The elemental chemistry of reef coral skeleton is an important ocean temperature proxy that has been used to address some particularly controversial climate change issues. Yet, while corals have potential to provide records of past climate unsurpassed in their combination of temporal resolution, continuity and length, there is a great deal of uncertainty in coral-based paleotemperature estimates because it is clear that biological processes confound the geochemical signal that we read as climate. In particular, skeletal calcification rate appears to influence skeletal chemistry and consequently, the temperature-dependence of trace-element incorporation may be non-linear over a broad temperature range. Corals with symbiotic zooxanthellae exhibit large diurnal fluctuations in calcification rate associated with the photosynthetic activity of their symbionts. We investigated the effects of daytime photosynthesis on Sr partitioning into the aragonite skeleton using secondary ion mass spectrometry (SIMS) to separately analyse the elemental composition of discrete daytime and nighttime skeletal accretions. The Sr/Ca content of daytime skeleton of the massive hermatypic coral Porites lutea is always lower than that of adjacent nighttime skeleton. We attribute the offset to differences in the growth rate of granular nighttime crystals versus needle-shaped daytime crystals. We also found that the slope of the nighttime Sr/Ca-SST correlation is particularly shallow (-0.04) and close to that determined for inorganic aragonite precipitates (-0.039). On the contrary, the slope of the daytime correlation is >4 times as steep indicating heightened sensitivity of Sr/Ca to changes in temperature during photosynthesis. Our results indicate that Sr/Ca of bulk coral skeleton is related principally to daytime calcification rate rather than directly to temperature and likely to cause exaggerated Sr/Ca changes during periods of SST cooling. More reliable estimates of past SST may be arrived at through selective analysis of nighttime skeleton.