Marine organisms with calcium carbonate skeletons are known to vary both their growth rate, and the chemistry of their shells in response to changes in sea temperature and changes in seawater chemistry. Palaeoclimatologists can such shells to investigate ancient climates, and in particular, can use the shells of long-lived organisms that grow throughout their lives to build up detailed records of climate variability in specific areas. Corals have been widely used to this purpose in tropical seas, and large, slow growing Bivalves in cold Arctic waters, but both groups are somewhat restricted in geographical range. Coralline Red Algae (Corallinales) are found in shallow waters in almost all marine environments from the tropics to the polar oceans, and many species are extremely long lived, which leads to the possibility of using such Algae as palaeoclimatic indicators, although their biology is generally less well known than that of Corals and Bivalves.
A sample of the Coralline Red Alga Lithophyllum kotschyanum from the Gulf of Aden. Caragnano et al. (2014).
Lithophyllum kotschyanum is known to vary the magnesium/calcium ratio in its skeleton in response to changes in temperature and salinity. Brachiopods and some Corals have been shown to vary the ratio of lithium/calcium in their skeletons in response to changes in temperature and salinity, while other Corals and Foraminiferans have been shown not to do this. Foraminiferans and Corals have been shown to vary the barium/calcium ration in their shells in response to nutrient availability. Lithophyllum kotschyanum is also known to have a variable growth rate, though it is not known what drives this.
Caragnano et al. examined specimens of Lithophyllum kotschyanum collected from the Gulf of Aden to attempt to determine any relationships between magnesium/calcium ratio, lithium calcium ratio, barium/calcium ratio and growth rate, and whether these could be connected to seawater chemistry or temperature.
The Gulf of Aden is located between the Red Sea and the Indian Ocean, and receives waters from both sources. Water from the Red Sea is significantly more saline than water from the Indian Ocean. The amount of water from each source is variable, with more water coming from the Indian Ocean during the winter monsoon (November-March) and more water coming from the Red Sea during the summer monsoon (June-September). The seawater temperature is also variable, at its highest (31–32℃) in May-June, then cooling to 29-30℃ in July-August, rising to about 30℃ in September, and cooling again to 24-25℃ in October-January, before rising slowly from February-April.
Caragnano et al. found a strong ratio between magnesium/calcium ratio, lithium/calcium ratio, growth rate and temperature in Lithophyllum kotschyanum. The barium/calcium ratio was variable, but not in response to any environmental factor studied.
Lithophyllum kotschyanum grew fastest in warmer conditions, although the temperature was not the only factor affecting its growth; which is also influenced by light levels, nutrient supply and grazing by herbivores.
Histological section of a sample of Lithophyllum kotschyanum from the Gulf of Aden: (A) longitudinal section of a protuberance showing band periodicity (arrowhead) and three growth stages (black arrows). Note the growth interruption (white arrow); (B) magnification of A showing different cell length along the same filament. (C–F) SEM images of the high-Mg calcite thallus of the sample in longitudinal section: (C) shorter cells and longer cells alternate along the growth direction, from bottom to top; (D) magnification of C showing length variability in cells of the same filament (white arrow to indicate the growth direction of one cell filament). Note the secondary pit-connection (black arrow); (E) magnification of the longest cells showing a thin cell wall; (F) magnification of the smallest cells showing a thick cell wall. The double arrow indicates the cell lumen, and the arrowhead the mineralized cell wall. Caragnano et al. (2014).
The rate at which magnesium is absorbed into calcium carbonate (as a replacement for calcium, forming a small proportion of magnesium carbonate) produced by
Lithophyllum kotschyanum has been studied at other (cooler) locations, and has been shown to increase with temperature. This remained true in the warm waters of the Gulf of Aden, though the rate at which the increase occurred was significantly higher than predicted from previous studies. This confirms that magnesium in calcium carbonate in the skeleton of this algae is a good proxy for temperature, but also that a better understanding of this relationship is needed; had the origin and temperature history of the algal skeleton not been known, then based on previous data the temperature at which it had grown would have been significantly overestimated.
The rate at which lithium is incorporated into calcium carbonate (again replacing calcium to form lithium carbonate) formed by
Lithophyllum kotschyanum has not previously been studied. In this instance the lithium absorption rate closely followed the magnesium absorption rate, suggesting that this is also controlled by temperature, and that lithium ratios can be used to predict temperature in the same way as magnesium ratios.
The ratio at which barium was absorbed into calcium carbonate appeared to be only weekly related to temperature. It also peaked during the summer monsoon, suggesting that it also rose while there was an increased nutrient supply coming from the Red Sea, however there was also other peaks in absorption
which were less easy to explain, notable a sharp increase in an area of the skeleton believed to have been formed in 2007. This corresponds to a period of building on the Yemeni coast close to where the sample was collected, as well as to dredging activities in the sea close to the site, suggesting that either of these actions could have increased nutrient levels in the water. This high susceptibility to localized events probably makes barium ratios in
Lithophyllum kotschyanum skeletons a poor palaeoclimatic tool.
See also
A hypercalcified Chaetetid Demosponge from the Late Carboniferous of northwest Spain,
Two new species of calcareous Sponge from the Weddell Sea,
Examining an Ordovician Stromatolite with a tool to look for life on Mars,
A new species of heavy metal Tolerant Bacterium from the Dabaoshan Mining Area in Guangdong Province, China and
Angiosperm-like pollen from the Middle Triassic of Switzerland.
