Effects of future climate conditions on photosynthesis and biochemical component of Ulva pertusa (Chlorophyta)

Abstract

Ulva pertusa, a common bloom-forming green alga, was used as a model system to examine the effects of elevated carbon dioxide (CO2) and temperature on growth and photosynthetic performance. To do this, U. pertusa was grown under four temperature and CO2 conditions; ambient CO2 (400 μatm) and temperature (16°C) (i.e., present), elevated temperature only (19°C) (ET; i.e., warming), elevated CO2 only (1,000 μatm) (EC; i.e., acidification), and elevated tempera ture and CO2 (ET and EC; i.e., greenhouse), and its steady state photosynthetic performance evaluated. Maximum gross photosynthetic rates (GPmax) were highest under EC conditions and lowest under ET conditions. Further, ET conditions resulted in decreased rate of dark respiration (Rd), but growth of U. pertusa was higher under ET conditions than under ambient temperature conditions. In order to evaluate external carbonic anhydrase (eCA) activity, photosynthesis was measured at 70 μmol photons m -2 s -1 in the presence or absence of the eCA inhibitor acetazolamide (AZ), which inhibited photosynthetic rates in all treatments, indicating eCA activity. However, while AZ reduced U. pertusa photosynthesis in all treatments, this reduction was lower under ambient CO2 conditions (both present and warming) compared to EC conditions (both acidification and greenhouse). Moreover, Chlorophyll a and glucose contents in U. pertusa tissues declined under ET conditions (both warming and greenhouse) in conjunction with reduced GPmax and Rd. Overall, our results indicate that the interaction of EC and ET would offset each other’s impacts on photosynthesis and biochemical composition as related to carbon balance of U. pertusa.