A multistressor model of carbon acquisition regulation for macroalgae in a changing climate

Abstract

It is widely hypothesized that noncalcifying macroalgae will be more productive and abundant in increasingly warm and acidified oceans. Macroalgae vary greatly in the magnitudes and interactions of responses of photosynthesis and growth to multiple stressors associated with climate change. A knowledge gap that exists between the qualitative "macroalgae will benefit" hypothesis and the variable outcomes observed is regulation of physiological mechanisms that cause variation in the magnitudes of change in primary productivity, growth, and their covariation. In this context, we developed a model to quantitatively describe physiological responses to coincident variation in temperature, carbonate chemistry and light supply in a representative bicarbonate-using marine macroalga. The model is based on Ulva spp., the best understood dissolved inorganic carbon uptake mechanism among macroalgae, with data enabling synthesis across all parameters. At boundary layer pH < 8.7 most inorganic carbon is taken up through the external carbonic anhydrase (CA ext) mechanism under all conditions of photosynthetic photon flux density, temperature, and boundary layer thickness. Each 0.1 unit decline in pH causes a 20% increase in the fraction of diffusive uptake of CO 2 thereby lessening reliance on active transport of bicarbonate. Modeled downregulation of anion exchange-mediated active bicarbonate transport associated with a 0.4 unit decline in pH under ocean acidification is consistent with enhanced growth up to 4% per day without increasing photosynthetic rate. The model provides a means to quantify magnitudes of change in productivity under factorial combinations of changing temperature, pCO 2 , and light supply anticipated as climate changes.

Ocean acidification and ammonium enrichment interact to stimulate a short-term spike in growth rate of a bloom forming macroalga

“…Increased pCO 2 levels and low-light intensities are two processes that can result in a down-regulation or loss of CCMs (Hepburn et al, 2011;Raven et al, 2011;Cornwall et al, 2012;Dudgeon and Kübler, 2020). Hepburn et al (2011) found that conspecific macroalgal species grown in low-light habitats had a higher reliance on CO 2 diffusion than those grown at high-light habitats suggesting that under high-light intensities CCMs are necessary for keeping up with demand for inorganic carbon for photosynthesis.…”

“…It has been previously shown that the effect of warming is also generally greater than the effect of increasing pCO 2 on Ulva sp. (Dudgeon and Kübler, 2020).…”

“…Either method of DIC utilization requires the use of nutrients and energy to synthesize CA proteins from amino acids (Mitsuhasi et al, 2000). CCM-using algae are CO 2saturated at present day pCO 2 , but increased passive uptake of CO 2 with OA could result in downregulation of the production of CA resulting in energetic savings that could be diverted to enhanced growth and reproduction, fueling the initiation and duration of algal blooms (Raven, 1997;Giordano et al, 2005;Hepburn et al, 2011;Raven et al, 2011;Cornwall et al, 2012;Young and Gobler, 2016;van der Loos et al, 2019;Dudgeon and Kübler, 2020). The increased availability of NH 4 + and CO 2 for passive uptake in macroalgae with OA could lead to synergistic increases in growth rates through energetic cost savings in acquiring those nutrients required for growth.…”

Physiological Responses of Sargassum muticum, a Potential Golden Tide Species, to Different Levels of Light and Nitrogen

“…The growth of macroalgae is closely related to algal photosynthesis (Lobban et al, 1985;Xu and Gao, 2009). Light provides the initial energy for photosynthesis, and the increase in culture light level can usually promote the photosynthesis of macroalgae by affecting electron transfer, pigment synthesis, and/or the activity of enzymes associated with carbon utilization (Xu et al, 2014;Wu et al, 2018;Dudgeon and Kübler, 2020;Eismann et al, 2020). In this study, a higher culture light level induced a greater maximum net photosynthesis rate, which indicated a higher photosynthetic capacity, and ultimately contributed to the higher growth rate of S. muticum.…”

The combined effects of ocean acidification, warming and light on Caulerpa spp.: The role of inorganic carbon physiology and species’ ranges

“…Calcareous macroalgae are negatively impacted while noncalcareous macroalgae are hypothesised to be either unaffected or benefit from anthropogenic climate change. However, the responses of non-calcareous macroalgae are highly variable within or between species, especially among generic co-varying traits such as rates of photosynthesis and growth Dudgeon & Kübler, 2020). Studies based on laboratory experiments and modelling (Dudgeon & Kübler, 2020) suggest that the responses of macroalgae to climate change stressors may be influenced by local habitat profile and may be species-specific (Ji & Gao, 2021).…”