Photosynthetic trait composition in freshwater plant communities

The Madagascar laceleaf Aponogeton madagascariensis is restricted to bicarbonate poor swamps and streams in Madagascar.

I have a new study out in Science today on photosynthesis in aquatic plants. The paper is the outcome of a three year collaboration with people from the freshwater laboratory at the University of Copenhagen and others. In the paper we explore global responses to carbon limitations in aquatic plants and show how photosynthesis is linked to catchment properties.

In contrast to their terrestrial ancestors, growth in aquatic plants is not limited by water-supply problems. However, life under the surface introduces a set of other problems unique for this specific environment. Inorganic carbon potentially limits photosynthesis in aquatic systems, because the diffusion of CO2 is 104-fold lower in water than in air. Consequently, the CO2 concentration needed to saturate photosynthesis is up to 12 times the air equilibrium concentration.

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

The fast-growing watereed Elodea nuttallii is capable of utilizing bicarbonate in photosynthesis.

Moreover, rapid photosynthesis can reduce CO2 in water substantially below air saturation. In response to frequent CO2limitations, many aquatic plants have developed carbon concentrating mechanisms via the use of bicarbonate (HCO3‑). Bicarbonate availability is a product of catchment geology and geological history and largely decoupled from present day climate. Consequently, if carbon uptake strategies in aquatic plants are linked to bicarbonate concentrations it might present an environmental link contrasting plant growth in terrestrial plants.

In this study we used a range of data sources to document the global distribution of bicarbonate uptake in aquatic plants. Using photosynthesis trait information from ~130 species, representing ~10% of the known species pool, we were able to establish a global link between bicarbonate concentrations and bicarbonate uptake in aquatic plants. At a global scale, the proportion of study species utilizing bicarbonate increased with increasing bicarbonate concentrations (this correlation persisted when adjusting for climate differences between regions).

Aquatic plants ability to use bicarbonate as a carbon source are positively correlated with bicarbonate concentrations. The map shows some interesting patterns with low concentrations in major outwash zones (big river catchments and postglacial melt zones).

However, at regional scales seemingly different factors could create this pattern. Using 967 northern hemisphere lakes and streams we showed that the global environment-trait relationship was habitat-dependent. In lakes, which often undergo CO2 limitations, bicarbonate use did increase with increasing bicarbonate and decreasing CO2concentrations. In streams, the presence of the trait changed independently of bicarbonate, but decreased with CO2 concentrations. Together this suggest that a pure CO2 strategy is only maintained in environments with stable and high rates of CO2 available.

Photosynthesis strategies along bicarbonate gradients in northern hemisphere aquatic plant communities. Due to inflow of CO2-rich water streams rarely experience CO2 depletions and bicarbonate use are only selected towards when at low CO2 concentrations (e.g. in downriver stretches). Lakes often experience CO2 depletions and bicarbonate use show a high response to increasing bicarbonate concentrations.

Altogether, we provide important inputs to a general model for plant growth and distributions in aquatic plant. Our results can explain the systematic change in lake ecosystems following acid rain in the northern hemisphere during the 1970s and 1980s and predict how these lakes will respond to the present days recalcification trends. Among terrestrial plants, the evolution of leaf traits and different photosynthetic pathways that enables rapid carbon assimilation and improved water economy has resulted in global biogeographical patterns that are linked to variations in climate. In contrast, for freshwater plants, we show that biogeographical patterns of bicarbonate-use exist and that these are caused by catchment properties that determine the concentration of bicarbonate and CO2. This insight will help evaluate the repercussions of future changes in concentration of bicarbonate and CO2 on the biodiversity and ecosystem function for fresh waters.

Read the full paper here.

And a perspective written by Rafael Marcé and Biel Obrador here

Citations:

Iversen L. L., Winkel A., Baastrup-Spohr L., Hinke A. B., Alahuhta J., Baattrup-Pedersen A., Birk S., Brodersen P., Chambers P A., Ecke F., Feldmann T., Gebler D., Heino J., Jespersen T S., Moe S J., Riis T., Sass L., Vestergaard O., Maberly S C., Sand-Jensen K., & Pedersen O. (2019): Catchment properties and the photosynthetic trait composition of freshwater plant communities. Science DOI 10.1126/science.aay5945.