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Why might high seawater temperatures have harmed photosynthetic symbionts living in marine invertebrates?

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Line graph showing the number of marine animal families from the Precambrian era to the present, with illustrations of key groups such as trilobites, ammonoids, marine reptiles, and diatoms. The graph highlights major extinction events and a rise to 1,900 living families today. — Permian extinction The diversity of marine animal families since late Precambrian time. The data for the curve comprise only those families that are reliably preserved in the fossil record; the 1,900 value for living families also includes those families rarely preserved as fossils. The several pronounced dips in the curve correspond to major mass-extinction events. The most catastrophic extinction took place at the end of the Permian Period.

During the Permian extinction, rising seawater temperatures are believed to have disrupted the vital relationship between marine invertebrates and their photosynthetic symbionts.12

When ocean temperatures reach levels outside of a specific, sustainable range, this mutualistic symbiosis can break down. Under such heat stress, these symbionts may produce toxins that are harmful to both the algae and their hosts. To protect themselves or as a result of this physiological stress, the host invertebrates expel the symbionts, a process often referred to as bleaching.3

Without these photosynthetic partners—which provide essential energy through the conversion of sunlight, water, and carbon dioxide into sugars—the host invertebrates are deprived of a critical nutrient source. If the water temperatures do not return to a normal range to allow for recolonization, the hosts may eventually die from starvation or disease. This mechanism highlights how thermal crises can lead to widespread ecological collapse by severing the foundational biological partnerships that support marine life.312