Ocean Acidification Accelerates the Growth of Two Bloom-Forming Macroalgae

• Published in: PloS one Vol. 11; no. 5; p. e0155152
• Main Authors: Young, Craig S, Gobler, Christopher J
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 13.05.2016; Public Library of Science (PLoS)
• Subjects: Acidification; Acids - chemistry; Algae; Atmospheric sciences; Biology and Life Sciences; Calcification; Carbon; Carbon - analysis; Carbon dioxide; Carbon Dioxide - analysis; Carbon Isotopes; Climate change; Coastal ecosystems; Earth Sciences; Estuaries; Estuarine environments; Eutrophic estuaries; Eutrophic rivers; Eutrophication; Exposure; Geography; Gracilaria; Gracilaria - growth & development; Gracilaria - metabolism; Grasses; Growth; Growth rate; Hydrogen-Ion Concentration; Influence; Inorganic carbon; Marine algae; Marine biology; Marine ecosystems; Models, Biological; New York; Nitrogen - analysis; Nutrient enrichment; Nutrients; Ocean acidification; Oceans; Oceans and Seas; Phosphorus; Phosphorus - analysis; Photosynthesis; Physical Sciences; Physiological aspects; River discharge; Salinity; Seawater; Seaweed - growth & development; Seaweed - metabolism; Seaweeds; Temperature; Ulva; Ulva - growth & development; Ulva - metabolism; Ulva lactuca; Upwelling; Water quality; United States; New York; United States > US
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0155152

While there is growing interest in understanding how marine life will respond to future ocean acidification, many coastal ecosystems currently experience intense acidification in response to upwelling, eutrophication, or riverine discharge. Such acidification can be inhibitory to calcifying animals, but less is known regarding how non-calcifying macroalgae may respond to elevated CO2. Here, we report on experiments performed during summer through fall with North Atlantic populations of Gracilaria and Ulva that were grown in situ within a mesotrophic estuary (Shinnecock Bay, NY, USA) or exposed to normal and elevated, but environmentally realistic, levels of pCO2 and/or nutrients (nitrogen and phosphorus). In nearly all experiments, the growth rates of Gracilaria were significantly increased by an average of 70% beyond in situ and control conditions when exposed to elevated levels of pCO2 (p<0.05), but were unaffected by nutrient enrichment. In contrast, the growth response of Ulva was more complex as this alga experienced significantly (p<0.05) increased growth rates in response to both elevated pCO2 and elevated nutrients and, in two cases, pCO2 and nutrients interacted to provide a synergistically enhanced growth rate for Ulva. Across all experiments, elevated pCO2 significantly increased Ulva growth rates by 30% (p<0.05), while the response to nutrients was smaller (p>0.05). The δ13C content of both Gracilaria and Ulva decreased two-to-three fold when grown under elevated pCO2 (p<0.001) and mixing models demonstrated these macroalgae experienced a physiological shift from near exclusive use of HCO3- to primarily CO2 use when exposed to elevated pCO2. This shift in carbon use coupled with significantly increased growth in response to elevated pCO2 suggests that photosynthesis of these algae was limited by their inorganic carbon supply. Given that eutrophication can yield elevated levels of pCO2, this study suggests that the overgrowth of macroalgae in eutrophic estuaries can be directly promoted by acidification, a process that will intensify in the coming decades.