Diurnal Changes in Microstructure and Microscale Chemistry of Reef Coral Skeletons
A.L. Cohen, et al, Dept of Geology and Geophysics, Woods hole of Oceanographic Institution
Abstract:
Calcification by corals with symbiotic zooxanthellae occurs 3 times faster in daylight than it does at night. We investigated the effects of light enhanced calcification on the microstructure and elemental chemistry of the aragonite skeleton over the diurnal cycle. The morphology of nighttime crystals accreted in the absence of photosynthesis imitates that of slow growing inorganic cements in a high CO2 environment. The morphology of daytime crystals accreted during the photosynthetic period imitates that of fast growing inorganic cements in a low CO2 environment. We used an Cameca IMS 3f ion microprobe to measure changes in the strontium-calcium content (Sr/Ca) of the growing skeleton of the tropical reef coral, Porites lutea, over the diurnal cycle. Sr/Ca in nighttime skeleton is close to equilibrium values but a large decrease in Sr/Ca is observed as the daytime crystals grow to fill the extracellular calcifying space in summer. The amplitude of change in skeletal Sr/Ca between night and day is as large as the annual cycle in Sr/Ca. During summer, at peak water temperature and symbiont photosynthesis, the amplitude of the diurnal Sr/Ca cycle is 3 times greater than that incurred only by the diurnal change in water temperature. During winter, the amplitude of the diurnal Sr/Ca cycle is equivalent to that incurred by temperature alone. Our data show that processes linked to symbiont photosynthesis exert significant influence on both skeletal microstructure
and microchemistry, and that temperature is not the primary control of diurnal Sr/Ca variability in reef coral skeleton.
P. Spencer Davies1, 2
(1) Department of Zoology, University of Glasgow, G12 8QQ Glasgow, Scotland, UK
(2) Discovery Bay Marine Laboratory, Jamaica
Accepted: 16 January 1989
Communicated by J. Mauchline, Oban
Abstract An accurate method for determining the growth rates of the skeleton of isolated branch tips (nubbins) of corals over intervals of less than 24 h is described. The skeletal weight of the coral was estimated from its buoyant weight in seawater whose density had been accurately determined. The coral tissues accounted for between 1 and 5% of the total buoyant weight in Pocillopora verrucosa and Acropora humilis with differing relative tissue biomass. After correcting for tissue buoyant weight, predictions of skeletal weight were accurate to within 1%. The method was used to estimate the growth of sample nubbins of Porites porites of similar diameter, in 2 m of water at Discovery Bay, Jamaica. Since growth of these branch tips is apical, growth rate could be expressed without correction for the size. The mean 24 h skeletal growth rate ranged between 40 and 47 mg. Differences could be measured between day-time and night-time growth, the day: night ratio being 3.7. The method also showed that P. porites virtually ceases calcification during the 4 to 5 d periods that it becomes enclosed in a mucus tunic. Nubbins of P. porites attached to the reef at different locations showed clear differences in growth rate with depth, and between clear and turbid water sites. The growth rate of nubbins was compared with that of branch tips of whole corals by measuring the linear extension after staining with Alizarin Red S. After 3 1/2 mo, the mean linear extension was 4.1 mm in each case, indicating that the growth rate of nubbins is the same as that of branch tips of the whole colony. It is suggested that this buoyant weighing technique will find applications in laboratory experiments with calcification mechanisms and as a bioassay on reefs exposed to environmental stress.