The Effects of Ocean Acidification on Coralline Algae

by Rebecca Guenther

Rebecca is a Ph.D. candidate at the University of British Columbia under the supervision of Dr. Patrick Martone. Her Ph.D. research is being completed at Friday Harbor Laboratories under the supervision of Dr. Emily Carrington.

The consequences of global climate change will have profound implications for marine ecosystems, along with the social and economic systems that they support. Coastal marine ecosystems are among the most ecologically and socio-economically important systems on the planet. However, these systems are threatened by global climate change. Over the past 200 years, the oceans have taken up nearly half of the anthropogenic carbon emissions. Uptake of CO₂ by the oceans leads to a decrease in seawater pH and lowers the saturation state for carbonate minerals. Thus, absorption of CO₂ alters ocean chemistry, with potentially serious consequences for marine life. It has been suggested that marine calcifying organisms such as corals, coralline algae, molluscs, and foraminifera will have difficulties producing their skeletons and shells under future climate scenarios if current acidification rates continue.

Coralline algae are an important component of the marine ecosystem and perform many ecosystem services. They provide key ecological functions such as cementing carbonate fragments into reef structures, providing settlement cues for several invertebrate taxa, and are major producers of carbonate sediments. In addition, articulated coralline algae provide an effective refuge for invertebrates from predation, wave exposure, and desiccation.

In the intertidal zone, coralline algae must contend with large hydrodynamic forces with every crashing wave, and so the strength of their materials is important in determining whether such hydrodynamic forces will dislodge and/or fragment algae. The changing pH and/or temperature of seawater may lead to differences in material properties of coralline algae. This may result in weaker tissues being produced in future climate conditions, leading to increased dislodgement and breakage.

The intertidal zone of temperate, rocky shores is one of the most stressful habitats on Earth. Yet, we know very little about the variation of pH in this habitat. As the tide rises and falls, organisms must contend with both marine and terrestrial conditions on a daily basis. Such fluctuations of submergence and emergence bring extreme swings in physiological challenges for many intertidal organisms. The lack of baseline data on the natural variation of physical factors makes it challenging to predict how intertidal marine ecosystems will respond to ocean acidification.

The overall aim for my Ph.D. is to document the growth and biomechanical responses of coralline algae to climate perturbations. My projects include work on the effects of pH and temperature on the growth and material properties of coralline algae, the effect of pH on the adhesion of red algal spores, and the seasonal variation in abiotic factors experienced by corallines in the field and any resulting changes in coralline growth and/or material properties throughout the season.

I am investigating how acidified seawater and increasing water temperatures compromise the biomechanical strength and flexibility of both calcified and un-calcified algal structures. Articulated coralline algae are segmented seaweeds, composed of alternating calcified segments and un-calcified joints (Figure 1). Calcified segments rely upon calcium carbonate for protection against grazers, and un-calcified joints provide flexibility which is essential for surviving crashing waves. Changes in either material may therefore impact seaweed survival. I am growing coralline fronds of two species of articulated corallines in the Ocean Acidification Environmental Laboratory (OAEL) at FHL under various ocean acidification and temperature conditions. I then quantify the growth and changes in material properties of these corallines to determine if corallines will be weaker in future climate conditions. I found that in both species, growth was stunted with decreasing pH, but that pH did not affect the quality of the un-calcified material made until coralline fronds were grown at a very low pH. Data is currently being collected on the strength of the calcified material and I am running an experiment which varies both temperature and pH to determine if increasing temperature results in weaker materials in these corallines, and whether temperature interacts with pH.

To complement my studies on adult corallines, I am also assessing the effect of ocean acidification on the performance of algal spores. This project was started by Dr. Kevin Miklasz while he was a post-doc at FHL, and I have continued collecting data to further explore the effect of pH on red algal spore adhesion. In the OAEL, I induced spore release from two species of red algae (one calcified and one un-calcified) to compare the effects of ocean acidification on both fleshy and calcified seaweeds. I quantified spore performance in two ways. First, I measured the time required for algal spores to attach. Then, I estimated the attachment strength of spores to the substratum. To do this I attached adult algal fronds to a carriage system that moves the alga across a settlement plate, releasing spores along the way (Figure 2). After a certain amount of time, a shear flume was attached to the settlement plate and secured. The spores were located with a microscope, and pictures were taken with an attached digital camera. Then, we applied shear forces to the settled algal spores and then re-shot the pictures to determine how many spores remained attached. We found that both alga species had delayed attachment times under low pH conditions, and interestingly, that the magnitude of this delay was the same in both the fleshy alga and the calcified alga. We also found that pH did not affect the fleshy alga in its final attachment strength, but that the calcified alga exhibited weaker attachment in low-pH water. Results show that seawater pH affects spore success, and this may also ultimately affect community composition and abundance of seaweeds along our shores.

The natural variation of physical factors in tidepools makes it challenging to predict how intertidal marine ecosystems will respond to ocean acidification. Thus, the third part of my thesis aims to document the natural variation of physical factors such as pH, temperature, light, and wave exposure in tidepools dominated by articulated coralline algae. Additionally, I am measuring the growth and biomechanical properties of articulated corallines over the course of a year to document any seasonal changes in coralline material properties or growth. This work is being completed at Dead Man’s Bay on the west side of San Juan Island, where corallines are common in the intertidal zone (Figure 3).

Overall, we are finding that coralline algae are impacted by ocean acidification in several life history stages. Consequently, the potential negative effects of ocean acidification may have cascading effects on marine communities, impacting other species and interactions between species.