Macroalgae are responsible for a significant fraction of CO2 uptake by the ocean. Consequently, the potential of macroalgae for ocean-based carbon removal (Ocean CDR) in engineered systems needs to be assessed. This study provides a fundamental understanding of the significance of bulk fluid velocity on specific uptake rate of dissolved inorganic carbon during cultivation of the red seaweed Agarophyton vermiculophyllum.
A clonal culture of A. vermiculophyllum was immobilized onto mesh panels and cultivated in a raceway recirculation tank. The thalli tissue proliferated to form a branched, highly-porous mat aligned in the direction of bulk flow (Fig. 1). Fluid velocity around and through the seaweed biomass was measured by a pitot tube. The tissue mat was subjected to bulk velocities ranging from 10 to 56 cm/s. The carbon uptake rate at each velocity was estimated from the difference of CO2 partial pressure in the aeration gas entering and leaving the tank.
The specific carbon uptake rate of 3.6 ± 0.32 mmol C/ g AFDW-day did not change when the bulk velocity was increased (Fig 2). Velocity distribution measurements across the width of the fluid channel showed increased flow around the tissue and decreased flow through the tissue. Bulk fluid velocities were attenuated within the tissue to a narrow range of flow spanning 12 cm/s to 32 cm/s, and so the internal sections of the tissue mat were not experiencing the same fluid velocity as the exterior of the biomass. Therefore, increasing bulk velocity beyond 10 cm/s had no significant effect on the CO2 uptake rate. This study has shown that in order to better predict the effects of current flow on ocean CDR by seaweeds, hydrodynamic conditions around and within the seaweed tissue mat need to be thoughtfully considered.