On land, photosynthetic plants form the base of the food chain, trapping the sun's energy and passing it along to a web of primary and secondary consumers. So too, in shallow seas and in the upper layer of the deep oceans, a wide variety of marine plants, from tiny phytoplankton to giant kelp, support a fabulous diversity of sealife.
But at depths below 450 feet, the solar radiation is too weak to allow photosynthesis and, below this "green line," living plants do not exist. All life in the deep, dark ocean waters is bacterial or animal, sustaining itself in a variety of ways. Much of the food arrives as an endless "snowfall" of animal and plant debris from the upper ocean waters, including occasional windfalls such as dead sharks and whales. Many deep ocean inhabitants supplement this diet by nightly trips to the rich surface waters where, under cover of darkness, they feed on phytoplankton, krill and other small organisms. Of course, secondary consumers of the deep ocean sustain themselves by feeding on the smaller fish and invertebrates that inhabit their realm.
Along the mid ocean ridges, where ocean crust is forming, colonies of bacteria feed on hydrogen sulfide that spews from the hot, volcanic vents. These, in turn, support an amazing diversity of exotic creatures, most of which are not yet known to science. In similar fashion, other bacteria feed on methane that seeps from the ocean floor at many locations across the globe; these bacteria also form the base for a web of sealife, adapted to their unique environment.
We humans are just beginning to explore the mysteries of the deep, dark ocean, the largest ecosystem on Earth and the least understood.
