Seaweeds alter ocean carbon chemistry and carbon access through changes in seawater alkalinity

Background/Question/Methods

Changes in the global carbon cycle arising from anthropogenic CO₂ release to the atmosphere may affect, and be affected by, many species within ecosystems. For example, marine macroalgae employ various mechanisms to concentrate dissolved inorganic carbon (DIC) for photosynthesis, including using bicarbonate (HCO₃^-), which can change seawater pH. However, changes in carbon budgets arising from shifts in carbon concentration mechanisms (CCMs) are not currently incorporated into models of climate change. Seaweeds change seawater carbon chemistry via respiration and photosynthesis and contribute to variation in ocean pH in nearshore environments. In addition to these two mechanisms, there is growing evidence that organisms can also affect total alkalinity (TA), which reflects the acid buffering capacity of the water and feeds back into the ocean carbonate system. To determine how macroalgae influence seawater chemistry, we used pH drift experiments, which measure the ability to deplete DIC, and concomitant TA measurements to assess the variability in CCMs in marine macroalgae from the rocky intertidal of the Northeast Pacific, an area currently undergoing rapid declines in ocean pH. Relationships of CCMs, TA alteration and organismal traits, including functional group, macroalgal evolutionary lineage, surface area to biomass ratio, and vertical habitat range limit, were also explored.

Results/Conclusions

Our pH drift experiments confirmed CCMs, in the form of HCO₃^- use, in 31 of 39 Northeast Pacific intertidal species. Analysis of seawater for 22 species revealed changes in TA of –1170 to +60 micromole/kg seawater (SW) relative to control seawater, indicating that macroalgae not only change pH, but alter the expected relationship between pH and alkalinity. Decreases in TA can shift DIC composition from carbonate (CO₃^2-) toward increasing HCO₃^- and CO₂ concentrations, forms which can be used for photosynthesis. Calculated changes in HCO₃^- ranged from -181 to +358 micromole/kgSW, corresponding to 70-1600% of expected values. Carbon dioxide changes ranged from -3 to +5 micromole/kgSW, proportionately large increases (35-83000%). Functional group, macroalgal lineage and habitat range limits were significant predictors of pH and TA shifts. Changes in seawater alkalinity may thus influence carbon availability in boundary layers and areas of low water mixing. Furthermore, altering alkalinity may provide an alternative mechanism of carbon dioxide concentration in species that cannot utilize bicarbonate. Macroalgal-driven changes in alkalinity present a novel biological pathway affecting ocean carbon chemistry.