In some classifications, a phylum (or group, i.e. the bacillariophytes) of protist algae comprising the diatoms and belonging to the stramenopiles. These microscopic marine or freshwater unicellular organisms have cell walls (frustules) composed of pectin impregnated with silica and consisting of two halves, one overlapping the other. Diatoms are found in huge numbers in plankton and are important in the food chains of seas and rivers. Past deposition has resulted in diatomaceous earths (kieselguhr) and the oil reserves of these species have contributed to oil deposits. The global abundance of diatoms means that they play a significant role in absorbing carbon dioxide from the atmosphere. This has led to the proposal that encouraging blooms of diatoms by fertilizing the oceans with essential nutrients may be a means of capturing this greenhouse gas and storing it in the bodies of diatoms on the ocean floor.