At the moment, we are especially interested in the Rhizaria, one special zooplankton group. Rhizaria can not only be very beautiful, but they may also play an important role for the climate. On this page we present you an overview of what Rhizarians are and why we think they are important. Here we present the different categories.
Rhizaria are single-celled organisms with a nucleus. Their size ranges between a few hundred micrometres and several centimetres. All Rhizaria have pseudopods, which are used for feeding or locomotion.
Rhizaria are extremely diverse in their structure and shape. Some Rhizaria build housings made of calcite, others have spines or whole skeletons made of silica. Already in the 19th century, Ernst Haeckel documented this impressive diversity in very well-known drawings. You can find these drawings here.
Rhizaria are not only diverse in their morphology - they also have very different feeding strategies: Many species feed on other organisms, such as microzooplankton, or collect particles by secreted mucus. Some, however, can additionally harbor tiny algae and gain energy via symbiosis. This enables them to live in nutrient-poor regions of the ocean. They contribute 0.1-0.4% of the ocean’s yearly primary productivity.
Especially in the tropical and subtropical ocean, a substantial part of the zooplankton abundance and biomass is made up by Rhizaria. For example, in the Californian upwelling region, 81% of the zooplankton are Rhizaria. Because Rhizaria - in contrast to other single-celled organisms - can become very big, they may play an important role in the marine food web and in the transport of carbon from the surface ocean to the deep sea.
Rhizaria are an important nutritional basis for other zooplankton. Because they - in contrast to other single-celled organisms - can become very big, they may represent a shortcut in the marine foodweb. This means that several steps in the energy transfer between primary producers (phytoplankton) and top predators (e.g. fishes) are left out, resulting in a smaller energy loss.
Rhizaria may provide an important vector for carbon transport from the surface to the deep sea and thus may play an important role for our climate. The ocean takes up about one third of the anthropogenically produced carbon dioxide. An important process for this uptake is the biological carbon pump.
CO2 is introduced to the surface ocean by exchange processes with the atmosphere. Primary producers, such as phytoplankton and the symbiotic algae of the Rhizaria, take up the dissolved carbon and convert it into organic material and oxygen with the aid of sunlight (photosynthesis). Phytoplankton are eaten by small zooplankton, which are eaten by larger zooplankton and so on. All this life produces small to large particles - be it dead cells or excreted fecal pellets - and a part of these particles sinks through the surface layer down to the deep sea and finally accumulates as sediments on the bottom of the ocean. This way, the carbon is removed from the carbon cycle and only reemerges after thousands of years.
One group within the Rhizaria, the Phaeodarea, was found to be related to very high particle numbers and fluxes in the North Atlantic Ocean. Especially concerning climate change, it is important to find out as much as possible about such climate-relevant processes. That is why we want to investigate whether the enhanced concentrations of Rhizaria in upwelling areas are also correlated with high particle fluxes and what their role in particle consumption is.
