Light and temperature niches of the large benthic foraminifer Heterostegina depressa

Reef calcifiers, such as symbiont-bearing large benthic foraminifera (LBF), play a major role in shaping (sub)tropical ecosystems. LBF are projected to experience larger fluctuations in light levels as well as elevations in sea surface temperatures, which might be accommodated by a shift in their species distribution towards cooler subtropical regions. While foraminifera are highly sensitive to environmental changes, their light and temperature niches have not been well characterized. Here we examined the ability of the diatom-bearing Heterostegina depressa to acclimate to a range of light levels (from 0 to 50 μmol photons m⁻² s⁻¹) and temperatures (from 15.6 to 33.1 °C) through a 4-week experimental study. Micro-CT scanning of live foraminifera prior to experimental incubations was successfully used to quantify calcification. A thermal niche between 24.4 and 30.3 °C was identified for H. depressa based on assessment of growth, calcification and photosynthetic yield. High sensitivity was observed outside this range, as also confirmed by declining photosynthetic efficiency. H. depressa showed a light optimum for growth between 25 and 38 μmol photons m⁻² s⁻¹, and first signs of photoinhibition at 50 μmol photons m⁻² s⁻¹, while decreased growth was observed in darkness. Acclimatization to higher light intensity is explained by a relative increase in the pigment violaxanthin, as detected by high-performance liquid chromatography (HPLC). Our results suggest that LBF survival could become compromised under future range expansions, however examining the interactive effects of light, temperature, and other environmental stressors on LBF performance are urgently needed as a basis for predicting their responses to future ocean projections.