Corals primarily expel their endosymbiotic algae:

Question

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Question: Corals primarily expel their endosymbiotic algae:

Options:
 a. when sea temperatures become elevated, algal photosynthesis generates reactive oxygen species that damage the coral cells.
 b. in response to increased sun exposure, as UV damages and stresses the coral.
 c. in response to increased sun exposure corals are able to photosynthesise themselves, so no longer require algae.
 d. when sea temperatures become elevated, the high temperatures cause the ocean to acidify, stressing the coral.

Analysis:
Option a: This statement aligns with the widely accepted mechanism of coral bleaching. When sea temperatures rise, the photosynthetic apparatus of the endosymbiotic algae (dinoflagellates) experiences stress and produces reactive oxygen species. These ROS can damage both algal cells and host coral cells, leading to the expulsion of the algae and resulting in bleaching. This explanation reflects a causal link between thermal stress, algal dysfunction, and loss of symbionts.

Option b: While increased sun exposure can contribute to light and thermal stress, bleaching is more directly tied to elevated water temperatures rather than UV exposure alone. UV can play a role, but the core trigger is thermal stress causing algal dysfunction, not just sunlight.

Option c: This option claims corals can photosynthesize independently when exposed to more sunlight, eliminating the need for symbiotic algae. In reality, corals rely on their algal partners for a significant portion of their photosynthetic products; they do not switch to self-synthesis to the extent implied, so this is biologically inaccurate.

Option d: Elevated temperatures can be a driver of bleaching, but the statement inaccurately links temperature rise to ocean acidification as the direct cause of stress in this context. Ocean acidification is primarily driven by elevated CO2 levels altering carbonate chemistry, a separate issue from the ROS-mediated stress of algal photosynthesis. The causal chain described here is incorrect for explaining the expulsion of endosymbiotic algae.

In summary, option a correctly identifies the mechanism by which corals expel their endosymbiotic algae under thermal stress, whereas options b, c, and d present partial or incorrect causal relationships that do not accurately explain the bleaching process.

BIO1022 MA S2 2025 On Campus Test 2

Corals primarily expel their endosymbiotic algae:

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