Responses of macroalgae to CO₂ enrichment cannot be inferred solely from their inorganic carbon uptake strategy

Abstract

Increased plant biomass is observed in terrestrial systems due to rising levels of atmospheric CO2, but responses of marine macroalgae to CO2 enrichment are unclear. The 200% increase in CO2 by 2100 is predicted to enhance the productivity of fleshy macroalgae that acquire inorganic carbon solely as CO2 (non‐carbon dioxide‐concentrating mechanism [CCM] species—i.e., species without a carbon dioxide‐concentrating mechanism), whereas those that additionally uptake bicarbonate (CCM species) are predicted to respond neutrally or positively depending on their affinity for bicarbonate. Previous studies, however, show that fleshy macroalgae exhibit a broad variety of responses to CO2 enrichment and the underlying mechanisms are largely unknown. This physiological study compared the responses of a CCM species (Lomentaria australis) with a non‐CCM species (Craspedocarpus ramentaceus) to CO2 enrichment with regards to growth, net photosynthesis, and biochemistry. Contrary to expectations, there was no enrichment effect for the non‐CCM species, whereas the CCM species had a twofold greater growth rate, likely driven by a downregulation of the energetically costly CCM(s). This saved energy was invested into new growth rather than storage lipids and fatty acids. In addition, we conducted a comprehensive literature synthesis to examine the extent to which the growth and photosynthetic responses of fleshy macroalgae to elevated CO2 are related to their carbon acquisition strategies. Findings highlight that the responses of macroalgae to CO2 enrichment cannot be inferred solely from their carbon uptake strategy, and targeted physiological experiments on a wider range of species are needed to better predict responses of macroalgae to future oceanic change.

Effects of elevated pCO2 and nutrient enrichment on the growth, photosynthesis, and biochemical compositions of the brown alga Saccharina japonica (Laminariaceae, Phaeophyta)

“…On the other hand, the net photosynthetic rate and chlorophyll content were reduced under the high pCO 2 levels. Similar results have been shown for Alaria esculenta (Gordillo et al, 2015) and Lomentaria australis (Van der Loos, Schmid & Leal, 2019). Elevated pCO 2 could accelerate the degradation of chlorophyll synthesis immoderately, which is not essential for light harvesting (Gordillo et al, 1998).…”

Adjustments in fatty acid composition is a mechanism that can explain resilience to marine heatwaves and future ocean conditions in the habitat‐forming seaweed Phyllospora comosa (Labillardière) C.Agardh

“…The decrease in δ 13 C values detected in the future scenario relative to the current scenario at the final sampling indicates a down‐regulation of the CCM. This can provide energetic savings in seaweeds, as the use of CO 2 as an inorganic carbon source requires less energy than (Cornwall et al, 2017; van der Loos et al, 2019). Any saved energy was not invested in growth (in contrast to that reported for a red seaweed by van der Loos et al, 2019), but instead could have been invested in the adjustment of fatty acid composition in response to elevated temperatures.…”

Seaweed nutrient physiology: application of concepts to aquaculture and bioremediation

“…It is generally considered that nitrogen incorporation during the day is energetically less expensive than at night. This is because during daylight, the energy and carbon necessary for the assimilation process are provided directly by photosynthesis; whereas, in the dark, accumulated carbohydrates are the energy source (Huppe & Turpin 1994;Turpin 1991). Several studies have revealed higher daytime uptake rates of NH 4 + and NO 3 − compared to nighttime.…”