New in situ sampling method used in study
Unicellular planktonic organisms play a more important role in the marine ecosystem than previously thought. This was the conclusion drawn by marine scientists from France and the GEOMAR Helmholtz Centre for Ocean Research Kiel, who published their findings in the international journal Nature recently.
Their discovery will affect our understanding of marine food webs and the important biogeochemical cycles in our oceans.
From microscopic bacteria to the meter-long jellyfish, the term “plankton” (coined by Kiel marine biologist Victor Hensen in 1880 to mean “free-drifting”) covers a large variety of different organisms. Despite the small size of many species of plankton, they play central roles in the food webs and biogeochemical cycles of our oceans. They produce oxygen and absorb carbon dioxide from the atmosphere, while the larger species serve as a food source for fish, seabirds and whales. However, their small size and high sensitivity has proved to be a challenge for researchers in the study of plankton.
According to this study, unicellular planktonic organisms called Rhizaria comprise a much larger proportion of zooplankton than previously assumed. “So far, animal plankton was primarily equated with small copepods. This image needs to be revised,” said co-author Dr Rainer Kiko from GEOMAR.
The study is based on data collected during 20 expeditions in the past eight years in the Mediterranean, Atlantic, Pacific and Indian Oceans, using the newly developed “Underwater Vision Profiler” (EIA). This device comprises a light source and integrated camera; the light source would illuminate and photograph multiple images of a precisely defined volume of water under the ocean surface. This in situ imaging system can survey the planktonic organisms, allowing scientists to identify them and quantify their frequency.
This sampling method has significant advantages over previous methods, as organisms that are at least half a millimetre can be studied without having to remove them from their habitats and interfere with their environment. Co-author Dr Helena Hauss from GEOMAR commented that before this method, plankton had been caught with fine-mesh nets, brought on board and then counted in the laboratory. However, this had led to the loss of many Rhizaria as they were destroyed in the course of the sampling.
As the sampling has revealed, the proportion of Rhizaria in the oceans is far greater than what one would have anticipated based on the previous net catches – on average, they account for one-third of zooplankton. In addition, they have been found to be strongly represented in the nutrient-poor tropical oceans.
In biogeochemical models to food webs, or the influence of plankton on the carbon cycle and climate, the Rhizaria has been hardly mentioned - this must change now. In the open tropical oceans, they play a similarly important function as the corals in coastal areas.
Link to study: www.nature.com/.../nature17652.html