Algae and marine phanerogams are eaten by sea urchins, some species of fish, some gastropod and polyplacophoran molluscs, and some crustaceans.
In sandy seafloors not colonized by marine phanerogams nor by green (chlorophycean) algae, unicellular algae develop that play no structural role and are consumed by sediment-eaters.
This might occur by the fixation of sediment by meadows of marine phanerogams, the building of organogenic structures by the accumulation of carbonated algae that live free in sedimentary seafloor, or (at least partially) by the creation of a hard substrate by the hermatypic corals and carbonated algae of coral reefs.
The growth of seaweeds and marine phanerogams is often determined by their capacity to assimilate nutrients, and their seasonal cycle often reflects variations in the water's nutrient concentration.
Other communities where competition for space clearly occurs include rocky areas with seaweed, meadows of marine phanerogams, coral reefs, and in general any environment where the availability of food is not limiting.
Some typical adaptations to terrestrial life make no sense in the marine medium (showy flowers adapted to pollinating insects, stomata to control transpiration, impermeable cuticles) and have not been maintained by marine phanerogams. However, the following adaptations to terrestrial life have acquired new purposes.
The marine phanerogams have colonized extensive areas, but they have done so mainly on substrates where seaweeds could not prosper: the sedimentary ocean bottoms.
The greater part of the surface area is permanently covered with ice and only in some marginal lands which thaw during summer do we find mosses, lichens, terrestrial algae and no more than three species of phanerogams
. The marine systems which surround this continent are very productive and it is for this reason that there is a very abundant fauna (penguins, seals, etc.) which feeds in the waters and only uses the continent as a place to rest and nest, in other words as a purely physical, solid base and not as a productive landscape into which they become integrated.