A compost in the sea that produces oxygen

Researchers from the University of Liège (BE) studied the fate of the material produced by Posidonia seagrass meadows. This study, carried out in the Mediterranean Sea at STARESO, shows that the dead leaves of what is commonly known as Neptune grass accumulate in shallow areas, where they break down like a compost, remineralising the organic matter. This has a previously underestimated effect on carbon fluxes in the Mediterranean coastal environments. Surprisingly, alongside this CO₂ emission, oxygen production was also measured. This is linked to the presence of photosynthetic organisms living in this compost in the sea, which fundamentally differentiates it from compost on land.

Posidonia, a flowering plant emblematic of the Mediterranean Sea, commonly known as Neptune grass, forms vast meadows (underwater prairies) in shallow waters (less than 40m deep). “It is a terrestrial plant that recolonised the marine environment several million years ago, a small quirk of Evolution,” explains Alberto Borges, an oceanographer at ULiège. “Like most terrestrial plants in our regions, Posidonia loses its oldest leaves in autumn. These dead leaves accumulate as litter (like at the base of trees) in large patches near the seagrass meadows." It is these accumulations of dead leaves and their breakdown and transformation that interested the researchers who travelled to STARESO, an underwater and oceanographic research station located in Calvi, Corsica, to conduct a study on the primary production and degradation of organic matter in Posidonia litter.

“In the litter, the organic matter breaks down and releases nutrients and CO₂, like compost in gardens,” explains Gilles Lepoint. “The litter accumulates in open, sun-light areas. Every gardener knows that to grow plants, you need nutrients and light. It is on this basis that we conducted our study, which led to a surprising first result: in the litter resulting from the accumulation of material that one would initially imagine as dead and inert, we measured oxygen production, a consequence of the photosynthetic activity of macroalgae drifted from rocks, living Posidonia shoots detached from the nearby meadow, and diatoms (microscopic algae) present in the litter."

To summarise: in this nutrient-rich environment, all living plants associated with the litter thrive and photosynthesize. This oxygen production is significant but does not offset the oxygen consumption by the decomposition of the dead leaves. These accumulations, therefore, remain net consumers of oxygen and, consequently, net emitters of CO₂, much like compost and litter in terrestrial environments.

The second result from this study somewhat surprised the researchers. “While we thought that Posidonia litter degraded relatively quickly, this study showed us the opposite, based on measurements of litter mass loss – it degrades more slowly,” says Alberto Borges. “We measured respiration through short-term (1-day) incubations based on very precise oxygen measurements.” These measurements provided a more realistic and accurate estimate, with lower values than those traditionally obtained by monitoring mass loss over very long periods (several months). This result could modify the current carbon balance calculations for these ecosystems, which are based on traditional mass loss measurements.

As part of this study, the researchers also examined the primary production and degradation of organic matter from the macroalgae growing on rocks adjacent to the Posidonia meadows. “We hypothesized that there might be exchanges between the two systems, which one might initially imagine to be separate and compartmented. Once again, we obtained an unexpected result,” says Willy Champenois with satisfaction. “These macroalgae, despite undergoing photosynthesis, were net consumers of oxygen rather than net producers! This means that the communities of bacteria and invertebrates living within the algae community consume more organic matter than the algae produce. This necessarily implies that this excess organic matter must come from an external source.” By calculating a mass balance, the researchers concluded that this excess organic matter was likely provided by the Posidonia in the form of dissolved organic molecules diffusing from the seagrass meadow and litter to the rocks.

In summary, there is a two-way exchange between the macroalgae on the rocks and the Posidonia meadows. The macroalgae drifting from the rocks can accumulate in the Posidonia litter and contribute to primary production there. In turn, the seagrass can supply organic molecules that diffuse to the rocks and are assimilated by the bacterial communities associated with the macroalgae on the rocks. A mutually beneficial relationship, indeed!

This study provides new insights into the quantification and understanding of the organic carbon balance of Posidonia seagrass meadows in the Bay of Calvi, which has been the subject of research by oceanographers and marine biologists at the University of Liège since the 1980s, notably through the STARESO marine research station.

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