Algae, like higher plants, obtain energy by photosynthesizing; essentially producing their food by capturing the energy from the sun. Many algae store energy in the form of starch. Other algae use other compounds and distinction between some algae groups is based upon which compound that they use to store energy. For example, Rhodophyta (red algae) store energy in the form of floridean starch, whereas the Phaeophyceae (brown algae) store their energy in the form of laminarin.
The green color of algae derives from the dominance of chlorophyll as a photosynthetic pigment. The other colors of many algae does not mean that they lack chlorophyll but that the chlorophyll is masked. These algae have accessory photosynthetic pigments which conceal the green color of chlorophyll. Rhodophyta contains the pigment phycobilin, giving it a red color while the group Chrysophaceae contain the pigment fucoxanthin which gives them a golden color.
Phytoplankton are microscopic floating photosynthetic organisms in aquatic environments, both freshwater and seawater. In seawater, the most common types of phytoplankton are diatoms and dinoflagellates. In the oceans, they are responsible for most of the primary production (photosynthesis). Their photosynthetic activities remove carbon dioxide from the environment and release oxygen; thus they are responsible for mitigating some of the effects of increased carbon dioxide in the atmosphere. Since they need sunlight in order to photosynthesize, they are found only in the upper, sunlit layers of the water. When excessive nutrients are present, there may be excessive blooms of phytoplankton, which when they die and sink to the bottom, may use up much of the oxygen in the deeper water and create a hypoxic layer. Some species of phytoplankton may produce toxins, and a bloom of such a species is referred to as a "harmful algal bloom" since they may poison other marine organisms.
Algae come in a variety of shapes and in varied colors due to their different photosynthetic pigments. Algae can be unicellular and microscopic or colonial forming plate-like colonies, thread-like filaments, net-like tubes, or hollow balls. Many planktonic algae species bear horns, ridges or wings to increase their surface area to volume ratio which not only increases their ability to obtain scarce nutrients from the environment, but also protects them from herbivores. Some multicellular brown algae form branched filaments or foliose plants many meters long with complex anatomy. Individual diatoms range in size from 2 microns to several millimeters, although there are only a few species that are larger than 200 microns. Some algae, like diatoms, are encased in a siliceous cell wall which takes the form of a box and comes in many unique and beautiful forms.
Reproduction in algae is either sexual or asexual. Cyanobacteria only reproduce asexually, with genetic recombination accomplished through transformation or conjugation. During transformation DNA, released from donor cells, is incorporated into recipient cells. Conjugation is the process by which two cells become connected by a narrow tube, through which DNA moves from one cell into the other. Other algal groups use various sexual or asexual methods which are fairly group specific, but some features are shared. When reproducing asexually most groups develop spores within a parent cell which are released into the environment. Each of these spores develops into a single algal cell or they divide mitotically into numerous cells. For example, reproduction in the cryptomonads is asexual and primarily via longitudinal cell division with the cell dividing in either a free-swimming or nonmotile condition. Sexual reproduction is not rare but occurs less often than asexual reproduction. Gametes are produced in different algal cells and released into the environment. Male gametes usually attach themselves to female gametes and their genetic material is absorbed through the cell coat of the female. In freshwater species, this is followed by the formation of a cyst in which meiosis takes place.