Author(s): Rothe, Nina
Date: 2015
Persistent ID: http://hdl.handle.net/10400.1/7892
Origin: Sapientia - Universidade do Algarve
Subject(s): Biologia marinha; Corais; Oceanos; Acidificação; Sistema esqueleto-muscular; Domínio/Área Científica::Ciências Naturais::Ciências Biológicas
Dissertação de mestrado, Biologia Marinha, Faculdade de Ciências e Tecnologia, Universidade do Algarve, 2015
Ocean acidification terms the process of increased CO2 uptake by surface ocean waters and a subsequent decrease in pH and carbonate ion concentration. The average pH has decreased by 0.1 since preindustrial times and is expected to drop by a further 0.3 - 0.4 until the end of the century if current anthropogenic CO2 emissions persist. This in turn will further decrease the degree of aragonite saturation in the seawater and affect the thermodynamic equilibrium of carbonate minerals (aragonite and calcite). Coral biomineralization is assumed to be strongly sensitive to the degree of saturation in the surrounding seawater, however marine carbonates are affected differently by ocean acidification and responses vary among different studies. Coral skeletons are composed of both inorganic components, usually aragonite, and organic compounds. The exact role of these organic compounds is still unknown but they are assumed to be involved in the control of carbonate precipitation. Some studies suggest, that the amount of organic compounds increases when corals grow under high CO2 and low pH conditions. In this study, the long-term effects on corals growing under high CO2 conditions at natural CO2 seeps in Papua New Guinea and under ambient CO2 conditions (control sites) were investigated as a case study and compared in terms of the amount of organic compounds in their skeletons. Three coral species were sampled and analyzed: Acropora millepora, Pocillopora damicornis and Seriatopora hystrix. Two analytical methods were investigated for their suitability to obtain information on the amount and type of organic compounds inside coral skeletons and whether any differences exist in samples collected from sites with ambient CO2 compared to increased CO2 conditions. Thermogravimetric Analysis (TGA) was used to record weight losses of powdered coral skeleton to determine the amount of hydrated organic compounds lost during gradual heating. Confocal Raman microscopy (CRM) mapping was used to investigate the microstructural arrangement and organic matrix distribution within the coral skeleton. Both analytical techniques were optimized in this study as no standardized technique was available. Here we show that four weight losses are recorded by TGA during the gradual heating of powdered coral skeleton, around 100 °C, 200 °C, 300 °C and 430 °C. The heating rate used during TGA measurements influences the reaction temperature which means that with increasing heating rate, the reaction is shifted to higher temperatures. We show that the ‘true’ temperature of a reaction can be determined by plotting different heating rates against the respective reaction temperature obtained by the TGA measurements. A combination of TGA with IR (infrared spectroscopy) and MS (mass spectrometry) shows that the highest amount of water in coral skeletons is lost around 300 °C and that CO2 released from the calcium carbonate skeleton is continuously rising with a gradual increase in temperature, however the release of CO2 peaks at 300 °C and 430 °C which indicates the release of organic compounds. The amount of organic compounds released during TGA does not differ for corals grown under high and low CO2 conditions in Papua New Guinea which may be explained by acclimatization via different processes obviating the necessity to alter the organic matrix. There are however differences between species comparing the weight losses of their skeleton during TGA: the weight loss from 30 °C until 550 °C (prior to decomposition) is highest in P. damicornis samples from both control and seep sites which suggests the highest amount of organic matrix for this species. Loss of matter during calcium carbonate decomposition is calculated to be 44% of weight, however in the coral standard sample used in this study it constitutes less (42.5 – 43.3%). This may be explained by organic compounds which are retained in the skeleton during heating and are only released during calcium carbonate decomposition. The heating rate used during TGA was found to additionally influence the amount of weight which is lost during decomposition, meaning a decreased weight loss with higher heating rates. CRM did not result in any differences in organic compound distribution or relative crystallographic orientation for corals from high and low CO2 sites. Raman point measurements resulted in small signals corresponding to CH-bands and OH-bands which indicate organic compounds, however no differences were obtained when comparing samples from control and seep sites or from different coral species. The results obtained by a combination of TGA and CRM show that aragonite conversion in biogenic carbonates occurs at lower temperatures than for inorganic aragonite. The coral standard used in this study transforms into calcite at ~430 °C, the inorganic aragonite only at ~470 °C. The data presented in this case study propose new approaches using both TGA and CRM to obtain information on the organic matrix inside coral skeletons. Combining TGA with IR and MS would additionally allow the investigation of compounds released during TGA and hence increase the possibility to chemically determine the compounds that are involved in weight loss during heating. Additional information on the organic matrix could aid in determining their role in biomineralization and whether differences in the amount exist between corals grown under different CO2 conditions.
