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Boron isotope sensitivity to seawater pH change in a species of Neogoniolithon coralline red alga

Boron isotope sensitivity to seawater pH change in a species of Neogoniolithon coralline red alga
Boron isotope sensitivity to seawater pH change in a species of Neogoniolithon coralline red alga
The increase in atmospheric carbon dioxide (CO2) observed since the industrial revolution has reduced surface ocean pH by ∼0.1 pH units, with further change in the oceanic system predicted in the coming decades. Calcareous organisms can be negatively affected by extreme changes in seawater pH (pHsw) such as this due to the associated changes in the oceanic carbonate system. The boron isotopic composition (δ11B) of biogenic carbonates has been previously used to monitor pH at the calcification site (pHcf) in scleractinian corals, providing mechanistic insights into coral biomineralisation and the impact of variable pHsw on this process. Motivated by these investigations, this study examines the δ11B of the high-Mg calcite skeleton of the coralline red alga Neogoniolithon sp. to constrain pHcf, and investigates how this taxon’s pHcf is impacted by ocean acidification. δ11B was measured in multiple algal replicates (n = 4–5) cultured at four different pCO2 scenarios – averaging (±1σ) 409 (±6), 606 (±7), 903 (±12) and 2856 (±54) μatm, corresponding to average pHsw (±1σ) of 8.19 (±0.03), 8.05 (±0.06), 7.91 (±0.03) and 7.49 (±0.02) respectively. Results show that skeletal δ11B is elevated relative to the δ11B of seawater borate at all pHsw treatments by up to 18‰. Although substantial variability in δ11B exists between replicate samples cultured at a given pHsw (smallest range = 2.32‰ at pHsw 8.19, largest range = 6.08‰ at pHsw 7.91), strong correlations are identified between δ11B and pHsw (R2 = 0.72, p < 0.0001, n = 16) and between δ11B and B/Ca (R2 = 0.72, p < 0.0001, n = 16). Assuming that skeletal δ11B reflects pHcf as previously observed for scleractinian corals, the average pHcf across all experiments was 1.20 pH units (0.79 to 1.56) higher than pHsw, with the magnitude of this offset varying parabolically with decreasing pHsw, with a maximum difference between pHsw and pHcf at a pHsw of 7.91. Observed relationships between pHsw and calcification rate, and between pHsw and pHcf, suggest that coralline algae exhibit some resilience to moderate ocean acidification via increase of pHcf relative to pHsw in a similar manner to scleractinian corals. However, these results also indicate that pHcf cannot be sufficiently increased by algae exposed to a larger reduction in pHsw, adversely impacting calcification rates of coralline red algae.
0016-7037
240-253
Donald, Hannah K.
574e955f-ec6a-49e1-b86a-8bf7d1ab877c
Ries, Justin B.
e7b09c48-88cf-49ae-8eb2-f1a2f54c2449
Stewart, Joseph A.
3d7f8398-d59b-458b-a3c9-e6a54d87e9b1
Fowell, Sara E.
04bb3910-4057-4a1a-89bb-701c5c0782eb
Foster, Gavin L.
fbaa7255-7267-4443-a55e-e2a791213022
Donald, Hannah K.
574e955f-ec6a-49e1-b86a-8bf7d1ab877c
Ries, Justin B.
e7b09c48-88cf-49ae-8eb2-f1a2f54c2449
Stewart, Joseph A.
3d7f8398-d59b-458b-a3c9-e6a54d87e9b1
Fowell, Sara E.
04bb3910-4057-4a1a-89bb-701c5c0782eb
Foster, Gavin L.
fbaa7255-7267-4443-a55e-e2a791213022

Donald, Hannah K., Ries, Justin B., Stewart, Joseph A., Fowell, Sara E. and Foster, Gavin L. (2017) Boron isotope sensitivity to seawater pH change in a species of Neogoniolithon coralline red alga. Geochimica et Cosmochimica Acta, 217, 240-253. (doi:10.1016/j.gca.2017.08.021).

Record type: Article

Abstract

The increase in atmospheric carbon dioxide (CO2) observed since the industrial revolution has reduced surface ocean pH by ∼0.1 pH units, with further change in the oceanic system predicted in the coming decades. Calcareous organisms can be negatively affected by extreme changes in seawater pH (pHsw) such as this due to the associated changes in the oceanic carbonate system. The boron isotopic composition (δ11B) of biogenic carbonates has been previously used to monitor pH at the calcification site (pHcf) in scleractinian corals, providing mechanistic insights into coral biomineralisation and the impact of variable pHsw on this process. Motivated by these investigations, this study examines the δ11B of the high-Mg calcite skeleton of the coralline red alga Neogoniolithon sp. to constrain pHcf, and investigates how this taxon’s pHcf is impacted by ocean acidification. δ11B was measured in multiple algal replicates (n = 4–5) cultured at four different pCO2 scenarios – averaging (±1σ) 409 (±6), 606 (±7), 903 (±12) and 2856 (±54) μatm, corresponding to average pHsw (±1σ) of 8.19 (±0.03), 8.05 (±0.06), 7.91 (±0.03) and 7.49 (±0.02) respectively. Results show that skeletal δ11B is elevated relative to the δ11B of seawater borate at all pHsw treatments by up to 18‰. Although substantial variability in δ11B exists between replicate samples cultured at a given pHsw (smallest range = 2.32‰ at pHsw 8.19, largest range = 6.08‰ at pHsw 7.91), strong correlations are identified between δ11B and pHsw (R2 = 0.72, p < 0.0001, n = 16) and between δ11B and B/Ca (R2 = 0.72, p < 0.0001, n = 16). Assuming that skeletal δ11B reflects pHcf as previously observed for scleractinian corals, the average pHcf across all experiments was 1.20 pH units (0.79 to 1.56) higher than pHsw, with the magnitude of this offset varying parabolically with decreasing pHsw, with a maximum difference between pHsw and pHcf at a pHsw of 7.91. Observed relationships between pHsw and calcification rate, and between pHsw and pHcf, suggest that coralline algae exhibit some resilience to moderate ocean acidification via increase of pHcf relative to pHsw in a similar manner to scleractinian corals. However, these results also indicate that pHcf cannot be sufficiently increased by algae exposed to a larger reduction in pHsw, adversely impacting calcification rates of coralline red algae.

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Accepted/In Press date: 9 August 2017
e-pub ahead of print date: 24 August 2017
Published date: 1 November 2017

Identifiers

Local EPrints ID: 414216
URI: http://eprints.soton.ac.uk/id/eprint/414216
ISSN: 0016-7037
PURE UUID: 7da937e4-8693-4cbe-bcba-57267c867dfd
ORCID for Gavin L. Foster: ORCID iD orcid.org/0000-0003-3688-9668

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Date deposited: 19 Sep 2017 16:31
Last modified: 07 Oct 2020 04:23

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