Abstract
The effects of climate change are likely to be dependent on local settings. Nonetheless, the compounded effects of global and regional stressors remain poorly understood. Here, we used CO2 vents to assess how the effects of ocean acidification on the seagrass, Posidonia oceanica, and the associated epiphytic community can be modified by enhanced nutrient loading. P. oceanica at ambient and low pH sites was exposed to three nutrient levels for 16 months. The response of P. oceanica to experimental conditions was assessed by combining analyses of gene expression, plant growth, photosynthetic pigments and epiphyte loading. At low pH, nutrient addition fostered plant growth and the synthesis of photosynthetic pigments. Overexpression of nitrogen transporter genes following nutrient additions at low pH suggests enhanced nutrient uptake by the plant. In addition, enhanced nutrient levels reduced the expression of selected antioxidant genes in plants exposed to low pH and increased epiphyte cover at both ambient and low pH. Our results show that the effects of ocean acidification on P. oceanica depend upon local nutrient concentration. More generally, our findings suggest that taking into account local environmental settings will be crucial to advance our understanding of the effects of global stressors on marine systems.
Highlights
Anthropogenic ocean acidification is among the major climate-related stressors in marine coastal ecosystems (IPCC 2014)
Glutathione-related genes were up-regulated at ambient pH (Fig. 1A; p < 0.001 for glutathione peroxidase (GPX) at both moderate and high nutrient levels and for glutathione reductase (GR) at high nutrient levels) and were down-regulated at low pH (Fig. 1B; p < 0.001 for GPX in both moderate and high nutrient enrichment and for glutathione S-transferase (GST) and GR at high nutrient levels, p < 0.01 for GR at moderate nutrient levels)
In plots not exposed to nutrient addition (CTRL), there were no significant changes, except for GPX that was down-regulated at ambient pH condition (Fig. 1A; p < 0.001) and GST down-regulated at low pH (Fig. 1B; p < 0.001)
Summary
Anthropogenic ocean acidification is among the major climate-related stressors in marine coastal ecosystems (IPCC 2014). Lauritano, et al.[27] compared the expression of genes involved in the response to stress between plants collected at low natural pH, near volcanic vents, and plants collected at normal pH conditions These authors found increased expression of some antioxidant and stress-related genes in epiphyte-free leaves of P. oceanica close to vents, suggesting overall negative effects of increased CO2 and low pH on plant physiology, possibly triggered by extensive leaf epiphyte loss[22]. In this light, long-term studies encompassing seasonal variations in plant metabolism[28] and epiphytic assemblage structure[29], as well as multiple levels of nutrient loading, appear necessary to improve our understanding of seagrass response to OA
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