Water motion and pH jointly impact the availability of dissolved inorganic carbon to macroalgae

Abstract

The supply of dissolved inorganic carbon to seaweeds is a key factor regulating photosynthesis. Thinner diffusive boundary layers at the seaweed surface or greater seawater carbon dioxide (CO2) concentrations increase CO2 supply to the seaweed surface. This may benefit seaweeds by alleviating carbon limitation either via an increased supply of CO2 that is taken up by passive diffusion, or via the down-regulation of active carbon concentrating mechanisms (CCMs) that enable the utilization of the abundant ion bicarbonate (HCO3−). Laboratory experiments showed that a 5 times increase in water motion increases DIC uptake efficiency in both a non-CCM (Hymenena palmata, Rhodophyta) and CCM (Xiphophora gladiata, Phaeophyceae) seaweed. In a field survey, brown and green seaweeds with active-CCMs maintained their CCM activity under diverse conditions of water motion. Whereas red seaweeds exhibited flexible photosynthetic rates depending on CO2 availability, and species switched from a non-CCM strategy in wave-exposed sites to an active-CCM strategy in sheltered sites where mass transfer of CO2 would be reduced. 97–99% of the seaweed assemblages at both wave-sheltered and exposed sites consisted of active-CCM species. Variable sensitivities to external CO2 would drive different responses to increasing CO2 availability, although dominance of the CCM-strategy suggests this will have minimal impact within shallow seaweed assemblages.

The diversity and coevolution of Rubisco and CO2 concentrating mechanisms in marine macrophytes

“…S1), consistent with previous studies (Cornwall et al ., 2015; Kübler & Dudgeon, 2015; Stepien et al ., 2016). However, it should be noted that this trend may not be universally applicable, as variations can occur depending on both the species and their specific environmental conditions (Marconi et al ., 2011; James et al ., 2022). Overall, the majority of data reported support the notion that light limitation at greater depths favours a diffusive CO 2 supply to Rubisco, as opposed to the more energetically demanding carbon uptake using CCMs (Raven et al ., 2005; Hepburn et al ., 2011).…”