What was done
Portions of colonies of Stylophora pistillata that originated in the Gulf of Aqaba (Red Sea), but which had been maintained for several years in laboratory aquaria, were transferred to four 15-liter tanks that were maintained at conditions to which the corals had become accustomed over the prior several-year period.  One of the tanks was kept at the original salinity level of 38 psu, while the other three were slightly altered to new levels of 36, 34 and 40 psu.  After three weeks at these conditions, measurements of photosynthesis and respiration were made, after which the corals were destructively harvested for chlorophyll and protein content analyses.

What was learned
The photosynthesis/respiration ratio of the corals cultivated at 38 psu was similar to values measured in the wild (1.5); but the ratio dropped significantly at salinities on either side of the 38 psu value to which the corals had become accustomed over the prior several years.  Indeed, gross maximal photosynthetic rates were 50% lower at 34, 36 and 40 psu than at 38 psu.  Similar results were obtained for protein content, which was 25 to 30% higher at 38 psu than at 34 and 40 psu.

What it means
This work clearly demonstrates that, with respect to environmental parameters, change alone, independent of its direction (higher or lower, more or less, etc.) can sometimes negatively impact the functioning of corals.  Hence, it is possible that many of the negative consequences (such as coral bleaching) that are often imputed to be due to an increase in this or that parameter - salinity, temperature, etc. - may not be due to the fact that the parameter increased, but rather to the fact that it merely changed.