Sustainable Exploitation of Posidonia oceanica Sea Balls (Egagropili): A Review

Abstract

Posidonia oceanica (L.) Delile is the main seagrass plant in the Mediterranean basin that forms huge underwater meadows. Its leaves, when decomposed, are transported to the coasts, where they create huge banquettes that protect the beaches from sea erosion. Its roots and rhizome fragments, instead, aggregate into fibrous sea balls, called egagropili, that are shaped and accumulated by the waves along the shoreline. Their presence on the beach is generally disliked by tourists, and, thus, local communities commonly treat them as waste to remove and discard. Posidonia oceanica egagropili might represent a vegetable lignocellulose biomass to be valorized as a renewable substrate to produce added value molecules in biotechnological processes, as bio-absorbents in environmental decontamination, to prepare new bioplastics and biocomposites, or as insulating and reinforcement materials for construction and building. In this review, the structural characteristics, and the biological role of Posidonia oceanica egagropili are described, as well as their applications in different fields as reported in scientific papers published in recent years.

Keywords: Posidonia oceanica; cellulose; egagropili; holocellulose; lignin; marine waste.

Figure 1

Pictures of POEGs of both oval and elongated shapes of different sizes, with a ruler used as a reference. They were collected by the authors on the beach of Marzamemi (Sicily, Italy; 36°44′34″ N, 15°7′1″ E).

Figure 2

Map of the sites in the Mediterranean Sea where the POEG samples mentioned in this review were collected.

Figure 3

SEM pictures of the fibers of POEGs collected in Poetto Beach (Cagliari, Sardinia, Italy; 39°12′00″ N, 9°09′33″ E) (A) at low magnification (207X, scale bar 200 mm) and (B) at high magnification (1000X, scale bar 20 mm); (C) pictures of the crystals and (D) of the diatoms found on these POEG fibers (magnifications 14,7 KX and 15 KX, scale bar 1 and 2 mm), and (E) of the lignin and (F) of the holocellulose fractions extracted from them (magnification 10KX, scale bar 1 and 2 µm). [The preparation procedure of the samples for SEM analyses was similar to the one reported before (Restaino et al., 2022 [8]): Samples were suspended in 4% formalin in PBS for 18 h, dehydrated in increasing ethanol concentrations (from 30% to 100% for 5–15 min), dried in a critical point dryer, and sputtered with platinum-palladium (sputter coater Denton Vacuum Desk V). Fe-SEM Supra 40 Zeiss (5 kV, detector InLens) and Smart SEM Zeiss software were used for observation].

Figure 4

Fields of applications of POEGs.

Figure 5

Bruna Esposito (Rome, 1960. The artist lives and works in Rome). Venti di rivolta o rivolta dei venti 2009. Fans, galvanized iron pipes, an electrical system, sea straw balls, and a breeze. (Reproduced with permission from the artist and by courtesy of Studio Stefania Miscetti-associazione culturale Mantellate).

Figure 6

Steps of POEG recovery, transportation, and pre-treatment in a biorefinery (black arrows) before being used in several applications and fields, such as whole POEG (blue arrow); as fibers or powder after grinding (violet arrow); or after chemical extraction (green arrow) of its cellulose (light green arrow) and lignin fractions (yellow arrow). The recovered sand is transported back to the beaches (grey arrow).