No light - no plants

Plants and some bacteria can produce their own food. The process they are using is called photosynthesis. They just need water; the gas carbon dioxide (CO₂) and they need light. They have green pigments called chlorophyll, which they need to catch the light (the photons). With the light as energy source, they synthesize sugar and starch, which the organisms can turn into energy or into proteins and fat. This kind of metabolism is called autotrophy and this is the way these organisms can grow and proliferate.

The most important autotrophic organisms in the ocean water are many very tiny algae and cyanobacteria. At the bottom of the ocean you find in shallow waters sea grass, large algae. At interface between land and sea at the coast you find mangrove trees and marsh grasses. Remarkably is at the coast where all these tiny algae and bacteria produce 50% of the global so-called primary production through photosynthesis. Most of it comes from the tiny algae and cyanobacteria in the water.

Animals have to eat food produced by others, because they cannot make it themselves; they are heterotrophic. There are also some protozoa and bacteria, who are heterotrophic and therefore need aliment.

The food, which is available in the deep sea, is almost entirely made in the upper, shallow water and on land. Only there is light and only there photosynthesis can happen. The single big exceptions are the hot vents! At these sites food is produced abundantly. We will explain how this works later.

The bulk of food for deep-sea organisms is floating slowly down in form of small particulate material. We call this process pelagic rain. Everybody down there is longingly awaiting it. But the pelagic rain does not only consist of food. There are also very tiny stones raining down and accumulate in the deep sea since millions and millions of years. Wood and other parts of plant as well as big dead animals like corpses of fish and whales are also sinking down. Still, since just little food is reaching the deep sea, only few animals can live there. They can look really creepy, though most of them are tiny little critters. Formerly it was thought, that there are just few different species living in the deep-sea environment. Today we know that it is completely different. There are many, many species of bacteria, protozoa and animals living there. The biodiversity (the species richness) is as high as in a tropical forest or a coral reef. Scientists are still puzzled why it is like this.