Two invasive algae put the future of Posidonia in the Balearic Islands at risk

PalmaTwo invasive algae species present in the Mediterranean can hinder the natural regeneration of Posidonia seagrass meadows. A study by the Mediterranean Institute for Advanced Studies (Imedea) has found that the presence of Caulerpa cylindracea and Lophocladia trichoclados reduces the biomass of leaves and rhizomes of new seagrass plants by between 40% and 50%, in addition to affecting their energy reserves and the associated microbial communities in the roots.

The research analyzed the early stages of seagrass development for over five months in a scenario subjected to two of the main changes affecting marine ecosystems: increased CO₂ concentration, associated with ocean acidification, and the expansion of invasive macroalgae.

The results show a clear difference between the two factors. While the increase in CO₂ did not cause significant improvements in the growth or photosynthesis of the new plants, exposure to invasive algae did have significant effects on development. Seedlings that grew in contact with these species developed fewer leaves and a smaller leaf surface area, while the amount of biomass accumulated in the leaves and rhizomes decreased by between 40% and 50%. Reserves of sugars and starches, essential for young plants to have energy during the early stages of development, were also significantly reduced.

CO₂ does not compensate for the effects of invasives

One of the questions researchers wanted to determine was whether increased CO₂ availability could benefit new seagrass plants and at least partially compensate for the effects of invasive algae.

However, the experiment did not detect any significant improvement in photosynthetic activity, growth, or biomass production. Only a slight tendency to increase starch reserves in the rhizomes and some changes in the microorganisms associated with the roots were observed. Therefore, increased CO₂ does not seem to offer seedlings enough advantage to counteract the competition from invasive species.

They also alter root microorganisms

The effects of algae are not limited to the visible growth of plants. The study also detected changes in the microbiome of seagrass roots.

Invasive macroalgae reduce the bacterial diversity associated with the roots and alter their composition. These communities of microorganisms play an important role in processes such as nutrient acquisition and the plant's response to stress situations.

This is particularly relevant in the early stages of seagrass development, when new plants are still particularly vulnerable and need to accumulate reserves to grow and consolidate.

"In the Balearic Islands, where the conservation of seagrass is an environmental priority, controlling the spread of invasive algae can be one of the most effective actions to promote the natural regeneration of meadows," points out Imedea researcher Iris Hendriks. The study's conclusion thus focuses on the need to control the spread of these invasive species as one of the ways to promote the natural recovery of seagrass meadows.

Seagrass meadows of Posidonia oceanica also perform an essential function in the coastal ecosystems of the Balearic Islands. They are a refuge and habitat for a great diversity of species, contribute to protecting beaches from erosion, help maintain water quality, and have a great capacity to capture and store carbon. The research shows that the pressure on these meadows does not solely come from ocean warming and acidification: the proliferation of invasive species can add a new difficulty precisely at the moment when new plants are trying to regenerate existing meadows.