In situ imaging reveals the biomass of giant protists in the global ocean

Abstract

Planktonic organisms play crucial roles in oceanic food webs and global biogeochemical cycles. Most of our knowledge about the ecological impact of large zooplankton stems from research on abundant and robust crustaceans, and in particular copepods. A number of the other organisms that comprise planktonic communities are fragile, and therefore hard to sample and quantify, meaning that their abundances and effects on oceanic ecosystems are poorly understood. Here, using data from a worldwide in situ imaging survey of plankton larger than 600 μm, we show that a substantial part of the biomass of this size fraction consists of giant protists belonging to the Rhizaria, a super-group of mostly fragile unicellular marine organisms that includes the taxa Phaeodaria and Radiolaria (for example, orders Collodaria and Acantharia). Globally, we estimate that rhizarians in the top 200 m of world oceans represent a standing stock of 0.089 Pg carbon, equivalent to 5.2% of the total oceanic biota carbon reservoir. In the vast oligotrophic intertropical open oceans, rhizarian biomass is estimated to be equivalent to that of all other mesozooplankton (plankton in the size range 0.2-20 mm). The photosymbiotic association of many rhizarians with microalgae may be an important factor in explaining their distribution. The previously overlooked importance of these giant protists across the widest ecosystem on the planet changes our understanding of marine planktonic ecosystems.

Optics-based surveys of large unicellular zooplankton: a case study on radiolarians and phaeodarians

“…2L, M). Their colonies have also been photographed in other optics-based surveys (Ichikawa 2008, Biard et al 2016. Previous studies based on plankton net sampling examined aggregated specimens (e.g.…”

“…It is, therefore, suggested that these unicellular zooplankton can play an important role in the silica cycle and the ecosystem in the oceans (Riemann 1989, Lampitt et al 2009, Nakamura & Suzuki 2015a. Phaeodarians and collodarians are the most important contributors to the biomass of Rhizaria, which has been estimated to comprise 33% of the total zooplankton abundance worldwide (Biard et al 2016). The vertical distribution also differs among the groups.…”

“…Despite tremendous efforts, very little data are available to estimate the abundance of collodarians (Swanberg 1974). In situ imaging has proved to be a useful tool to quantify members of this group (Dennett et al 2002, Biard et al 2016.…”

“…The order Spumellaria, possessing a skeleton made of amorphous silica, is solitary and their biomass is usually low . On the contrary, Collodaria, which also have siliceous skeletons in general, contain many colonial species, and this order sometimes exhibits high biomass (Dennett et al 2002, Biard et al 2016. Another radiolarian order, Acantharia, has a skeleton mainly composed of strontium sulfate, and therefore this order is known to contribute to the strontium cycle in the oceans (Suzuki & Aita 2011b.…”

“…Therefore, unicellular zooplankton, such as radiolarians and phaeodarians, have been largely ignored. However, studies on such unicellular plankton are important from the viewpoint of the material cycle and understanding ecosystems in the oceans (Biard et al 2016), as explained above. We, hence, focus here on the unicellular plankton that have usually been classified simply as Radiolaria in previous optics-based surveys, examine their ecology, and construct identification criteria for use in optics-based plankton studies.…”