Macroalgal responses to ocean acidification depend on nutrient and light levels

Paula S M Celis-Plã¡,Fã©Lix L Figueroa,Paulo Antunes Horta,Nathalie Korbee,Christopher E Cornwall,Marco Milazzo,Jason M Hall-Spencer

doi:10.3389/fmars.2015.00026

Abstract

Ocean acidification may benefit algae that are able to capitalise on increased carbon availability for photosynthesis but is expected to have adverse effects on calcified algae through dissolution. Shifts in dominance between primary producers will have knock-on effects on marine ecosystems and will likely vary regionally, depending on factors such as irradiance (light vs shade) and nutrient levels (oligotrophic vs eutrophic). Thus experiments are needed to evaluate interactive effects of combined stressors in the field. In this study, we investigated the physiological responses of macroalgae near a CO2 seep in oligotrophic waters off Vulcano (Italy). The algae were incubated in situ at 0.2 m depth using a combination of three mean CO2 levels (500, 700-800 and 1200 µatm CO2), two light levels (100 and 70% of surface irradiance) and two nutrient levels of N, P, and K (enriched vs non-enriched treatments) in the non-calcified macroalga Cystoseira compressa (Phaeophyceae, Fucales) and calcified Padina pavonica (Phaeophyceae, Dictyotales). A suite of biochemical assays and in vivo chlorophyll a fluorescence parameters showed that elevated CO2 levels benefitted both of these algae, although their responses varied depending on light and nutrient availability. In C. compressa, elevated CO2 treatments resulted in higher carbon content and antioxidant activity in shaded conditions both with and without nutrient enrichment - they had more Chla, phenolic and fucoxanthin with nutrient enrichment and higher quantum yield (Fv/Fm) and photosynthetic efficiency (αETR) without nutrient enrichment. In P. pavonica, elevated CO2 treatments had higher carbon content, Fv/Fm, αETR, and Chla regardless of nutrient levels - they had higher concentrations of phenolic compounds in nutrient enriched, fully-lit conditions and more antioxidants in shaded, nutrient enriched conditions. Nitrogen content increased significantly in fertilised treatments, confirming that these algae were nutrient

Highlights

Ocean acidification due to increased atmospheric CO2 levels is altering the concentrations of dissolved inorganic carbon (DIC)in surface waters; CO23− levels are corrode marine carbonates, whilst falling, which is expected to CO2 and HCO−3 levels are rising which can stimulate photosynthesis (Connell et al, 2013; Cornwall et al, 2015)
Macroalgae are thought to be more likely to rely on carbon uptake via diffusion than use energetically expensive carbon concentrating mechanisms (Raven and Beardall, 2014; Raven et al, 2014) which has led to the idea that any benefits of ocean acidification on growth would only be seen at lower light levels for the majority of species (Hepburn et al, 2011)
Recent reviews surmise that ocean acidification is likely to increase macroalgal productivity due to beneficial effects of increased dissolved inorganic carbon (DIC) levels which can stimulate the growth of algae and allows them to divert more resources into anti-herbivore and photo-protective compounds (Harley et al, 2012; Brodie et al, 2014)

Summary

Introduction

Ocean acidification due to increased atmospheric CO2 levels is altering the concentrations of dissolved inorganic carbon (DIC)in surface waters; CO23− levels are corrode marine carbonates, whilst falling, which is expected to CO2 and HCO−3 levels are rising which can stimulate photosynthesis (Connell et al, 2013; Cornwall et al, 2015). Algae minimize damage from high irradiance by down-regulating photosystemsthey produce chemicals, such as phenolic compounds in the brown algae, which screen ultraviolet light and dissipate energy (Figueroa et al, 2014a). In oligotrophic waters, such as those of the Mediterranean, nutrient availability generally limits macroalgal growth (Ferreira et al, 2011), photosynthetic capacity (Pérez-Lloréns et al, 1996) and photoprotective mechanisms (Celis-Plá et al, 2014a)

Methods

Results

Conclusion