Highlights
- Dwarf Posidonia oceanica (bonsai) observed in hydrothermal vents off Panarea.
- Bonsai shoots are from 61–75% smaller in leaf biomass than regular-sized shoots
- Bonsai shoots lacks the regular seasonal pattern in sheath thickness (lepidochronology) of normal-sized shoots
Abstract
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.
Gambi M. C., Esposito V. & Marín-Guirao L., 2023. Posidonia bonsai: dwarf Posidonia oceanica shoots associated to hydrothemal vent systems (Panarea Island, Italy). Aquatic Botany 185: 103611. doi: 10.1016/j.aquabot.2022.103611. Article (subscription required).
