Once food has reached the sea floor, currents carry this food and organisms filter it without having to use their own energy to go and get food. Another way organisms use energy is by coming out at night and rising to feed on upper-level organisms.
Physical Science redirects here. It is not to be confused with Physics. Physical science is a branch of natural science that studies non-living systems, in contrast to life science. It in turn has many branches, each referred to as a physical science, together called the physical sciences.
Physical science usually has a prereq of pre-algebra, and the topics include chemistry, physics, and a smattering of related topics including astronomy and weather. It depends on what curriculum as to the topics. I've seen some books that are primarily physics and chemistry with little else, and some that go broarder.