Why is there red algae in the water depth?

The so-called red algae is that the algae are red when they are taken above the water surface. Among the various colors of white light, red light has long wavelength, low energy and weak penetration ability, so it is difficult to reach the deep sea. Only colored light with short wavelength, high energy and strong penetrating power (such as blue-violet light) can be transmitted into deep water. So it is difficult for algae that mainly absorb red light to survive here. Algae living here mainly absorb short-wavelength colored light. When the algae is brought above the water surface, the red light is not absorbed but reflected under the irradiation of white light, so the algae you see is red. It also explains the color of brown algae. Because all colors of light can be transmitted to shallow water, algae here, like plants on the ground, mainly absorb red light and blue-purple, and it is difficult to absorb green light; Under the irradiation of white light, green light is reflected into green.