The effect of light on the Mg/Ca ratio in the planktic foraminifer Orbulina universa: implications for a light driven vital effect on Mg incorporation into foraminiferal calcite

Many paleotemperature reconstructions rely heavily on the Mg/Ca ratio of planktic foraminifers. The incorporation of Mg is highly dependent on temperature and is also subtly affected by other water column parameters (e.g. pH and salinity), but the nature of the biological control over Mg uptake into foraminiferal calcite has not been fully determined. The distribution of Mg within a single foraminifer test is often organized in intercalated bands of high and low-Mg/Ca ratio calcite. Initial studies showed that low Mg/Ca calcite is added during the day light hours and high Mg/Ca calcite during the night (Spero et al., in press). Banding has been attributed to 1) modification of pH in the microenvironment around the shell via symbiont photosynthesis/respiration, 2) an unknown circadian mechanism, and/or 3) differential mitochondrial pumping of Mg away from the calcifying shell surface during day and night phases.

We conducted several culture-based experiments to assess the affect of light on Mg/Ca variability in the planktic symbiont-bearing foraminifer Orbulina universa by culturing the foraminifers in different photoperiods including: 1) reversing the day/night cycle, 2) 24-hour constant light, and 3) 24-hour constant dark. With a reversed light/dark photoperiod, there is no 'lag' in the timing or amplitude of the Mg/Ca bands. In 24-hour constant light, the high Mg ‘night’ bands are reduced in amplitude and are no longer paced by a 24-hour cycle. This produces a mean shell Mg/Ca ratio that is lower than in shells growing on a normal light:dark cycle. Under 24-hour constant dark conditions, high Mg/Ca bands dominate the shell calcite. This produces a mean shell Mg/Ca ratio that is higher than shells grown on a normal light:dark cycle. These results suggest that a light cue, independent of photosymbiont activity, is responsible for controlling the presence/absence of high Mg/Ca banding and that ambient irradiance changes in the water column could influence overall Mg/Ca in O. universa shells. Additional experiments to investigate the light cue are underway and results will be presented at the fall meeting.