2. Effects of ocean acidification on the photosynthetic performance, carbonic anhydrase activity and growth of the giant kelp Macrocystis pyrifera

Pamela A. Fernandez (1)*, Michael Y. Roleda (2), Catriona L. Hurd (3)

1 Department of Botany, University of Otago, Dunedin 9054, New Zealand.
2 Bioforsk Norwegian Institute for Agricultural and Environmental Research, Kudalsveien 6, Bodø 8049, Norway.
3 Institute for marine and Antarctic Studies (IMAS), University of Tasmania, TAS 7001, Australia.

Background
Temperate kelp forests play an important role in coastal environments, contributing 50% of primary production and providing food and habitat for a variety of marine invertebrates. By 2100, an increase of 192% and 14% in CO2(aq) and HCO3, respectively, is expected due to ocean acidification (OA). Most seaweeds can take up both CO2 and HCO3 as an inorganic carbon (Ci) source for photosynthesis. The projected changes in seawater carbonate chemistry may affect the ability of seaweeds to acquire Ci and, consequently, their photosynthetic performance and productivity. However, a seaweed’s response to OA will depend on their carbon physiology. This study evaluates how the relative increase in different Ci species will affect the enzyme carbonic anhydrase (CA) (external and internal), which plays an essential role in Ci acquisition, and hence photosynthesis, and growth rates of the habitat-forming giant kelp Macrocystis pyrifera.

Methods
Response variables were measured in blade discs of individuals of M. pyrifera sporophytes cultivated under two CO2(aq) concentrations (400 vs. 1,200 µatm) for 10 days in laboratory. Sampling was conducted in a time series, i.e. every 3 days, to determine regulatory and/or acclimatory processes in the carbon physiology of M. pyrifera in response to OA.

Findings
Photosynthetic and growth rates were not affected by CO2(aq). Internal CA was considerably higher than external CA under both CO2 treatments. However, external CA was reduced on day 7 of incubation in the elevated CO2(aq) treatment, but it remained active. In contrast, internal CA remained high and active across all sampling days under both CO2 treatments. Stables isotopes (Ϩ15N and Ϩ13C), carbon and nitrogen content and C:N ratio were not affected by CO2(aq).

Conclusions
The giant kelp, M. pyrifera, has two effective mechanisms for HCO3 utilization and it is not limited at current Ci concentrations. Therefore, increased CO2(aq) under an OA scenario will not affect the growth and photosynthetic performance of the giant kelp.