# The temperate coral Astrangia poculata maintains acid–base homeostasis through heat stress

## TL;DR

Temperate coral Astrangia poculata maintains stable internal pH during heat stress, unlike tropical corals, but still shows some tissue damage.

## Contribution

Identifies thermally resilient acid–base homeostasis in a temperate coral under heat stress.

## Key findings

- A. poculata maintained intracellular pH and calcification despite 30°C heat stress.
- Tropical corals typically experience cellular acidification and reduced growth under similar stress.
- Heat-treated A. poculata showed tissue damage and lower egg densities, indicating fitness threats.

## Abstract

Heat stress can disrupt acid–base homeostasis in reef-building corals and other tropical cnidarians, often leading to cellular acidosis that can undermine organismal function. Temperate cnidarians experience a high degree of seasonal temperature variability, leading us to hypothesize that temperate taxa have more thermally robust pH homeostasis than their tropical relatives. To test this, we investigated how elevated temperature affects intracellular pH and calcification in the temperate coral Astrangia poculata. Clonal pairs were exposed to elevated (30°C) or control (22°C) temperatures for 17 days. Despite causing damage to host tissues and symbiont cells, elevated temperature did not affect intracellular pH or inhibit calcification in A. poculata. These responses contrast with those of tropical cnidarians, which experience cellular acidification and decreased growth during heat stress. Astrangia poculata therefore appears to have thermally resilient cellular acid–base homeostasis mechanisms, possibly because of adaptation to large seasonal temperature variations. However, we also observed tissue damage and lower egg densities in heat-treated individuals, suggesting that increasingly severe marine heatwaves can still threaten temperate coral fitness. These results provide insight into corals’ nuanced adaptive capacity across latitudes and biological scales.