CHANGES IN PRIMARY PRODUCTION AS A RESULT OF SYMBIONT-LOSS IN THE DIATOM-FORAMINIFER HOLOBIONT AMPHISTEGINA GIBBOSA FROM THE FLORIDA KEYS (Invited Presentation)

Photosynthetic response to variations in irradiance (P-E curves) was measured for the diatom-bearing foraminifer Amphistegina gibbosa collected from the Florida reef tract. The analyses compared changes in primary production among three levels of symbiont-loss: no bleaching, partial bleaching, and severe bleaching. These protists were acclimated to laboratory, non-stressful conditions for at least two weeks before experimental trials. During light curve generation, specimens were exposed sequentially to 12 levels of irradiance between 0–688 µmol photons m^-2 s^-1, with oxygen measurements after 10 minutes of illumination at each level. Values were normalized by surface area (five specimens per replicate) and fitted to an empirical equation of photosynthesis as a function of light. Gross-photosynthesis maxima (P^g_max) ranged from 7–27 nmol O₂ h^-1 mm^-2. P^g_max was significantly lower in severely bleached foraminifers than for those with no or partial bleaching; specimens in the latter categories were not significantly different with respect to P^g_max. Photosynthetic efficiency as reflected by α and onset of saturation (E_k) were not significantly different between any of the categories of bleaching. Although α and E_k were not statistically different, the overall trend of the curves indicates that symbionts in the inner chambers (i.e., in “severely bleached” trials) may require higher irradiances before reaching a light-saturated state. In addition, severely bleached specimens showed a reduction of about half the daily net-photosynthesis, compared to the other two categories that remained in a similar range. These results are consistent with laboratory observations of normal growth in partly bleached individuals and reduced survival of severely bleached organisms, even when maintained in non-stressful conditions.