Photosynthesis can buffer ocean acidification (OA) by raising seawater pH, favouring calcification. Thus, incorporating macroalgae into bivalve farms may help mitigate OA impacts, while also enhancing biomass production. We present two laboratory experiments addressing this potential.
First, we evaluated the effect of the blue mussel Mytilus chilensis and three macroalgae (Ulva sp., Chondracanthus chamissoi , Macrocystis pyrifera) on seawater chemistry in mono- and co-cultures under ambient and OA conditions. Macroalgal monocultures increased pH and aragonite saturation state (Ωara) compared to mussel monoculture, while co-cultures showed intermediate values. A daytime buffering effect (pH > 7.7, Ωara > 1) occurred in co-cultures, while nighttime conditions remained unfavourable for calcification (Fig. 1).
Second, we assessed optimal initial biomass ratios in four macroalga–bivalve combinations (C. chamissoi:Magallana gigas , Sarcopeltis skottsbergii:M. gigas , C. chamissoi:M. chilensis , Gracilaria chilensis:M. chilensis) over 28–30 days using the De Wit replacement series. A synergistic interaction emerged, with 2:1 (macroalga:bivalve) as the optimal ratio (Fig. 2) , driven by the faster growth rates of macroalgae (2–8 times greater than that of bivalves.
These results indicate that macroalgae can enhance biomass production in co-cultures but do not guarantee favourable calcification conditions under OA. Further long-term (>30 days) studies in laboratory and field settings are needed to assess co-culture benefits fully.