The primary contributors to the global silicon cycle are single-celled algae called diatoms, which use silicon dissolved in seawater to build elaborate, rigid shells called frustules of silicon dioxide, or silica. In a new study, Llopis Monferrer et al. show that another group of planktonic organisms, known as Rhizaria, may produce up to 19% of the total amount of biogenic silica in the ocean.
Rhizaria are a diverse group of single-celled organisms that, like diatoms, use dissolved silicon to construct silica frustules. These shells can serve a variety of functions, such as protecting cell structure and providing armor against predators.
Although diatoms are photosynthetic and reside in upper water layers, most Rhizaria are heterotrophic (they rely on external sources for food) and live throughout the open ocean. A growing amount of evidence suggested that Rhizaria may play an unrecognized role in biogenic silica production, but until now, estimates of their contribution remained highly speculative.
To generate the first estimates of the contribution of Rhizaria to the world’s biogenic silica production, the researchers collected plankton samples at 22 sites in the Mediterranean Sea. Focusing on two major groups of Rhizaria—polycystines and phaeodarians—the team conducted experiments using radioactively labeled silicon to measure the organisms’ silica production rates.
Then the scientists combined the results of their experiments with previously published data on the abundance of polycystines and phaeodarians throughout the world’s oceans. The analysis revealed that the shell-building activities of Rhizaria could account for 1%–19% of the total amount of oceanic silicon incorporated into biogenic silica every year.
These findings challenge the view that diatoms have total control over oceanic silicon cycling, a process that is coupled with other biogeochemical cycles, such as the carbon and nitrogen cycles. (Global Biogeochemical Cycles, https://doi.org/10.1029/2019GB006286, 2020)
—Sarah Stanley, Science Writer