RNA/DNA ratio
The RNA/DNA ratio is based on the principle that the amount of DNA within a cell remains invariant whereas the amount of RNA varies as a function of metabolic processes, such as protein synthesis related to growth. Under conditions favourable for coral growth, such as in clear-water with high benthic irradiance, the RNA/DNA ratio is expected to be greater than conditions where growth is limited by available light. Indeed, recent studies have demonstrated positive relationships between RNA/DNA ratio and light intensity along depth and turbidity gradients (Meesters et al. 2002; Buckley and Szmant 2004; Table 2). However, interspecific and seasonal variability, coupled with complex responses to increased heterotrophy in turbid conditions, suggests that although RNA/DNA ratios may be sensitive to changes in water quality, further research is required to better understand the sources of variability. The RNA/DNA ratio ranked a medium-priority bioindicator for use in long- and short-term monitoring programmes (Table 3), until a better understanding of interspecific and seasonal variability in this ratio is obtained. This bioindicator may, however, prove useful in EIA to measure effects of acute changes in water quality within species when values from impact locations are compared with those from reference locations.