Answer:
The answer is Non-photosynthetic because very little light is available; small because there are so few nutrients.
Step-by-step explanation:
Very little to no light reaches below 200 m, but below 1000m it's completely dark. Therefore, photosynthesis is impossible, autotroph organisms depend on organic matter and they obtain energy through chemosynthesis. Most organisms are small and passive, they don't waste energy in hunting, but they wait for the food to reach them. Although there's a condition called deep-sea gigantism, it's due to the high pressure and not to the low salinity. These gigantic organisms are mainly composed of water and that way they avoid being compressed, like organisms with air cavities like humans.