Algal density mediates the photosynthetic responses of a marine macroalga Ulva conglobata (Chlorophyta) to temperature and pH changes

Abstract Growing of macroalgae increases their biomass densities in natural habitats. To explore how the altered algal density impacts their photosynthetic responses to changes of environmental factors, we compared the photosynthesis versus irradiance characteristics of a marine green macroalga Ulva conglobata under low [2.0 g fresh weight (FW) L−1], medium (6.0 g FW L−1) and high biomass densities (12.0 g FW L−1), and under a matrix of temperatures (20, 25, 30 and 35 °C) and pH levels (7.8, 8.2 and 8.6). Increased algal densities decreased the photosynthetic O2 evolution rate among all combined temperature and pH treatments, in parallel with the decrease of light-utilizing efficiency (α, the initial slope) and maximum photosynthetic rate (Pmax) and the increase of light saturation point (EK). Rising temperature interacted with lowered pH to increase the α under low but not under high algal densities. Rising temperature increased the Pmax and decreased the EK under low algal density, but not under high density. Lowered pH promoted the Pmax and EK under all three algal densities. The increased temperature enhanced the dark respiration (Rd) and light compensation point (EC), while the altered pH showed a limited effect. Moreover, the increased algal density reduced the Rd, and had a limited effect on the EC. In addition, our results indicate that changing algal densities caused the complex photophysiological changes in responses to the temperature and pH changes, and these complex responses resolved into a close relation between Rd and Pmax across the matrix of temperatures and pH levels.