Exogenous diatoms ameliorate thermal bleaching of symbiont bearing benthic foraminifera.

Many marine calcifiers engage in obligatory algal symbiosis which is threatened by ocean warming. Large benthic foraminifera are prominent carbonate and sand producers in shallow environments with a wide range of species-specific thermal tolerances assumed to be related to their diverse algal symbionts. We examine two diatom-bearing benthic foraminifera species which differ in their thermal physiological tolerance and symbiont community composition. Our findings demonstrate that the less thermally tolerant host, Amphistegina lobifera Larsen, 1976, 'shuffles' the dominant players of the internal symbiont community with increasing temperature while the more thermally tolerant host Pararotalia calcariformata McCulloch, 1977, is dominated by Arcocellulus cornucervis Medlin, 1990, at all temperatures. Although this diatom species was present in A. lobifera from all treatments, it became more abundant only under the most severe temperature stress. Symbionts were isolated from the thermally tolerant foraminifera P. calcariformata, with only one species of symbiont surviving at 35°C, while the others failed to survive at 32°C. Supplementation of isolated symbionts reduced bleaching of A. lobifera under heat stress suggesting that while increased temperature creates shuffling at the family level, heat tolerance of the holobiont is related to changes at the species level of the symbiont algae.