Sinking diatoms trap silicon in deep seawater of acidified oceans

The seas are acidifying as a result of carbon dioxide emissions. It now emerges that this will alter the solubility of the shells of marine organisms called diatoms — and thereby change the distribution of nutrients and plankton in the ocean.

The ecologically dominant phytoplankton in much of the ocean are a group of unicellular organisms known as diatoms. Writing in Nature, Taucher et al. present a study that uses a combination of experimental, observational and modelling approaches to examine how the diatom-driven effects of ocean acidification — a consequence of rising carbon dioxide concentrations in seawater — will affect biogeochemical cycles. The separate lines of evidence suggest that ocean acidification will have far-reaching effects on the export of elements to the deep ocean.

Diatoms are highly efficient at converting dissolved CO2 into organic carbon through photosynthesis, whereupon this organic carbon becomes incorporated into particles that sink rapidly to the deep ocean. Diatoms therefore serve as primary engines of a ‘biological pump’ that exports carbon to the deep ocean for sequestration. Each diatom cell is enclosed in a shell of silica (SiO2, where Si is silicon), and the solubility of the silicon in this biomineral is pH-sensitive — it becomes less soluble as seawater acidity rises. Although these features of diatoms are familiar to marine scientists, their combined implications for future biogeochemical cycles in the context of ocean acidification had not been explored.

Enter Taucher and colleagues. They carried out a series of five experiments in various parts of the ocean in which natural phytoplankton communities were grown in large enclosures (with volumes of 35–75 cubic metres) known as mesocosms, which simulated future ocean acidification. When the authors measured the elemental composition of the diatom-derived debris at the bottom of the mesocosms, they observed much higher ratios of silicon to nitrogen than the ratios of particles suspended near the surface. This suggested that, at low seawater pH, diatom silica shells were dissolving much more slowly than nitrogen-containing compounds in the same sinking material. In other words, silicon was being exported from the surface to deeper waters preferentially to nitrogen. The authors validated this finding using records of silicon-to-nitrogen ratios in sinking biological detritus in the open ocean, measured as a function of seawater pH, and obtained from particle-collecting sediment traps deployed by research vessels.

Oceanographers have long known that silicon in diatom shells has a deeper remineralization depth profile than those of elements such as carbon and nitrogen in sinking particles, meaning that silicon is converted more slowly to dissolved forms as particles sink. Silicon thus becomes progressively enriched in particles as they descend the water column, relative to the concentrations of other elements. The advance in Taucher and colleagues’ study is the finding that ocean acidification substantially magnifies the existing difference in the dissolution rates of elements in sinking diatom-cell debris.

The authors integrated their findings into a sophisticated biogeochemical model that extrapolated this differential remineralization to the year 2200. Their modelling suggests that widespread acidification could result in much of the marine silicon inventory becoming trapped in the deep ocean, as a result of the downward transport of highly silicon-enriched particles by the biological pump (Fig. 1). This implies that the amount of dissolved silicon returned to the upper ocean by large-scale, mid-depth ocean circulation patterns will decrease, thereby reducing the amount of this essential nutrient that is available to support diatom growth in sunlit surface waters. The model therefore predicts precipitous declines in worldwide diatom abundance in the future, because diatoms will be starved of the dissolved silicon required to build their shells.

Figure 1 | Solubility of diatom shells in acidified oceans affects marine silicon fluxes.

Hutchins D. A., 2022. Sinking diatoms trap silicon in deep seawater of acidified oceans. Nature 605: 622-623. Article (subscription required).

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