The effects of rising ocean temperature and pCO2 on the physiology and growth of giant kelp, Macrocystis pyrifera, and grazing by purple urchins, Strongylocentrotus purpuratus

As climate change rapidly alters the world’s oceans, marine life will have to acclimate and/or adapt to warmer and more acidic conditions. While there is a growing body of literature on the individual effects of elevated temperature and CO₂ on marine biota, few studies have examined the synergistic effects of these factors, especially regarding how they impact species interactions. In coastal environments of temperate latitudes, forests of kelp (large brown seaweeds in the Order Laminariales) provide habitat and food for numerous species, support enhanced biodiversity, and provide important ecosystem services. Consequently, impacts to these important ecosystem engineers can have disproportionately large effects on coastal ecosystem functioning. To determine how climate change might impact kelp forest ecosystems, I examined two of the more conspicuous and ecologically important kelp forest species, namely the giant kelp, Macrocystis pyrifera, and the purple sea urchin, Strongylocentrotus purpuratus. First, I performed three separate experiments in order to determine the effects of elevated temperature and pCO₂ on M. pyrifera growth and photosynthetic performance. In my first experiment I cultured M. pyrifera meristematic tissues under three pCO₂ levels (500, 1000, 1500 matm CO₂) and examined how this impacted their growth, steady-state photosynthetic oxygen evolution, and changes in their tissue carbon:nitrogen ratios. In my second experiment, I used a fully factorial design with two temperatures (12°C and 15°C) and two pCO₂ levels (500 matm and 1500 matm CO₂), and examined how these impacted kelp growth, steady-state photosynthetic carbon uptake, and tissue carbon:nitrogen ratios. In my third experiment, I used the same fully factorial design (12°C and 15°C; 500 matm and 1500 matm CO₂), but examined changes in kelp photosynthetic pigment composition and carbonic anhydrase activity (an estimate of their ability to use HCO3 – in photosynthesis). Counter to my expectations, elevating only pCO₂ in the water had no effect on kelp growth rates, photosynthesis or tissue carbon:nitrogen ratios in either of the first two experiments. In contrast, in the second experiment, elevating only seawater temperature resulted in a significant reduction in both photosynthesis and growth, and an increase in tissue carbon:nitrogen ratios. However, when seawater temperature and pCO₂ were increased together, the kelps exhibited significant increases in photosynthesis and growth relative to the other treatments This suggested that rising ocean temperatures may interact with rising pCO₂ to elicit responses that are different than when either of these factors is increased by itself. In my third experiment, elevating pCO₂ in the water significantly reduced carbonic anhydrase activity, suggesting a reduction in HCO₃ –based photosynthesis (i.e. a down regulation of carbon concentrating mechanisms) and an increase in CO₂-based photosynthesis. In contrast, elevating temperature and/or CO₂ alone had littleto- no impact on photosynthetic pigment concentrations. Following the experiments on M. pyrifera, I then examined how climate change will impact the interactions between S. purpuratus and M. pyrifera. Here, I cultured these two species separately under both “present day” conditions (i.e. 12°C and 500 matm CO₂) and “future” conditions (i.e. 15°C and 1500 matm CO₂) for three months. During this period, urchins were fed kelp from either their own water conditions or the alternate conditions, resulting in a fully factorial design with four treatment combinations (urchins held under either present day or future conditions being fed kelps grown under either present day or future conditions). My results indicate that urchins held under future conditions exhibited reduced feeding and growth rates, and smaller gonads than urchins held under present day conditions regardless of the conditions in which their food was grown. In contrast, urchins held under present day conditions and fed kelp grown under future conditions showed higher feeding and growth rates compared to similar urchins fed kelps grown under present day conditions. Together, my data suggest that M. pyrifera may benefit physiologically from a warmer, more acidic (i.e. higher pCO₂) ocean while S. purpuratus will likely be impacted negatively. Given that S. purpuratus can exert a strong deterministic influence on M. pyrifera distribution and abundance, changes to either of their populations that might arise from climate change can alter how they interact and thus have serious consequences for many coastal environments.

Brown M. B., 2014. The effects of rising ocean temperature and pCO2 on the physiology and growth of giant kelp, Macrocystis pyrifera, and grazing by purple urchins, Strongylocentrotus purpuratus. MSc thesis, San Diego State University, 54 pp. Thesis.

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