Diversity, Ecology and Biogeochemistry of cyst-forming acantharia (Radiolaria) in the oceans

Sampling

Marine planktonic organisms that undertake active vertical migrations over their life cycle are important contributors to downward particle flux in the oceans. Acantharia, globally distributed heterotrophic protists that are unique in building skeletons of celestite (strontium sulfate), can produce reproductive cysts covered by a heavy mineral shell that sink rapidly from surface to deep waters. We combined phylogenetic and biogeochemical analyses to explore the ecological and biogeochemical significance of this reproductive strategy. Phylogenetic analysis of the 18S and 28S rRNA genes of different cyst morphotypes collected in different oceans indicated that cyst-forming Acantharia belong to three early diverging clades from the orders Chaunacanthida and Holacanthida that are mainly heterotrophic. Environmental high-throughput V9 tag sequences and clone libraries showed that the three clades are widely distributed in the Indian, Atlantic and Pacific Oceans at different latitudes, but seem more abundant in regions of higher primary productivity. Moreover, sequences of cyst-forming Acantharia were distributed evenly in both the photic and mesopelagic zone, a vertical distribution that we attribute to their life cycle where flagellated swarmers are released in deep waters from sinking cysts. Bathypelagic sediment traps in the subantarctic and oligotrophic subtropical Atlantic Ocean showed that downward flux of Acantharia was only large at high-latitudes and during a phytoplankton bloom. High organic carbon export in cold waters would be a putative nutritional source for non-symbiotic juveniles ascending in the water column. This study improves our understanding of the life cycle and biogeochemical contribution of Acantharia, and brings new insights into a remarkable reproductive strategy in marine protists

Method steps

1. Pipeline used: https://www.ebi.ac.uk/metagenomics/pipelines/4.1