How do kelp forests change water conditions and microbial communities?

Charismatic kelp forests

In honor of World Algae Day, which is celebrated annually on October 12^th, my post today will focus on kelp. Kelps are a group of brown seaweeds (a type of marine algae) that form majestic, underwater forests in cool-water areas of the world. Kelp forests are important ecosystems that are home to hundreds of species of invertebrates, fish, and mammals. On the Pacific coast of the United States, two species of kelp are particularly well-known: the giant forest-forming kelp Macrocystis pyrifera (see featured image) which lives for multiple years and the bull kelp Nereocystis leutkeana (Figure 1) that only lives for a single year appearing each spring and disappearing each fall.

 

In recent decades, there has been a rising concern over the effects of ocean acidification due to global climate change. Ocean acidification is the result of increasing atmospheric carbon dioxide, much of which is absorbed by the ocean, resulting in a decrease in ocean pH. Why is ocean acidification a problem? Well for organisms that contain shells, such as mussels, oysters, sea urchins, and corals, it becomes harder to form shells when the pH of the ocean decreases. So many researchers have turned to kelp and other photosynthetic organisms such as sea grass, which have the effect of increasing pH in their local environments through photosynthesis. During photosynthesis, kelps take carbon dioxide out of seawater which drives pH up in the local environment.

But how much can kelp change the pH of seawater and how long will potential changes last? A recent study by Cathy Pfister from the University of Chicago and her colleagues set out to determine how giant kelp and bull kelp alter the chemistry of seawater and impact microbial communities in the Pacific Northwest. You may be thinking, why would you want to examine the microbial communities in kelp forests? The importance of microbes to ecosystems, including the human body, has increased significantly in recent years. Microbes play important roles in a variety of functions, but one that is particularly important is nutrient cycling.

Nutrient cycling is how nutrients essential to life on Earth such as carbon and nitrogen move through ecosystems. For example, imagine you add nitrogen fertilizer to your garden to help your plants grow. That nitrogen molecule is taken out of the soil by the plant and used to make a leaf. At the end of the growing season, the plant dies and the leaf falls to the ground. Over time, microbes in the soil break down the leaf into its nutrient components, returning the nitrogen that was held in the leaf to the soil to be used by another plant in the next growing season. This cycling of nutrients is extremely important for your garden to be successful year after year. In the same respect, nutrient cycling is essential to fuel successful kelp forests year after year.

Kelp forests alter ocean chemistry and enhance microbial communities

Pfister and her colleagues selected three large kelp beds off of the coast of Washington state that were composed of the giant kelp Macrocystis pyrifera and the bull kelp Nereocystis leutkeana to sample in their study. Over the course of two summers, they collected water samples both inside and outside of each of these kelp beds to determine what impact kelp forests have on water pH and microbial communities.

Pfister and her colleagues reported that the seawater pH was indeed higher inside kelp forests. However, they point out that there was high variability in pH levels throughout the study and that the pH decreased during the night. At night, photosynthesis does not occur, and kelp actually release carbon dioxide through respiration. So while kelp may create conditions beneficial to organisms with shells during the day, these conditions may only occur during day light and during the active growing season (spring-fall). Additional research is necessary to investigate the benefits of kelp forests on shell building.

Pfister and her colleagues also found some interesting differences between the microbial communities inside and outside of kelp forests. Kelp forests had distinct microbial communities and there was a higher diversity of microbes found inside kelp forests. In particular, they found nine different groups of microbes that were more abundant in kelp forests. Is it possible that these microbes directly live on the surface of the kelp or that they simply survive better in the conditions created by the kelp forests. Importantly, it is likely that the microbial community in kelp forests is associated with nutrient cycling. A more diverse microbial community in kelp forests could lead to more nutrient cycling and may have been previously overlooked by earlier studies. Further research is needed to determine the exact functions and identities of the microbial communities in kelp forest. But one thing remains clear. Kelp forests are important ecosystems that harbor diverse microbial communities and their impacts extend far beyond simply serving as habitat for invertebrates, fish, and mammals.

Source Article: Catherine Pfister, Mark Altabet, and Brooke Weigel. 2019. Kelp beds and their local effects on seawater chemistry, productivity, and microbial communities. Ecology 100: e02798. https://doi.org/10.1002/ecy.2798

Feature Image: The giant kelp Macrocystis pyrifera forms large underwater forests in cool-water areas. Photo credit: Fred Hsu (Wikipedia:User:Fredhsu on en.wikipedia)