Kelp gametophytes upregulate photosynthetic bicarbonate ( <scp> HCO ₃ </scp> ⁻ ) utilization in response to irradiance and temperature

Abstract Microscopic stages play an integral role in the kelp life cycle and ultimately determine settlement patterns of adult populations. Kelp gametophytes often exhibit broader thermal tolerances than adults and may enable the persistence of kelp forests under stressful conditions that adult sporophytes cannot survive. However, physiological responses to moderate increases in temperature can be complex in kelps and may depend on the ability to obtain sufficient inorganic carbon for photosynthesis. While most kelps use carbon concentrating mechanisms (CCMs) to convert abundant HCO 3 − to CO 2 for use in photosynthesis when CO 2 is limited, little is known about CCM functioning in their microscopic life stages. To better understand the role of CCMs in kelp gametophyte physiology, we measured steady‐state net and gross photosynthesis and external carbonic anhydrase (eCA)‐mediated CCM usage in gametophytes of two forest‐forming kelp species, Macrocystis pyrifera and Pelagophycus porra , under different irradiance and temperature conditions. Both species used eCA‐mediated CCMs to support 5%–40% of their photosynthetic carbon demand and increased HCO 3 − usage in response to elevated irradiance and temperature. Our results suggest that M. pyrifera and P. porra gametophytes adjust their inorganic carbon use efficiency to acclimate to environments with varying irradiance and temperature, which may enhance photosynthetic performance and survival across depth gradients.