Posidonia bonsai: Dwarf Posidonia oceanica shoots associated to hydrothemal vent systems (Panarea Island, Italy)

[EN] Very small-sized shoots of the Mediterranean seagrass Posidonia oceanica, defined as “bonsai” shoots, were found in areas with most intense CO2 emissions and low pH associated with four vents systems off Panarea island (Aeolian Archipelago, Sicily, Southern Tyrrhenian Sea). Bonsai shoots were sampled in September 2021 and October 2022: Bottaro crater (8 m depth), Camp 7 (16 m and 21 m), Black Point (20 m) and Hot/Cold Points (10 m). They had 2–6 leaves, and adult-intermediate leaves were 5–21 cm long and 3.5–7 mm wide, with leaf shoot surface ranging 4.8 and 44.5 cm2, and shoot leaf biomass between 16 and 89 mg (d.w.). These values were all significantly lower (t-test p < 0.006–0.0001) than those measured in normal-sized shoots collected within the vents and in control sites not affected by gas emissions. Bonsai shoots had 86–89% lower leaf surface, and 61–75% lower leaf biomass than all normal-sized shoots measured. The sheath thickness of the bonsai shoots was very low (0.1–0.8 mm), and the temporal trend of sheath thickness along the rhizome (lepidochronology) showed an irregular pattern, without the clear cyclical seasonal variation typical of normal-sized shoots. The reasons of size reduction and lack of temporal cycle in lepidochronology are discussed in the light of plant acclimatization and the constraints imposed by the continuous exposure to the stressful conditions of seawater acidification and presence of phytotoxic gases (e.g. hydrogen sulfide) in the vents.
We wish to thank the staff of the 5th and 6th School of scientific diving at Panarea (2021–2022) and the 2nd stage on Posidonia oceanica ecology at Salina (2022; founded by the Blue Marine Foundation), as well as the diving Amphibia coordinated by Andrea Fogliuzzi, for the support at sea and in the laboratory. Thanks to the ECCSEL NatLab Italy infrastructure of the OGS for the logistic support at Panarea. We wish to thank the anonymous reviewers, and the Editor B. van Tussenbroek, for the useful and constructive comments.