Plankton defines a group of free floating-organisms, living in the open waters in the ocean. They are unable to move actively and are subject to the flow and waves of the water. Some are teeny tiny animals or algae (i.e. multicellular), but most of them are protists, bacteria or archaea (unicellular). (See here for more on cells, bacteria vs. protists vs. archaea, and unicellular vs. multicellular).
Phytoplankton
Some of the protists of the plankton get their food through photosynthesis. Together with cyanobacteria (a group of photosynthesising bacteria) they are called phytoplankton. The other protists and animals are part of the zooplancton. Phytoplankton, much like terrestrial plants, need sunlight to process carbon dioxyde (CO2) and transform it into sugar and oxygen. The sugar is used by the organism for the energy they need to live and grow, while the oxygen is released in the water and in the atmosphere. Because they depend on light, phytoplankton is found only in the top 100m of water near the surface.
The importance of phytoplankton
Protists and cyanobacteria are unicellular organisms, and therefore very small. It has been estimated that all together phytoplankton makes up for less than 1% of the volume of terrestrial plants. One could easily think that their importance in the grand scheme of things is therefore insignificant, but in the last 20 years, since the development of high-throughput sequencing (a technology that allows the analyses of DNA extracted directly from the water or soil or any other source of biological material), it has been shown that the biodiversity of bacteria and protists in the oceans is to count in the hundred of thousands. (See here for more on Biodiversity). Moreover, it has been estimated despite their small size, all together they produce up to half of the oxygen in the atmosphere. I think that that makes them crucially indispensable.
Food-web
The importance of plankton is not limited to the oxygen production by the phytoplankton, they are also at the base of the ocean food-web. Phytoplankton is eaten by zooplankton. Zooplankton and phytoplankton are both eaten by bigger organisms, such as krill. Krill is eaten by even bigger animals, such as squids and penguins. And somewhere in between this web there are all the species of fish that make up the fish industries. Once again, phytoplankton is crucial to the survival of many organisms, including humans.
Climate change
Because of its importance in the ocean food-web and in the cycle of carbon dioxyde, oxygen, and other nutrients, phytoplankton has increasingly taken central place in studies concerned with the consequences of climate change. The frequency of episodes of toxic blooms, i.e. sudden and unusually big growth in populations of some algae or phytoplankton that temporarily decrease the oxygen level in the water, or even produce toxins that negatively affect populations of other organisms is increasing. The acidity of the water is increasing as a consequence of the higher concentration of carbon dioxyde in the atmosphere . This could lead to a significant change of the phytoplankton that feeds all of the ocean food-web, including humans. The decline of biodiversity that we are experiencing is going to have huge and mostly unknown consequences on the life as we know it today. It is of paramount importance to understand and protect the phytoplankton and their beautiful and magical ecosystem, the ocean.
Pörtner, Hans O., and Anthony P. Farrell. Physiology and Climate Change (2008). Science 322 (5902): 690–92. doi: 10.1126/science.1163156.
Scherner F, Pereira CM, Duarte G, et al. Effects of Ocean Acidification and Temperature Increases on the Photosynthesis of Tropical Reef Calcified Macroalgae. (2016). PLoS One 11(5): e0154844. doi: 10.1371/journal.pone.0154844.
Zoe V. Finkel, John Beardall, Kevin J. Flynn, Antonietta Quigg, T. Alwyn V. Rees, John A. Raven, Phytoplankton in a changing world: cell size and elemental stoichiometry (2010). Journal of Plankton Research 32 (1): 119–137. doi: 10.1093/plankt/fbp098.