Tracking single coccolith dissolution with picogram resolution and implications for CO2 sequestration and ocean acidification

Abstract

Coccoliths are micrometer scale shields made from 20 to 60 individual calcite (CaCO(3)) crystals that are produced by some species of algae. Currently, coccoliths serve as an important sink in the global carbon cycle, but decreasing ocean pH challenges their stability. Chalk deposits, the fossil remains of ancient algae, have remained remarkably unchanged by diagenesis, the process that converts sediment to rock. Even after 60 million years, the fossil coccolith crystals are still tiny (< 1 μm), compared with inorganically produced calcite, where one day old crystals can be 10 times larger, which raises the question if the biogenic nature of coccolith calcite gives it different properties than inorganic calcite? And if so, can these properties protect coccoliths in CO(2) challenged oceans? Here we describe a new method for tracking dissolution of individual specimens, at picogram (10(-12) g) resolution. The results show that the behavior of modern and fossil coccoliths is similar and both are more stable than inorganic calcite. Organic material associated with the biogenic calcite provides the explanation. However, ancient and modern coccoliths, that resist dissolution in Ca-free artificial seawater at pH > 8, all dissolve when pH is 7.8 or lower. Ocean pH is predicted to fall below 7.8 by the year 2100, in response to rising CO(2) levels. Our results imply that at these conditions the advantages offered by the biogenic nature of calcite will disappear putting coccoliths on algae and in the calcareous bottom sediments at risk.

Fig. 1.

SEM images of (A) chalk extracted from more than 60 million year old sediments below the North Sea and (B) calcite formed inorganically under ambient conditions in the laboratory (about a day).

Fig. 2.

Dissolving calcite. (A) Mass loss of pure, synthetic calcite crystals in pure DI water; (B) SEM of the single particle (mass ∼4.4 ng) glued to a cantilever; (C) glue residue after dissolution; (D) mass loss of one Iceland spar calcite cleavage fragment exposed to ASW in tests where Ca concentration was slightly above saturation and where the solution was Ca-free and finally pure DI water; (E) calcite mass approximately 420 pg; and (F) glue residue after dissolution was complete.

Fig. 3.

Dissolving coccoliths. (A) Mass loss from one cultured coccolith exposed to Ca-free ASW (Ω_calcite estimated in each solution aliquot at the end of the dissolution period, i.e., representing each data point, < 10^-8) and then to pure water (estimated Ω_calcite in each aliquot < 10^-12); (B) a coccolith from C. pelagicus imaged at t = 6,000 s; (C) after complete dissolution, an organic relic beautifully reflects coccolith structure and morphology; (D) mass loss from a 60 million year old coccolith (Arkhangelskiella sp. cf. cymibiformis); (E) initial SEM image, then exposure to Ca-free ASW, pH 8.4 (t > 1,500 s) removed loose particles (arrows), presumed to be nonbiogenic calcite, but the coccolith remained; (F) the same coccolith after 3,200 s, just before exposure to pure water; and (G) complete dissolution left an unstructured relic.