Limits to the Positive Effect of Ocean Acidification on Macroalgal Primary Production, Interactions with Light and Temperature

About one third of macroalgal species lack any carbon concentrating mechanism (CCM), which prevents carbon limitation under air equilibrium in other seaweed species. It is predicted that those species lacking CCM’s will benefit from ongoing ocean acidification in terms of primary productivity and growth. The absolute sizes and pattern of those benefits are not known. Here, we compare the results of a model based on composite data from the literature, with a growth experiment using Plocamium cartilagineum, a broadly distributed rhodophyte species lacking a carbon concentrating mechanism and hypothesized to be carbon limited under current conditions. We grew P. cartilagineum, at 15 and 20°C in seawater aerated with a total of 53 different pCO₂s (from 344 to 1053µatm), in 8 multiweek trials over 12 months. We measured growth and photosynthetic rates. A linear mixed model analysis was used to partition the effect sizes of drivers of variation in the experiment. The growth rates and maximum photosynthetic rates responded nonlinearly to OA, increasing with elevated pCO₂ from recent atmospheric level to up 450µatm and decreasing at higher pCO₂. Light harvesting efficiency was unaffected by pCO₂ and inversely related to temperature. We were able to compare the results of the growth experiment directly to the model based on the additive effects of temperature and pCO₂ on photosynthetic rates, finding concordance of the pattern of response. The size of the effect of pCO₂ on growth rate in the experiment was greater than the effect predicted by the model for net primary productivity. These results predict that the benefit of OA for macroalgal growth may disappear as ocean acidification continues through this century.