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. 2024 Nov 8;10(45):eadr5453.
doi: 10.1126/sciadv.adr5453. Epub 2024 Nov 8.

Microzooplankton grazing on the coccolithophore Emiliania huxleyi and its role in the global calcium carbonate cycle

Affiliations

Microzooplankton grazing on the coccolithophore Emiliania huxleyi and its role in the global calcium carbonate cycle

Chloe L Dean et al. Sci Adv. .

Abstract

Identifying mechanisms driving the substantial dissolution of biogenic CaCO3 (60 to 80%) in surface and mesopelagic waters of the global ocean is critical for constraining the surface ocean's alkalinity and inorganic carbon budgets. We examine microzooplankton grazing on coccolithophores, photosynthetic calcifying algae responsible for a majority of open-ocean CaCO3 production, as a mechanism driving shallow dissolution. We show that microzooplankton grazing dissolves 92 ± 7% of ingested coccolith calcite, which may explain 50 to 100% of the observed CaCO3 dissolution in supersaturated surface waters. Microzooplankton grazing on coccolithophores is thus a substantial, previously unrecognized biological mechanism affecting the ballasting of organic carbon to deeper waters, the ecology and fitness of microzooplankton themselves due to buffering of food vacuole pH, and ultimately the continued ability of the surface ocean to take up atmospheric carbon dioxide.

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Figures

Fig. 1.
Fig. 1.. MZP digestion promotes dissolution of coccolithophore PIC.
(A) PIC (micromoles) ingested and egested (PIC that remained after the digestion period) for each MZP. (B) % PIC dissolved based on the difference between PIC ingested and egested. (C) Summary plot of the nanomolar change in DI13C for all grazing experiment treatments, including uncalcified naked controls (purple boxes), prey only (“w/o grazing”; red median line), and grazing treatments (“w/ grazing”; black median line). Zero line is drawn on (C) to indicate no change in DI13C, where any points above the zero line signify molar enrichment of 13C and points below the zero line indicate molar depletion of 13C. All calcified and naked prey treatments where grazing occurred fall above the zero line, indicating release of 13C from the digestion of labeled biomaterials. All control treatments without grazing fall at or below the zero line.
Fig. 2.
Fig. 2.. LysoSensor tracks MZP vacuole pH evolution during digestion of calcified and noncalcified prey.
(A to D) Image series for O. mar food vacuoles 3 hours after ingesting calcified E. hux. (A) Bright-field image showing engulfed prey and free prey. Scale bar, 5 μm. (B) False-color blue light image showing chlorophyll fluorescence of ingested and free prey. (C) False-color ultraviolet (UV) light image showing LysoSensor probe fluorescence within MZP vacuoles containing prey. (D) False-color composite of images (A to C) overlaid to aid in the visualization of colocalized prey and LysoSensor fluorescence. (E) Scatterplot of corrected total cell fluorescence (CTCF); purple circles show CTCF for freshly engulfed prey (t = 0 hours), green triangles show ingestion of calcified prey (CCMP374-C) after 3 hours, and tan squares show ingestion of noncalcified prey (CCMP1323) after 3 hours.
Fig. 3.
Fig. 3.. Fate of biogenic calcium carbonate within the euphotic and mesopelagic ocean.
Values in boxes represent the upper and lower bounds for global CaCO3 fluxes. Arrows represent the flux of CaCO3 in percentage, with letters indicating each distinct process. The top dashed line corresponds to the euphotic zone (~200 m), the middle dashed line represents the aragonite saturation horizon, and the bottom dashed line represents the calcite saturation horizon (where all forms of calcium carbonate are undersaturated). Shallow CaCO3 production values are taken from (2, 10). (A) Portion of global CaCO3 production attributed to large calcifying organisms (10) which sink directly out of the euphotic zone with minimal shallow dissolution. (B) Portion of global CaCO3 production attributed to fish—now unconstrained (32). (C) Portion of global CaCO3 production attributed to coccolithophores (10). (D) Portion of coccolithophore production that is not directly grazed and is available for viral lysis and/or aggregation into sinking particles (16, 17). (E) Portion of coccolithophore production grazed by mesozooplankton [12%; (16, 17)] and subsequently dissolved [38%; (9)]. (F) Portion of coccolithophore production grazed by MZP [60%; (16, 17)] and subsequently dissolved (92%; this study). (G) Estimated extent of dissolution occurring in “mid-saturated” waters [where waters are supersaturated with respect to calcite but undersaturated with respect to aragonite; 29%; (6)].

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