[AOS27-P02] Effect of ocean acidification on bioavailability of iron bound to natural iron-complexing ligands
Keywords:Ocean acidification, Iron, Phytoplankton, Organic ligand
We performed laboratory culture experiments using a marine diatom Thalassiosira sp. isolated from the HNLC water of the western subarctic Pacific, and its growth responses at low and high pH/pCO2 were compared between surface waters collected from the western and eastern subarctic Pacific and subsurface water of the California coastal upwelling region. Before the inoculation of iron-limited diatom cells, the filtered seawater was spiked with 0.5 nM FeCl3 and allowed to equilibrate for 24 hour with natural iron-binding ligand, and pH/pCO2 was manipulated by bubbling the CO2:air mixed gases.
The specific growth rate of diatom in the surface water from the western subarctic Pacific (Stn. K2) decreased with decreasing pH from 7.9-8.0 to 7.5-7.6. The observed increase in the consumption ratio of silicic acid and nitrate (Si/N) supports decreased iron availability at lower pH. Opposite trend was observed in the subsurface water of California coastal region. Decreasing the pH increased the specific growth rate, indicating increased iron availability. The relatively low growth rate observed at higher pH suggests that diatoms can not efficiently use iron bound to natural iron-complexing ligands in the subsurface water. The effect of pH in experiments using surface seawater collected from the eastern subarctic Pacific (Stn. P) was not statistically significant, but there was an increasing trend in the specific growth rate with decreasing pH. These results suggest that ocean acidification over the next century will alter the availability of iron in these surface waters differently and may affect future primary production.