6ebbba77_Marine_habitats__principles__Living_in_the_sea
[Factor] Substrate [Living on the land] Plants need moisture and nutrients which are present in loose soil. Rocks are difficult places to live although some organisms are able to do just that (lichens). Plants need to transport moisture, nutrients, oxygen and carbondioxide through vessels from their roots to their leaves and back. [Living in the sea] 
The water is found all around. Seaweeds do not need roots for finding water and nutrients. They need solid rock to hold on to, which they do by means of root-like structures. Seaweeds cannot grow in sand in the open sea, although there are some exceptions (seagrass). Seaweeds do not need to transport liquids very far. The water movement helps to pump liquids around in their tissues.
Marine habitats, principles
Living in the sea