Question

Algae and other submerged green plants often float in water during daytime and sink at night because:
A. Trapping of oxygen bubbles during the clay in their photosynthesis
B. Release of absorbed air by warming water
C. They become light due to consumption of food
D. They come up to enjoy the sunshine

Answer 1

Hint:- Oxygen is provided by phytoplankton as a byproduct during the photosynthesis process. Algae and cyanobacteria are responsible for around half of all the oxygen present in the ocean and in our atmosphere, due to their large and widespread populations.

Complete answer:
Photosynthesis is the explanation why algae float to the surface during the day and sink at night. The algae generate oxygen during the daytime. It gets stuck in bubbles when enough oxygen is produced during the day and it can bring the clumps up to the surface. This oxygen is consumed at night, and $CO_2$ is generated. Then the algae sinks.

Hence, Option A is the right answer.

Additional Information:
Phytoplankton helps control inorganic carbon (carbon dioxide) in the atmosphere, in addition to supplying food and oxygen for almost all life on Earth. To make sugar for energy, carbon dioxide and water molecules are used during photosynthesis. Carbon fixation is called the method of integrating inorganic carbon into organic carbon (glucose and other biologically useful compounds) and is part of the biological carbon pump. Since carbon fixation and oxygen production are part of the same mechanism, the degree of involvement of phytoplanktons is of the same magnitude. A comparable amount of carbon dioxide is absorbed by phytoplankton as combined by all land plants. While phytoplankton can absorb carbon dioxide from the ocean or the atmosphere, it will have a similar impact. Carbon dioxide derived from water is replaced by ambient carbon dioxide.

Note:-
Aquatic plants have specialised sections, such as roots, stems and leaves, whether floating, submerged, or emerging (starting in the water and rising out). There are also vascular structures in most plants (xylem and phloem), which hold nutrients all over the plant. While algae (like plants) contain chlorophyll, they do not have these specialised structures.