Single Symbionts for Corals

Abstract

Tropical coral reefs are stressed by sea-level rise and higher water temperatures brought on by climate change. Stress prompts corals to shed their photosynthetic symbionts, or zooxanthellae, and large areas of reefs can “bleach,” sometimes killing the coral. Controversy has centered on whether bleaching is adaptive to enable bleached corals to acquire different symbionts that could endow their hosts with different physiologies to cope with different conditions, in particular greater temperature tolerance. Symbiont shuffling could happen only if the host coral can naturally tolerate a variety of symbionts. Goulet has undertaken a meta-analysis and review of 43 papers containing genotype data for 442 coral-zooxanthellae associations. It seems that most mature hard coral individuals harbor only one strain of symbiont and will retain the same genotype for decades, even after transplantation from one site to another. It remains unclear how the remaining 23% of corals that can host several symbionts respond to bleaching conditions. — CA