Photosynthetic rates verses growth rates of Acropora millepora at Lizard and Heron Island (MTSRF 2.5i.2, UQ)

Brief description

The aim of this component of the study was to determine how photosynthetic rates related to growth rates both into terms of skeletal deposition and organic carbon accrual associated with linear extension, and with organic carbon accrual associated with fat deposition or lipid enrichment of existent tissue. The study was based on the coral Acropora millepora located at Lizard Island (-14°40’, 145°27’) and at Heron Island (-23°26', 151°54') in the Great Barrier Reef (GBR) of Australia, with data collected for winter and summer months. Coral colonies were monitored for two years, with both summers breaching maximum mean monthly maximum sea surface temperatures, but by less than the NOAA Bleach Watch trigger of 1°C, Lizard Island colonies proved to be highly susceptible to whole colony mortality, with a loss of 2 of 5 colonies monitored. Heron Island colonies proved to be more robust with no whole colony mortality. Interestingly, comparisons of the growth rate of Lizard colonies in summer to that of Heron colonies in winter showed that despite winter heterotrophy in Heron colonies, compared to summer autotrophy in Lizard colonies: Corals had greater linear extension in the winter at Heron, than they did in the summer at Lizard. Heron and Lizard colonies at these times were equally fat with non-significant differences in lipid per surface area. Heron colonies appear to maintain their weight (areal lipid concentrations) and growth (positive linear extension) despite a potential reliance on heterotrophy, whilst Lizard colonies appear to struggle despite high rates of photosynthesis. The study therefore reinforces the notion that projections regarding coral health need to greater appreciate the mixotrophic lifestyle of corals. The following parameters were measured: Temperature in degrees C; Pnet max as micromol O2 h-1 cm-2; Lipid per surface area as mass per cm-2;

Notes

Purpose
Photosynthetic rates and bleaching status are typically used to determine the “health” status of corals. Despite this, corals can also obtain organic carbon heterotrophically, through capture of plankton, ingestion of suspended particulate matter (POM) and uptake of dissolved organic matter (DOM). How strongly feeding on all of these food sources contributes to coral energy budget is still unknown, but evidence in the literature demonstrates that feeding has a positive effect on tissue and skeletal growth and indirectly, also on the rates of photosynthesis (Houlbrèque&Ferrier-Pagès, 2008; Treignier, C. et al. 2008). Feeding has been shown to be very important for lipid stocks when translocation of algal photosynthates is greatly reduced (Grottoli et al. 2008).