# Exogenous diatoms ameliorate thermal bleaching of symbiont bearing benthic foraminifera

## TL;DR

Exogenous diatoms help reduce heat stress in foraminifera by altering their algal symbiont communities.

## Contribution

The study reveals that heat tolerance in foraminifera is linked to specific symbiont species rather than broader community shifts.

## Key findings

- Amphistegina lobifera reshuffles its symbiont community under heat stress, while Pararotalia calcariformata maintains a stable Arcocellulus cornucervis symbiont.
- Supplementing isolated diatoms reduced bleaching in A. lobifera, indicating the importance of specific symbiont species for heat tolerance.
- Only one diatom species from P. calcariformata survived at 35°C, highlighting species-specific thermal resilience.

## Abstract

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.