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Photosynthesis
Photosynthesis is an endothermic reaction that uses solar energy to create chemical bonds. These bonds release that energy in usable form when they break, allowing the organism access to it and driving the processes necessary to sustain life. The ability to convert the sun's light into usable energy defines plants and algae. Scientists call these organisms photoautotrophs, or "light self-feeders," as they fill their energy needs from the sun. Photosynthesis forms the basis of every food web, because heterotrophs, "other-feeders," get their energy from photosynthesis in algae and plants.
Chlorophyll C
Terrestrial plants and green algae primarily use the pigment chlorophyll A to absorb sunlight. Chlorophyll A absorbs weakly in the green portion of the electromagnetic spectrum; green leaves appear green because of this reflected light. Brown algae's light-collecting fronds contain chlorophyll C as well. Brownish chlorophyll C allows brown algae to make use of more available wavelengths of light. Brown algae varieties grow up to 30 meters in length, attaching themselves to the sea floor and growing to the water's surface. At a depth of 30 meters, the sun's rays provide less available energy, leaving these species' lower fronds in dim blue light that would limit photosynthesis using only chlorophyll A. These algae species therefore use chlorophyll C as well to make their own food.
Fucoxanthin
Secondary pigments, colorful chemicals that work alongside chlorophyll to make more use of the sun's energy, play no direct role in photosynthesis. Instead, these accessory pigments absorb energy and transmit it in usable form to the chloroplasts, organelles that contain the chlorophyll essential to photosynthesis. Fucoxanthin's name comes from the brown algae genus Fucus plus the Greek word for yellow, "xanthos." It absorbs blue-green and green light most efficiently, giving it a yellowish or brownish tint. Brown algae contain fucoxanthin to make use of the blue light in deeper marine environments.
Energy Storage
Though terrestrial plants store the energy-rich products of their photosynthesis as starches and sugars, brown algae species store them as laminarin, a polysaccharide that breaks down into glucose when the algae needs to access the stored energy bound in the chemical's molecular structure. Laminarin supplies the animals that feed on kelp and other brown algae with its energy. Laminarin forms the basis for many marine food webs just as terrestrial plants form the basis for land-based food webs.