High prevalence of diffusive uptake of CO2 by macroalgae in a temperate subtidal ecosystem

Productivity of most macroalgae is not currently considered limited by dissolved inorganic carbon (DIC), as the majority of species have CO₂-concentrating mechanisms (CCM) allowing the active uptake of DIC. The alternative, diffusive uptake of CO₂ (non-CCM), is considered rare (0–9 % of all macroalgal cover in a given ecosystem), and identifying species without CCMs is important in understanding factors controlling inorganic carbon use by eukaryotic algae. CCM activity has higher energetic requirements than diffusive CO₂ uptake, therefore when light is low, CCM activity is reduced in favour of diffusive CO₂ uptake. We hypothesized that the proportional cover of macroalgae without CCMs (red and green macroalgae) would be low (<10 %) across four sites in Tasmania, southern Australia at two depths (4–5 and 12–14 m); the proportion of species lacking CCMs would increase with decreasing depth; the δ¹³C values of macroalgae with CCMs would be more depleted with depth. We found the proportion of non-CCM species ranged from 0 to 90 % and included species from all three macroalgal phyla: 81 % of red (59 species), 14 % of brown (three species) and 29 % of green macroalgae (two species). The proportion of non-CCM species increased with depth at three of four sites. 35 % of species tested had significantly depleted δ¹³C values at deeper depths. Non-CCM macroalgae are more abundant in some temperate reefs than previously thought. If ocean acidification benefits non-CCM species, the ramifications for subtidal macroalgal assemblages could be larger than previously considered.