Question

........ are good sources of oxygen for those organisms that live in water.
A.Moss
B.Mushroom
C.Sea urchin
D.Algae

Answer 1

Hint:Algae are photosynthetic organisms which can produce oxygen as a bi-product. When considering an aquatic ecosystem, the algae occupy the first trophic level as either sea algae which float on the surface or present on corals, rocks, etc, or as phytoplankton within the water. As they can produce oxygen during photosynthesis, they provide a good source for oxygen for other organisms within the aquatic ecosystem.

Complete step by step answer:
Aquatic ecosystems are just as complex and energy-demanding when compared to other ecosystems and biomes. But the major difference is that the aquatic ecosystems are underwater and the process of absorbing energy from sunlight is tricky.
Even in aquatic ecosystems, several plants grow such as seaweeds, or other species of the bryophytes. But the major source of oxygen and food in such ecosystems are provided by the phytoplankton. Phytoplankton is a group of photosynthetic organisms mainly algae which absorb the sunlight and produce their food with oxygen as their bi-product.
Such phytoplankton is the base of all aquatic ecosystems, for without them the ecosystem would fail. Such vital importance to such algae is due to the production of oxygen as even underwaters creatures require oxygen to survive.

Phytoplankton can be present as algal deposits on corals, rocks, or can also be present as free-floating clumps in the water. Sometimes excessive growth of these algae can cause algal bloom which may drastically affect the marine ecosystem. Even low amounts of these organisms could cause phytoplankton die-off which will result in oxygen depletion in the water and ultimately kill the marine ecosystem.

Therefore, the correct option is D.

Note:
Mosses are a species of bryophytes that are found on damp surfaces and require water for reproduction. Even though these are aquatic plants, the oxygen produced by these plants is not dissolved in water as they grow on water surfaces, and any oxygen produced is released into the air and not the water. But other bryophytes do contribute to most of the oxygen production underwater. Mushrooms are a type of fungi that do not grow underwater nor do they produce oxygen. Sea urchins are small spiny animals found deep underwater which does not produce oxygen