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Ocean acidification may benefit algae that are able to capitalize on increased carbon availability for photosynthesis, but it 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, phenols and fucoxanthin with nutrient enrichment and higher quantum yield (F_v/F_m) and photosynthetic efficiency (α_ETR) without nutrient enrichment. In P. pavonica, elevated CO₂ treatments had higher carbon content, F_v/F_m, α_ETR, 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 fertilized treatments, confirming that these algae were nutrient limited in this oligotrophic part of the Mediterranean. Our findings strengthen evidence that brown algae can be expected to proliferate as the oceans acidify where physicochemical conditions, such as nutrient levels and light, permit.

Item Type:	Refereed Article
Keywords:	ocean acidification, macroalgae, photosynthesis, phenolic compounds, nutrient availability
Research Division:	Earth Sciences
Research Group:	Oceanography
Research Field:	Chemical oceanography
Objective Division:	Environmental Management
Objective Group:	Marine systems and management
Objective Field:	Marine biodiversity
UTAS Author:	Cornwall, CE (Dr Chris Cornwall)
ID Code:	117315
Year Published:	2015
Web of Science® Times Cited:	70
Deposited By:	Oceans and Cryosphere
Deposited On:	2017-06-07
Last Modified:	2017-09-26
Downloads:	140 View Download Statistics