routinely vetted prior to analysis to check for diagenetic alteration, such as recrystallization of skeletal aragonite to calcite (e.g., Enmar et al 2000;McGregor and Gagan 2003;Quinn and Taylor 2006), on the general assumption that coral skeleton that has not been exposed to freshwater (meteoric) diagenesis should retain reliable marine trace-element inventories.…”