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Elevated CO2 Simulates the Oxygen Production of Marsh Plants to the Benefit of Estuarine Heterotrophs

Paper Reviewed
Duarte, B., Santos, D., Silva, H., Marques, J.C., Cacador, I. and Sleimi, N. 2014. Light-dark O2 dynamics in submerged leaves of C3 and C4 halophytes under increased dissolved CO2: Clues for saltmarsh response to climate change. AoB Plants 6: plu067; doi:10.1093/aobpla/plu067.

Wetlands retain about one-half to one-third of the carbon they fix, ranking them among the most productive ecosystems on the planet. They also serve important functions in estuarine systems, including helping to stabilize shorelines, regenerate nutrients, improve primary production and provide habitat for animal species.

According to Duarte et al. (2014), a "key factor defining [wetland] species expansion, growth and productivity is their exposure to abiotic stresses, both environmental (for example, climate driven) and anthropogenic (for example, pollution with heavy metals)." And given concerns over possible future climate change, Duarte et al. say "it is of great importance to understand plant stress responses and adaptations at the molecular, biochemical, cellular and physiological levels." Thus, the team of six scientists set out to investigate the effects of future climate change on two halophytic plants common in Mediterranean estuaries. This was accomplished by measuring the photosynthetic responses of both light-exposed and dark-incubated leaves of the C4 plant Spartina maritima and the C3 plant Halimione portulacoides to rising atmospheric CO2 concentrations under submerged conditions.

Results of the experiment revealed that both plant species, while submerged, are inorganic carbon-limited at normal inorganic carbon concentrations in estuarine water, although they showed "very different feedbacks to increases in dissolved CO2." H. portulacoides, as a C3 plant, for example, experienced a much greater increase in photosynthesis (O2 production) in light-exposed leaves, and decline in respiration (O2 consumption) in dark-incubated leaves, as dissolved CO2 concentrations increased, than the C4 S. maritima.

In extrapolating their findings to the ecosystem level, Duarte et al. say "there is a general trend for increasing water column oxygenation during the daily tidal cycle (two tides, one in the daytime and another during the night-time), driven by plant underwater photosynthesis." The significance of this calculation, the authors note, "becomes of great importance if we consider salt marshes as one of the most important primary producers in an estuarine system." The CO2-induced increase in oxygen production by halophyte species benefits estuarine heterotrophic species (fishes, macro-invertebrates and bacteria) by raising the rate of air-water O2 diffusion and thereby introducing important amounts of oxygen required by heterotrophs.

Consequently, in the words of the authors, "a new saltmarsh service arises as a crucial O2 producer for the estuarine aquatic community to accompany the role of these marshes as important carbon-harvesting primary producers." In addition, they conclude that "salt marshes will play a crucial role in counterbalancing the effects of climate change, in terms of water column oxygenation and in buffering its acidification by withdrawing excess CO2" from their waters.

Posted 9 November 2015