A compost in the sea that produces oxygen

Posidonia, a flowering plant synonymous with the Mediterranean and commonly known as Neptune grass, forms vast meadows (underwater prairies) in shallow waters (less than 40 m deep). “It is a land plant that repopulated the marine environment several million years ago, a small quirk of evolution,” explains Alberto Borges, oceanographer at ULiège. “Like most land plants in our regions, Posidonia loses its oldest leaves in autumn. These dead leaves accumulate as litter (such as at the base of trees) in large areas near the seagrass beds.” It was these accumulations of dead leaves and their decay and transformation that interested the researchers who led to STARESO, an underwater and Oceanographic Research Station, based in Calvi, Corsica, traveled to conduct a study on the primary production and degradation of organic matter Posidonia litter.

“The organic matter decomposes in the litter and releases nutrients and CO.”₂“Like compost in gardens,” explains Gilles Lepoint. “Garbage accumulates in open, sunlit areas.” Every gardener knows that growing plants requires nutrients and light. On this basis, we conducted our study, which led to a surprising first result: in the waste created from the accumulation of material that would initially be considered dead and inert, we measured oxygen production, a consequence of photosynthetic activity from macroalgae drifting from rocks, live Posidonia sprouts detached from the nearby meadow, and diatoms (microscopic algae) present in the litter.”

In summary: In this nutrient-rich environment, all living plants associated with the litter thrive and carry out photosynthesis. This oxygen production is significant, but does not offset the oxygen consumption caused by the decomposition of the dead leaves. These accumulations therefore remain net consumers of oxygen and therefore net emitters of CO₂similar to compost and waste in terrestrial environments.

The second result of this study somewhat surprised the researchers. “While we thought that Posidonia waste would decompose relatively quickly, this study, based on measurements of waste mass loss, showed us the opposite – it decomposes 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). were achieved. This finding could change current carbon balance calculations for these ecosystems, which are based on traditional mass loss measurements.

As part of this study, researchers also examined the primary production and degradation of organic matter from the macroalgae growing on adjacent rocks Posidonia meadows. “We hypothesized that there could be an exchange between the two systems, which could initially be thought of as separate and compartmentalized. Once again we received an unexpected result,” says Willy Champenois with satisfaction. “Despite photosynthesis, these macroalgae were net oxygen consumers and not net producers! This means that the bacterial and invertebrate communities living in the algae community consume more organic matter than the algae produce. This necessarily means that this excess organic matter must come from an external source.” By calculating a mass balance, the researchers concluded that this excess organic matter likely came from the plant Posidonia in the form of dissolved organic molecules that diffuse from the seagrass meadow and waste to the rocks.

In summary, there is a two-way exchange between the macroalgae on the rocks and the Posidonia meadows. The macroalgae that drift from the rocks can accumulate there Posidonia Throw away trash and contribute to primary production there. The seaweed, in turn, can provide organic molecules that diffuse to the rocks and are taken up by the bacterial communities of macroalgae on the rocks. A mutually beneficial relationship indeed!

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