Changing light levels induce photo-oxidative stress and alterations in shell density of Amphistegina lobifera (Foraminifera)

Light is an important factor limiting the distribution of holobiont (host-algae) organisms in marine environments. Reef-dwelling large benthic foraminifera (LBF) are particularly sensitive to changes in light exposure, but their cryptic behaviour allows them to colonise different habitats across a light gradient. Yet, how foraminifera of the same species that live in different habitats respond to variations in light conditions, and the physiological and morphological attributes that govern that response, remain unclear. Here, we investigate how Amphistegina lobifera, a common and abundant LBF species in reef environments, collected from different reef sites across a light gradient, respond to changing light levels under lab-controlled conditions. Reduced light below their optimum level for growth and calcification caused a significant reduction in antioxidant capacity, and a significant decline in the density of newly added chambers. In contrast, elevated light caused significant reduction in survivorship, induced bleaching and photo-oxidative stress, and caused thickening of external chambers. Reef site did not have a significant effect on most of the responses, and populations collected from different reef sites showed similar sensitivity to changes in irradiance below and above their optimum level. The capacity to regulate symbiont density and to host a diversity of symbiotic diatoms within Amphistegina could be a reason for their ability to acclimate rapidly to different light levels. However, prolonged exposure to reduced light may result in a decrease in shell density and, as a consequence, a decline in carbonate production in reef environments.