Cultured Stylophora pistillata in Phosphate Loading Berth (PLB) sites in the Jordanian Gulf of Aqaba was investigated using Back-Scatter Scanning Electron (BSE) microscopy and Energy-Disperse X-ray (EDX) to determine trace elements and calcium concentrations’ distribution within the micrometer scale. Ca, C, and O in addition to six trace elements (Mg, Sr, S, Cl, Ni, and Na) were mapped within the sample cross section. Samples from the PLB were as expected poorly mineralized as previously reported and showed mineralization heterogeneity in the micrometer length scale in the BSE image. In four selected positions within the cross sections, Ca concentration found to range between 0.44 - 1.80 wt% in low-mineralized regions and between 3.99 - 4.66 wt% in mineralized regions. The average calcium concentrations were in accordance to previous study; about 10% of the Ca existed in the same coral species from other coastal sites in the Gulf of Aqaba. This could be attributed to the role of phosphate in inhibition of calcification and enhancement of photosynthesis. Amounts of trace elements in lower Ca concentration positions within the cross section were relatively very low except for chlorine, whereas positions with higher Ca contained amounts of Na and Sr. This study reports remarkable heterogeneity in mineral distribution within the microstructure of the coral Stylophora pistillata under phosphate pollution stress.
Terrestrial runoff into seawater leads to increasing loads of nutrients, sediments and pollutants discharged from the land like using and handling of fertilizers that contain phosphate [
Seawater contains trace elements in the form of K+, Na+, Mg2+, Ca2+, Cl− and
Conventional loading systems of phosphates produced as a constituent of fertilizer result in very high levels of dust pollution in the surrounding coastal area, diffused and transported along the sea by winds [
Phosphate pollution has also been implicated as a contributor to the decline of reef ecosystems in the Gulf of Aqaba [
The Stylophora pistillata is among the important common coral species in the Gulf of Aqaba as well as in the Red Sea and known to be the fastest of the scleractinian corals. It is also very suitable for research because it can also be sampled without causing major damage to the colony [
In the present study, we investigate the heterogeneity of mineral content and trace elements in Stylophora pistillata corals in the Phosphate Loading Berth (PLB) site using Back-Scatter electron images and EDX techniques for elemental analysis. We also show the heterogeneous structure of the phases with different Ca concentrations in the micrometer scale within the skeleton.
Small fragments of the model branching coral Stylophora pistillata were cultured in a mid-water floating nursery along the Jordanian coast of Gulf of Aqaba [
Coral samples were collected from the nurseries in the Phosphate Loading Berth (PLB) site (
Coral samples were cut perpendicular to the skeletal growth using low speed diamond saw (BUEHLER Isomat, Germany) and cross section surfaces were further polished using ethyleneglycol lubricant to provide a flat surface that was coated with Au for the SEM imaging.
Field-Emission Scanning Electron Microscope (JEOL JSM-7500F) with an Oxford Instruments detector was used for performing the EDX analysis to measure the elemental content in the coral samples and their corresponding emission spectra, as well as acquiring Back-Scatter Electron (VSE) microscopy images for the cross section of the sample. EDX analysis usually involves the generation of an X-ray spectrum from the entire scan area of the SEM, which shows the counts of X-rays received and processed by the detector and the energy level of those counts. The EDX instrument software associates the energy level of the X-rays with the elements found in the scan area.
Back-Scattered Scanning Electron Microscopy (BS-SEM) images of coral samples collected from the Phosphate Loading Berth (PLB) in the Jordanian Gulf of Aqaba (
In
Four positions in the cross-section in the BSE image (
In all locations, Ca concentrations were very low as expected and recorded in previous study on such coral in the same site [
1 | 2 | 3 | 4 | |
---|---|---|---|---|
C | 74.72 ± 0.11 | 57.86 ± 0.12 | 41.85 ± 0.14 | 37.93 ± 0.14 |
O | 24.67 ± 0.11 | 40.01 ± 0.12 | 52.85 ± 0.14 | 57.40 ± 0.14 |
Ca | 0.44 ± 0.01 | 1.80 ± 0.02 | 4.66 ± 0.03 | 3.99 ± 0.03 |
Na | 0.03 ± 0.01 | 0.14 ± 0.02 | 0.31 ± 0.02 | 0.30 ± 0.02 |
Mg | --- | 0.04 ± 0.01 | 0.05 ± 0.01 | 0.04 ± 0.02 |
S | --- | --- | --- | --- |
Cl | 0.14 ± 0.01 | 0.06 ± 0.01 | 0.05 ± 0.01 | 0.03 ± 0.01 |
Ni | --- | 0.03 ± 0.01 | --- | 0.02 ± 0.01 |
Sr | --- | 0.06 ± 0.02 | 0.24 ± 0.03 | 0.29 ± 0.03 |
averages over the range of 100 m. In our study, the focus area for the EDX analysis was in the range of 1 m in order to map the heterogeneity in trace element distribution within the skeleton microstructure.
In
elements needs to be further investigated based on expanded experimental work and literature survey. In the less-mineral region, position (2) has quite higher Ca than (1) because the last is located close to the less- and higher mineralized interface (
The amount of trace elements (N, Mg, S, Cl, Ni, and Sr) in the four scanned positions in the coral sample cross section (
Except for Chlorine (Cl), trace elements in the position 1 of the lowest calcium content were very low (<0.1 wt%) so their amounts can’t be considered due to possible uncertainty in their values. Trace elements concentrations can be reliably considered only in regions of high Ca concentrations.
The relatively high Cl concentration in position (1) which is 3 times more than it in other positions cannot be explained. Chlorine (Cl) exist in coral skeletons (but not in the aragonitic structure), as potassium chloride or as sodium chloride phases and is considered to be toxic for corals [
In
0.015. However, the method used for this study is not optimal for calculating trace- element/Ca (mmole/mole) ratios.
Although Na is not a divalent cation (monovalent ion), i.e. it does not satisfy the electrostatic valence requirements of the Ca position in the aragonite lattice, but being the second most abundant ion in seawater [
Strontium (Sr) ion is a divalent cation as magnesium (Mg), which can be secreted within the coral skeleton through direct replacement of calcium (or carbonate) in coral aragonite structure [
Mg is generally found in much lower concentrations in aragonitic skeletons due to its smaller ionic radius than Ca [
Due to its inhibition of calcification, high levels of phosphate are inversely associated with decreasing concentrations of strontium in the corals [
Sulphur (S) was not detected in our sample. It is the forth abundant ion in seawater (as sulfate
Nickel (Ni) concentrations in the four scanned positions were almost not detected. It is weakly incorporated into the aragonite skeleton relative to calcium concentrations due to structural incompatibility [
The low calcium in corals that are stressed by phosphate dust pollution, is due to the enrichment of phosphate as nutrient in seawater which cause inhibition of calcification in corals and coralline algae as well [
The environmental impacts of the deposited phosphate dust on marine ecosystem include not only siltation of the coral reef and depression of coral growth (Hawkins et al., 1991) but also increasing suspended solids and water turbidity, reduction of water clarity and light penetration [
Distribution of calcium in different very low concentrations was found in skeleton of corals that are exposed to phosphate dust and heterogeneous in the micrometer scale. This can be attributed to the well-known inhibition process of calcification by increase of phosphate concentrations at the time that the corals were formed. Dominating trace elements were only Na and Sr, which is typical for coral skeleton constituent. Chlorine was found to be higher in the lowest-calcium concentrations than other regions. Further elemental analysis and microstructure studies could be conducted on other coral species and several skeletal organisms that live in the Phosphate Terminal coastal site in order to find out their response to phosphate pollution. Moreover, trace element/Ca ratios should be considered in future similar studies using suitable investigation techniques for complete analysis.
The authors are indebted to Prof. Fuad Al-Horani for providing the coral samples used for this study and to Prof. Peter Fratzl for giving the chance to use the SEM laboratory at the Max-Planck Institute for Colloids and Interfaces (MPIKG). Many thanks to Susann Weichold and Heike Runge of the SEM lab Germany for their support and help during the measurements, also we are thankful to the divers Ali Al-Njadat and Omar Al-Momany for collecting the coral samples used in this study.
Al-Sawalmih, A. and Megdadi, J. (2016) Heterogeneous Microstructure and Distribution of Trace Elements in Coral Stylophora pistillata Nursed in the Phosphate Loading Berth Site in the Gulf of Aqaba. Natural Science, 8, 541-552. http://dx.doi.org/10.4236/ns.2016.812053