Brown algae are found all over the world. Almost all are marine organisms and prefer cold, active waters, though a few prefer tropical and subtropical climates. The phaeophytes prefer colder climates because colder water is able to hold higher concentrations of carbon dioxide, used in photosynthesis. They are found off the coast of almost every country. They are an important part of the marine flora as they provide food, shelter, spawning areas, and a substrate for numerous marine animals.
I will describe a kelp forest community, which will provide you with a general idea of the interactions therein.
In the forests, there is a stratified organization. For kelp forests, the structural layers are established by water depth, rocky outcrops, sandy patches, and seaweeds of various heights. The differing needs of sun and/or tolerance for shade as well as being tall/short, deep/shallow determines where the various kelps will grow. Within the kelp forest, there are various invertebrates and vertebrates that interact with one another. These interactions can range from being neutral, to beneficial, to hostile. As can be imagined, fish are found in the kelp forests, along with other organisms: sea urchins, sea otters, sunflower stars, crabs, barnacle parasites (among others), and anemones. In a food chain pattern, the kelp is eaten by thesea urchins, which in turn are eaten by sea otters and sunflower stars. This keeps the community in balance, so one doesn't totally destroy the population of the other. All of these various interactions are reminiscent of their mainland counterparts. The huge forests of kelp provide a great many things for the different organisms therein to survive. All are dependent on the other for various aspects of their survival.
Here is the fossil of a phaeophyte:
One of the reasons why it is difficult to find fossils of phaeophytes is due to the facts that they do not have hard parts. Another difficulty is the fact that over time, the pigment will go away, causing there to be difficulty in distinguishing between brown, red and green algae. Despite this, the oldest fossils which are definitely phaeophyta are kelps from the Tertiary. There are possible phaeophyta fossils from the Vendian, but the identity of these algae are skeptical.