"If there were no algae there would be no fish in the sea". Comment.

Algae are the main source of productivity in sea. They prepare their own food with the help of photosynthesis. They are food for many small aquatic animals living in the sea. Hence the algae are food for small fishes and small fishes are food for big fishes. It is like a food chain.

If there are no algae then:

• Ocean food webs would completely collapse. algae is the main driver of photosynthesis and without it the basic energy inputs for the rest of the food webs would be gone, causing mass die-offs in the ocean.

• Similarly, the oxygen output is significant from algae (about half of total production, so life on land would suffer immensely and quickly (faster than if food production from the ocean ceased alone).

• The death of other ocean organisms would cause the oxygen in the ocean to be used up as the organic material decomposes (this is the problem with algae blooms). This would lead to even less available oxygen for any remaining species, and an increase in CO2 levels adding to ocean acidification. I'm not sure it would wipe out all life in the ocean, but it would get close.

• This is a guess on my part, but since there are no more algae to absorb the sun's radiation, all that radiation previously photosynthesized into sugars would now be hitting the ocean and would instead transfer their energy to water molecules, causing them to vibrate and raise the temperature of the water. (It may be that since plants mostly synthesize using visible, specifically blue and red, light, this effect could be negligible) For those in the ocean, the environment becomes even more hospitable, and for those on land, the additional ocean warming would likely cause greater climate shifts.