When first studying seaweed at first sounded very boring because up to this point all I thought it was, was grass that grew in salt water instead of land. After some time though we began to realize that these "weeds" were very diverse and had many parts to them. Seaweed isn't actually just one type of plant but it instead summarizes the many other different types of these species found in our body of water's on earth. Among these sub groups there are Phaeophyta, Rhodophyta, and Chlorophyta and a fourth called Anthophyta. Phaeophyta are also known as the brown algae which are known to grow the largest in the kingdom. Rhodophyta are also known as red algae and can live the deepest out of any of the seaweeds. The Chlorophyta are also as the Green Algae and are by far the most common out of this kingdom. Anthophyta species are sea grasses and have true roots and the ability to produce flowers. 

        What makes seaweed so interesting is how the plants allow themselves to absorb different light pigments at different levels for each color of light. The pigments of algae are divided into three major groups: the chlorophylls, the carotenoids, and the phycobilins. . Also, the role of some pigments in these groups is not yet known. It is known, however, that photosynthesis in algae, as well as in higher plants, is associated with the complex molecule chlorophyll. Two very important pigments are chlorophyll a and b. Only chlorophyll a is a common factor in all photosynthetic plants because of its uses.The other chlorophylls, as well as some carotenoids and phycobilins, function in photosynthesis as accessory pigments, being involved either as part of the light reaction pigment systems or as donors of light energy to chlorophyll a. Over all though all these pigments produce the same idea, which is to allow them to survive in their environment. 
  

www.seaweed.ie and oceanservice.noaa.gov are two more great sites if your interested in more research