Which abiotic factor is essential to all aquatic ecosystems except ocean hydrothermal vents and why?

The abiotic factor that is essential for all aquatic ecosystem with the exception of hydro thermal vent is SUNLIGHT, THIS IS BECAUSE IT PROVIDES A SOURCE OF ENERGY. 
Sunlight is very essential for aquatic ecosystems because it is the principal source of energy from which aquatic plants trap energy to make their food and provided mean of sustenance for themselves and other organisms in the water.