Growth, ammonium metabolism, and photosynthetic properties of Ulva australis (Chlorophyta) under decreasing pH and ammonium enrichment

• Published in: PLOS ONE
• Main Authors: Reidenbach, LB, Fernandez, P, Leal, PP, Noisette, F, McGraw, CM, Revill, AT, Hurd, CL, Kubler, JE
• Format: Article in Journal/Newspaper
• Language: English
• Published: Public Library of Science 2017
• Subjects: Biological Sciences; Plant Biology; Phycology (incl. Marine Grasses); Ocean acidification
• Online Access: https://doi.org/10.1371/journal.pone.0188389; http://www.ncbi.nlm.nih.gov/pubmed/29176815; http://ecite.utas.edu.au/122793

The responses of macroalgae to ocean acidification could be altered by availability of macronutrients, such as ammonium (NH 4 + ). This study determined how the opportunistic macroalga, Ulva australis responded to simultaneous changes in decreasing pH and NH 4 + enrichment. This was investigated in a week-long growth experiment across a range of predicted future pHs with ambient and enriched NH 4 + treatments followed by measurements of relative growth rates (RGR), NH 4 + uptake rates and pools, total chlorophyll, and tissue carbon and nitrogen content. Rapid light curves (RLCs) were used to measure the maximum relative electron transport rate (rETR max ) and maximum quantum yield of photosystem II (PSII) photochemistry (F v /F m ). Photosynthetic capacity was derived from the RLCs and included the efficiency of light harvesting (α), slope of photoinhibition (β), and the light saturation point (E k ). The results showed that NH 4 + enrichment did not modify the effects of pH on RGRs, NH 4 + uptake rates and pools, total chlorophyll, rETR max , α, β, F v /F m , tissue C and N, and the C:N ratio. However, E k was differentially affected by pH under different NH 4 + treatments. E k increased with decreasing pH in the ambient NH 4 + treatment, but not in the enriched NH 4 + treatment. NH 4 + enrichment increased RGRs, NH 4 + pools, total chlorophyll, rETR max , α, β, F v /F m , and tissue N, and decreased NH 4 + uptake rates and the C:N ratio. Decreased pH increased total chlorophyll content, rETR max , F v /F m , and tissue N content, and decreased the C:N ratio. Therefore, the results indicate that U . australis growth is increased with NH 4 + enrichment and not with decreasing pH. While decreasing pH influenced the carbon and nitrogen metabolisms of U . australis , it did not result in changes in growth.