Most people think of algae as the bothersome green stuff that grows on the tops of ponds and needs to be removed from the inside of fish tanks, but algae also provides clues about the environment. The response of Harmful Algal Blooms (HABs) to climate change, for example, provides evidence that some algae are extremely efficient environmental adaptors.
HABs are formed when colonies of algae living in fresh or saltwater grow out of control and begin producing toxic effects that can threaten the health and lives of animals and humans. According to NOAA, they have occurred in every coastal state in the United States and are increasing in frequency due to rising temperatures associated with climate change. As a result, HAB responses to climate change, including changes in pH and CO2, have been increasingly studied.
These responses include the expansion of the blooms into larger areas and an increased release of toxic poisons with warming temperatures. In a recently published paper, Mardones et al examine a special type of algal bloom found to be an expert adaptor to climate change. This strain of algal blooms called Alexandrium catenella occurs in highly variable fjord systems in southern Chile.
These Chilean fjords have had to respond to fluctuations in CO2 and pH. They experience huge freshwater inputs from Patagonian ice fields and heavy precipitation events. When dissolved in water, CO2 forms carbonic acid, which has a low pH. Therefore, levels of CO2 and pH are inversely correlated. As CO2 increases due to climate change, algal blooms in the fjords produce more Paralytic Shellfish Toxin (PST). This toxin could have long-term effects on the fish population and therefore the entire food web and ecosystem in the fjord.
In an article by Pedro Costa, he describes how these neurotoxins can have a lasting impact: poisoned fish can be consumed by seals and humans, causing health issues or even death. As we expect CO2 to continue to rise, it is very likely harmful algal blooms like the ones in Chile will produce more PST, leading to more fish kills, disturbed ecosystems in the fjords, and possible human health issues.
During their research, Mardones et al explored six levels of CO2/pH and two light conditions to examine how the algal blooms react. The scientists identified key differences in the waters in the fjord closest to the melting ice fields and the waters in the fjord further offshore. The near-shore water in the fjord experiences the largest impact of the freshwater inputs from the ice fields. The freshwater means that the upper layers of the water are much less salty compared to lower layers. This creates an intense halocline (stronger layers of differences in salinity) in the water column. Strong winds in the region mix the layers, which produce highly fluctuating differences in carbonate chemistry. As Patagonian glaciers continue to melt, even more freshwater will be introduced into the fjords, which will continue to change the conditions of the water.
On the other hand, the more stable offshore waters have CO2 equilibrium with the atmosphere. The main environmental driver offshore is human-caused ocean acidification. As CO2 emissions increase in the atmosphere, it dissolves in oceans and lowers the pH of the water. For most species, this causes huge problems, but certain types of algal blooms are able to adapt to these conditions.
Previous studies done by Tatter et al. show that a type of the same algal bloom from Southern California have previously changed their physiological responses due to changing pCO2/pH. Under higher CO2 conditions, production of Paralytic Shellfish Toxin increased. In 2015, there was an unprecedentedly large algal bloom that stretched from Central California to the Alaskan peninsula.
Mardones et al. found similar results in the Chilean algal blooms by examining the strains of the bloom under lab conditions. The blooms had been previously harvested years before and kept in culture. They analyzed these HAB responses to changes in pH and CO2. While they had optimal physiological performance at near-equilibrium levels of pH/CO2, the algal blooms showed an ability to adapt to changing conditions. They found that the blooms are in fact able to adapt their cell size based on the pH/ CO2 levels. In conditions with high pH/low CO2, the blooms adapted to a smaller cell size. In conditions with low pH/high CO2, their cell size increased, which means they are able to change their shape to not only survive changing conditions, but to thrive in them. In low CO2, high pH could increase chain formation (they could increase their swim speed to maintain their location without being moved in the current).
These factors, in addition to others, contribute to the resiliency of the harmful algal blooms in changing conditions, demonstrating they are expert adaptors to climate change.