ASU study shows some aquatic plants depend on the landscape for photosynthesis

Runoff from soils and surrounding environments provide life-sustaining carbon

Water reeds are often the dominant plant cover in lakes and rivers

When present, water reeds like this Elodea nuttallii are often the dominant plant cover in lakes and rivers. They grow fast and are capable of using bicarbonate in photosynthesis. Photo courtesy Lars Iversen

By Sandy Keaton Leander |
November 14, 2019

All plants need carbon dioxide, or CO2, to live. They extract it from the air and use it during the photosynthesis process to feed themselves.

But what happens to aquatic plants? How do they get carbon dioxide?

Some have partial terrestrial forms, such as floating leaves or above-water growth, which allows them to use carbon dioxide from the atmosphere. But for plants that live completely submerged in water, CO2 is limited and many of these plants have developed a mechanism to tap into other carbon sources. In this case, they extract it from bicarbonate — a naturally occurring mineral that comes from the weathering of soils and rocks. The runoff of that process reaches the plants.

In a paper published today in Science, researchers from Arizona State University's School of Life Sciences found that not only are freshwater aquatic plants affected by climate, they are also shaped by the surrounding landscape.

“In this study, we’re able to show that yes, when in an environment where carbon dioxide is limited, then plants use strategies to extract carbon from bicarbonate,” said Lars Iversen, principal investigator for the study and a research fellow at the School of Life Sciences. “We see this in local rivers and lakes, but we also see this across the globe. We have identified patterns across ecoregions and there’s a direct link between the availability of catchment bicarbonate and the ability of aquatic plants to extract carbon from that bicarbonate.”

In aquatic environments, plants fight for light and carbon to maintain photosynthetic activity. Since CO2 is limited in freshwater, many species have developed alternative carbon resources. Some have partial terrestrial life forms, such as floating leaves, and thereby have access to atmospheric CO2. Others, like these green stoneworts, are able to use bicarbonate HCO3- as a carbon source.

Photo courtesy Lars Iversen
Photosynthesis activity and growth of aquatic plants in lakes and ponds are restricted by limited CO2 concentrations in these habitats. In order to maintain growth via photosynthesis, many plant species in standing waters have developed alternative carbon uptake strategies by using bicarbonate.

Photo courtesy Lars Iversen
Species of starworts are usually unable to use bicarbonate in their photosynthesis. Hence, these are primarily found in dense underwater cushions in streams.

Photo courtesy Lars Iversen
One of the study's products is a world map of local bicarbonate concentrations in aquatic environments (dark blue = high bicarbonate values, light yellow = low bicarbonate concentrations). The ability of aquatic plants to use bicarbonate as a carbon source are positively correlated with bicarbonate concentrations. The map shows patterns with low concentrations in major outwash zones.

Photo courtesy Lars Iversen
Lars Iversen, a research fellow with ASU School of Life Sciences, found that certain aquatic plants use bicarbonate as a source of carbon to use in photosynthesis. His current research is focused on how these findings scale to sustainability in freshwater systems. He's working to establish sustainability targets for global river systems and learn how freshwater systems are changing.

Photo courtesy Lars Iversen

The study, which focused specifically on aquatic plants that live completely submerged, also showed that when plants have easier access to carbon dioxide, they will use that as their carbon source, even if bicarbonate is available.

“One of the main points of this study is that aquatic plants are different. We cannot use our extensive knowledge about terrestrial plants in the same way as aquatic plants,” said Iversen, a researcher in Assistant Professor Ben Blonder’s ecology lab. “This is really important because, on a global scale, at least one-third of the human population is very closely linked to freshwater systems. So things like deltas, drinking water and fishing grounds are critical to human survival. If we are to understand how these systems will persist and change within the next 100 years, then we really need to know how some of the main components and structures in freshwater systems are working.”

Environmental changes caused by human activity, such as deforestation, land cultivation, and the use of fertilizers, are causing large increases in bicarbonate concentrations in many freshwater bodies around the world. Iversen said the insight from this study will help researchers evaluate how ecosystem functions change if concentrations of bicarbonate increase.

Iversen’s research is funded by the Carlsberg Foundation (CF-17-0155 and CF-18-0062). Colleagues from 14 additional institutions located in Finland, Denmark, Germany, Canada, Sweden, Estonia, Poland, Norway, the U.S., Kenya, the U.K. and Australia participated in this study.