Biodegradable nets for sustainable restoration of Posidonia oceanica meadows

6 months ago

For restoring meadows of Posidonia oceanica, an endemic plant of the Mediterranean Sea, an underwater greenhouse of cuttings is required.

Biodegradable nets for sustainable restoration of Posidonia oceanica meadows

To ensure that they can root it is necessary to anchor them firmly to the seabed in order to resist the waves and to do so, adequate supports are needed that do not turn into ghost nets that trap marine species or microplastics that pollute the sea, but are biodegradable in marine environment. Inside a tank of the Acquario di Livorno (Livorno, Italy), a net was installed consisting of a very thin filament of glass fibre covered with plastic material, capable of degrading in sea water in an adequate time (from 12 to 24 months, depending on the thickness of the coating) to the rooting of the Posidonia cuttings.

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The choice of the used bioplastic derives from a study undertaken a few years ago on different bioplastics and their biocomposites to establish their rate of biodegradation and decay over time of the mechanical properties in a tank of the Acquario di Livorno.

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This study led to the initial choice of both PBSA (polybutylene succinate-co-adipate) and PHBHV (poly(3-hydroxybutyrate-co-3-hydroxyvalerate), both commercial bioplastics used in different applications but never to make nets for marine restoration operations. Given the greater flexibility and better processability, the PBSA was chosen to produce, through a coating extrusion process, a textile fiber covered with a thermoplastic polymer (PBSA). The result is a high performing product, with great versatility of use and applications in multiple sectors.

PBSA is a semi-crystalline random co-polyester, synthetized by the reaction of 1,4-butanediol with aliphatic dicarboxylic acids such as succinic and adipic acids. All three building blocks can be produced either from renewable feedstock such as glucose and sucrose via fermentation or from petroleum-based feedstock. PBSA is a very attractive biopolymer because its very good biodegradability in various environments compared to other biodegradable polymers, mechanical properties and service temperatures comparable with those of widely used polyolefins. In comparison with polylactic acid (PLA), one of the most widely used biopolymers, it is much more flexible and therefore, does not require plasticizers. It also has a lower melting point (112°C vs 160°C) and thus is easier to process than PLA. PBSA has got a wide range of applications such as packaging films, agriculture mulch films, packaging materials, vegetation nets, compost bags and other commodity.

Here, PBSA is a biodegradable bioplastic able to offer mechanical resistance to waves and with the adequate flexibility for the cuttings, to make them grow correctly. The net produced was placed in a tank of the Livorno Acquario d Livorno, coupled to a metal frame and Posidonia cuttings were planted. The degradation of the net will be monitored over time as well as the growth of the cuttings.

In Spring the test will start in the open sea, near the Island of Elba, where the Posidonia meadows were threatened by the Capoliveri reverse osmosis seawater desalination plants, which release hypersaline water that is poorly tolerated by the Posidonia. Nets capable of covering 1500 square meters of seabed will be used for a massive repopulation of the P. oceanica meadows, which requires the presence of divers who will position the nets and monitor the state of degradation over time. The network supported by the metal frame anchored to the seabed, it will keep the cuttings in position, without preventing their correct growth during rooting.

There are a few reports on the degradation behaviour of PBSA under marine conditions. This project has great significance since the accumulation of plastic wastes in the oceans has emerged as a global challenge and only limited experimental data is available on the degradation of plastics in the aquatic environment.

Why is it important to save Posidonia? But what is special about Posidonia oceanica? Marine biologist Francesco Cinelli, professor of Marine and Underwater Ecology at the University of Pisa, involved in this project, explains: "It is like wondering what usefulness the woods have on land. Posidonia has an important role in the dynamics of sublittoral ecosystems, like the Amazon rainforest, it serves to produce oxygen, absorb anhydride, it offers shelter and support for various marine organisms such as fish and crustaceans that reproduce there and then depart towards the open sea". Yet there are several phenomena that cause its disappearance: the excavation of the sands, trawling, the construction of ports, dams or the change of currents along the coast. Even the Capoliveri’s desalinator with its submarine pipelines has created a trench in which the Posidonia meadows have been destroyed. Like all plants in autumn, Posidonia turns yellow and loses its leaves which are beached on the docks. "To many they bother - continues Cinelli - but they are very wrong. The leaves protect the beach from erosion: they are a sort of soft bed on which the waves break and do not take away the sand. Then slowly this material returns to sea, becomes organic substance and resumes its natural cycle. Without them the damage would be enormous both for the sea and for the land. It's time to think about it ".

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The Posidonia oceanica will thus return to populate the seabed along the coast of Capoliveri also thanks to the cooperation between the Department of Civil and Industrial Engineering of the University of Pisa, A.S.A. (Environmental Services Company) of Livorno, the marine biologist Francesco Cinelli, the Acquario d Livorno and the Coatyarn textile company.

At the same time, the Department of Civil and Industrial Engineering together with the Department of Biology of the University of Pisa are conducting a study on the biodegradation of PBSA by means of selected marine fungi and bacteria in order to identify the microorganisms involved in its biodegradation in marine environment and the mechanism of microbial/enzymatic degradation involved.

The potential use of PBSA-based net is vast, from fish farming to the so-called marine gardens. If we consider applications on the land, similar items could be used to consolidate landslides and slopes: nets capable of biodegrading once they have performed their function.

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