Catchment geology and human activity influence photosynthesis in aquatic plants

Like all plants, aquatic plants rely on carbon to photosynthesise. However, unlike in the open air, carbon dioxide (CO₂ ) is not a reliable source of carbon underwater. CO₂ doesn’t diffuse efficiently through the water column, and is rapidly depleted as a result. So, how do aquatic plants get the carbon they need to live and grow? And how does this shape the distribution of where different plants are found?

A major new study shows that many aquatic plants have evolved the ability to use inorganic carbon, largely bicarbonate derived from the weathering of rocks and soils, in photosynthesis. Writing in Science, Lars L. Iversen and colleagues show that around half of the 131 global submerged plant species they studied showed this bicarbonate adaptation.

The research team found that plant species with this trait were generally found in water bodies where bicarbonate concentration is high. In other words, where bicarbonate is widely leached from the weathering of soils and rocks (e.g. from limestone and dolomite), aquatic plants have adapted to use it in their life cycles. This is in contrast to terrestrial environments, where plant communities are largely determined by climate, rather than geology.

However, bicarbonate use is not ubiquitous in aquatic plants, for two reasons. First, using bicarbonate in photosynthesis is an active process, and requires energy to process. Second, photosynthesis rates at limiting concentrations of inorganic carbon are higher in plant species that use CO₂. In short, bicarbonate dependence might limit photosynthesis rates in some environments.

As a result, the research team found that where CO₂ concentrations in a water body are high (and substantially above the air equilibrium) – as is often the case in streams – the benefits of bicarbonate use are reduced. They observed that bicarbonate traits in aquatic plants were rarer in such environments.

The broad trend identified in this groundbreaking new study is that where CO₂ is limited in the water column – most often in lakes – aquatic plants have tended to adapt to use bicarbonate as a source of carbon in their life cycles. However, where CO₂ is available, plants preferentially use it in photosynthesis, like their terrestrial counterparts do. Thus, bicarbonate use by plants is a response to carbon limitation in aquatic environments, and observed more often in lakes than streams.

The study highlights the extent to which catchment geology and weathering processes can shape the distribution of aquatic plants in lakes and streams. However, human activities such as deforestation and agriculture have the potential to alter biocarbonate concentrations in aquatic environments, with resulting effects on the plant species they support.

The authors suggest that increases in bicarbonate concentrations – for example, as the result of nitrate fertiliser leaching – will have particularly severe impacts on biocarbonate-poor lakes. In such ecosystems, they suggest that plant species composition will significantly alter, as tall, fast-growing bicarbonate users colonise and suppress smaller species adapted to CO₂ use alone.

Elsewhere in the same Science issue, aquatic ecologists Rafael Marce and Biel Obrador highlight three key areas of significance in the new study. They write:

“[The study] paves the way for future studies of the impacts of global change on freshwater biodiversity and ecosystem functioning. It highlights the need to develop models for the dynamics of dissolved inorganic carbon in fresh waters that go beyond the mainstream focus on CO₂ emissions to the atmosphere. And it constitutes a powerful example of integrative ecology across spatial and temporal scales and knowledge domains.”

The broad-scale, interdisciplinary scope of the new study by Iversen and colleagues reveals new biogeochemical mechanisms that helps us understand the patterns and processes of freshwater biodiversity, and potentially predict global changes that can inform biodiversity conservation planning.

Accordingly, Marce and Obrador suggest that the study strengthens the argument for developing better biogeochemical models of river networks. “Although it is challenging to integrate complex geochemical and biological interactions at large scales, such models paint a more precise picture of the freshwater carbon cycle and better inform multidisciplinary research on biodiversity conservation and Earth-system modelling,” they write.

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Iversen L.L. et al (2019) “Catchment properties and the photosynthetic trait composition of freshwater plant communities”, Science, Vol 366, Issue 6467, 878-881

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Fig 1 detail: (A) Proportion of bicarbonate-using species across 52 plant ecoregions. Gray areas indicate regions where information on bicarbonate use in local plants is not available. (B) Relationship between mean bicarbonate concentration in plant regions and frequency of bicarbonate users. The line represents the mean proportion of bicarbonate users. (C) Density plots of bicarbonate preferences for bicarbonate users (n = 57) and obligate CO₂ users (n = 72). The central horizontal black lines represent the means, and the boxes indicate the 95% confidence intervals around the means.

Fig 2 detail: Distribution of two pondweed species with contrasting bicarbonate use in the British Isles. Potamogeton polygonifolius (obligate CO₂ user; black triangles) is found in areas with lower bicarbonate concentrations than are present where Potamogeton crispus (bicarbonate user; white circles) is found. The top left inset shows the density distribution of the two species across bicarbonate concentrations. Bicarbonate concentrations are from the global bicarbonate map (fig. S2), and species data were extracted from the geo-referenced plant occurrences.