All of the following are puzzling to reef scientists EXCEPT: 1\. how the zooxanthellae stimulate Scleractinia to secrete calcium carbonate. 2\. how barrier reef's become separated from land masses by vast expanses of water. 3\. how the reef environment reaches such high levels of oxygen saturation. 4\. how fringing reef's develop into barrier reef's and then into atolls.

Review the given options to understand the aspects of coral reefs that could be puzzling to reef scientists. 1. how the zooxanthellae stimulate Scleractinia to secrete calcium carbonate. 2. how barrier reef's become separated from land masses by vast expanses of water. 3. how the reef environment reaches such high levels of oxygen saturation. 4. how fringing reef's develop into barrier reef's and then into atolls.

Explore the key aspects of reef ecology to understand which of them might be puzzling to reef scientists. 1. Zooxanthellae and Scleractinia - Zooxanthellae are photosynthetic algae that live in a symbiotic relationship with coral polyps (Scleractinia). The coral provides the zooxanthellae with protection and nutrients, while the zooxanthellae generate food through photosynthesis. 2. Barrier reef formation - Barrier reefs are massive coral reefs that run parallel to the shore and are separated by a lagoon. The formation of barrier reefs involves various processes, such as the subsidence of landmasses, which slowly separate the coral reef from the shoreline. 3. Oxygen saturation in reef environments - Coral reefs are highly productive ecosystems which support an abundance of species. The photosynthesis performed by zooxanthellae (as well as other organisms like seaweed) leads to an increase in oxygen levels in the water. 4. Reef development - Fringing reefs, barrier reefs, and atolls represent different stages of reef development, which are often influenced by the geological activity and sea level changes. While these processes have been widely studied, some questions remain unanswered about the development of these different types of reefs.

Based on the information about the key aspects, determine which aspect is least puzzling to reef scientists. While reef scientists may have questions about all four aspects mentioned, the least puzzling aspect appears to be "how the reef environment reaches such high levels of oxygen saturation." This is because the process of photosynthesis in reef ecosystems is well-understood, and it has been established that zooxanthellae are responsible for much of the oxygen production in coral reefs. Unlike some of the other topics, there is extensive knowledge about this aspect of coral reefs. Therefore, the answer is: 3\. how the reef environment reaches such high levels of oxygen saturation.