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

Abstract

The responses of macroalgae to ocean acidification could be altered by availability of macronutrients, such as ammonium (NH4+). This study determined how the opportunistic macroalga, Ulva australis responded to simultaneous changes in decreasing pH and NH4+ enrichment. This was investigated in a week-long growth experiment across a range of predicted future pHs with ambient and enriched NH4+ treatments followed by measurements of relative growth rates (RGR), NH4+ 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 (rETRmax) and maximum quantum yield of photosystem II (PSII) photochemistry (Fv/Fm). Photosynthetic capacity was derived from the RLCs and included the efficiency of light harvesting (α), slope of photoinhibition (β), and the light saturation point (Ek). The results showed that NH4+ enrichment did not modify the effects of pH on RGRs, NH4+ uptake rates and pools, total chlorophyll, rETRmax, α, β, Fv/Fm, tissue C and N, and the C:N ratio. However, Ek was differentially affected by pH under different NH4+ treatments. Ek increased with decreasing pH in the ambient NH4+ treatment, but not in the enriched NH4+ treatment. NH4+ enrichment increased RGRs, NH4+ pools, total chlorophyll, rETRmax, α, β, Fv/Fm, and tissue N, and decreased NH4+ uptake rates and the C:N ratio. Decreased pH increased total chlorophyll content, rETRmax, Fv/Fm, and tissue N content, and decreased the C:N ratio. Therefore, the results indicate that U. australis growth is increased with NH4+ 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.

Responses of macroalgae to CO2 enrichment cannot be inferred solely from their inorganic carbon uptake strategy

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“…Target CO 2 levels were achieved in each of the 24 culture containers using a system similar to that described in Bockmon, Frieder, Navarro, White‐Kershek, and Dickson () with the modifications presented in Reidenbach et al (). Briefly, every 4 hr, seawater in each 650‐ml culture container was replaced by flushing the container with ~1 L of seawater using Jebao DP‐4 Auto Dosing aquarium pumps.…”

Peer Review #1 of "Future CO2-induced seawater acidification mediates the physiological performance of a green alga Ulva linza in different photoperiods (v0.1)"

“…For example, OA has a positive influences on the growth of macroalgae, such as the red algae Pyropia yezoensis, Pyropia haitanensis and Gracilaria lemaneiformis (Gao et al, 1991;Xu & Gao, 2015;Chen et al, 2018), green algae Ulva prolifera, Ulva lactuca, Caulerpa taxifolia Chen et al, 2015;Roth-Schulze et al, 2018) and brown algae Hizikia fusiforme, Ecklonia radiata, Sargassum muticum (Zou, 2005;Britton et al, 2016;Xu et al, 2017). However, research has shown that high CO 2 levels did not have a detectable effect on the relative growth rate of several species, for example, Sargassum henslowianum, Ulva rigida, Ulva australis, the giant kelp Macrocystis pyrifera and Ulva australis (Chen & Zou, 2014;Rautenberger et al, 2015;Fernández et al, 2015;Reidenbach et al, 2017). Furthermore, increasing seawater CO 2 concentration reduced the growth of Gracilaria tenuistipitata, Porphyra leucosticte, Fucus vesiculosus, Halimeda opuntia and Ulva linza (García-Sánchez et al, 1994;Mercado et al, 1999;Gutow et al, 2014;Johnson et al, 2014;Gao et al, 2018).…”

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

“…For diatoms, this energetic bonus of downregulation has been estimated to be too small to account for positive effects of OA on growth rate (Kranz et al 2015;Young et al 2015). Enhancement of growth rates and/or photosynthetic rates under OA could be due to other factors, such as changes in efficiency of nutrient uptake (Young and Gobler 2016;Kang and Chung 2017;Reidenbach et al 2017) or, at the ecosystem level, species biomass composition. Another reason for uncertainty is that the positive effects of OA that are expected for productivity of strictly CO 2 -using phototrophs that are carbon-limited at present appear small (Kübler and Dudgeon 2015).…”

“…Finally, recent experimental studies of macroalgae and seagrasses show highly variable effects of OA on photosynthetic metabolism (Xu and Gao 2012;Fernandez et al 2015;Gao et al 2016Gao et al , 2018Young and Gobler 2016;Reidenbach et al 2017;Xu et al 2017;Legrand et al 2018). Much of the variability between strong, weak, or null effects may be due to the different experimental conditions used, species studied, and their environmental histories prior to sampling.…”