Effects of pH, Temperature, and Light on the Inorganic Carbon Uptake Strategies in Early Life Stages of Macrocystis pyrifera (Ochrophyta, Laminariales)

The responses of seaweed species to increased CO₂ and lowered pH (Ocean Acidification: OA) depend on their carbon concentrating mechanisms (CCMs) and inorganic carbon (Ci) preferences. However, few studies have described these mechanisms in the early life stages of seaweeds or assessed the effects of OA and its interactions with other environmental drivers on their functionality and photophysiology. Our study evaluated the effects of pH, light (PAR), temperature, and their interactions on the Ci uptake strategies and photophysiology in the early stages of Macrocystis pyrifera. Gametophytes were cultivated under varying pH (7.80 and 8.20), light (20 and 50 µmol photons m^-2s^-1), and temperature (12 and 16 °C) conditions for 25 days. We assessed photophysiological responses and CCMs (in particular, the extracellular dehydration of HCO₃^- to CO₂ mediated by the enzyme carbonic anhydrase (CA) and direct HCO₃^- uptake via an anion exchange port). This study is the first to describe the Ci uptake strategies in gametophytes of M. pyrifera, demonstrating that their primary CCM is the extracellular conversion of HCO₃^- to CO₂ mediated by CA. Additionally, our results indicate that decreased pH can positively affect their photosynthetic efficiency and maximum quantum yield; however, this response is dependent on the light and temperature conditions.