ORIGINAL PAPER Photosynthetic activity of the non-dormant Posidonia oceanica seed David Celdra ´n • Arnaldo Marı ´n Received: 23 March 2010 / Accepted: 17 December 2010 / Published online: 13 January 2011 Ó Springer-Verlag 2011 Abstract The photosynthetic adaptive features of non- dormant seeds in Posidonia oceanica were studied in order to evaluate the effects of light on germination success. Transmission electron micrographs showed the presence of chloroplasts in the epidermal cells, close to the nucleus at the periphery of the cytoplasm. The well-developed thylakoid membranes and the presence of starch granules indicated that the chloroplasts were photosynthetically active. The relationship between photosynthesis versus irradiance in P. oceanica seeds incubated at 15 and 21°C was analysed. The net photosynthesis in the non-dormant seed of P. oceanica was positive and compensated its respiration demand (90 lmol quanta m -2 s -1 ) at both temperatures. Net photosynthesis was negative at the other irradiance values. To test the effects of light on germina- tion success, seeds were placed both in dark and light conditions. Germination success was significantly higher in light rather than in dark condition. The characteristics observed in the photosynthesis in P. oceanica seed could be a mechanism to guarantee seedling survival in temperate waters, demonstrating though the specialized nature of this species. Introduction Posidonia oceanica (L.) Delile lives in the Mediterranean Sea where it covers 2.5–5 million hectares in the form of extensive submarine meadows (Pergent-Martini and Per- gent 1995). It is a slow-growing species and constitutes ones of the most productive ecosystems in the word (Per- gent et al. 1994). This seagrass has the ability to reproduce sexually, through seeds, or asexually through the propa- gation of rhizome fragments (Molinier and Picard 1952). Sexual recruitment seems to be relatively rare, and popu- lation spread occurs mainly through clonal propagation (Procaccini et al. 2001). However, this low recruitment seems to be contradicted by some events of massive recruitments in particularly favourable local conditions (Balestri and Lardicci 2008). Sexual reproduction, even at a low rate, could play an important role in the colonization of new sites, postdisturbances recovery and the establish- ment of new genotypes in existing seagrass populations (Alberte et al. 1994; Orth et al. 1999). P. oceanica is monoecious species with hermaphroditic and male flowers arranged in a peduncle composed of three–four flowered spikes (Balestri et al. 2003). Inflorescences emerge at the start of autumn (September–October) and fruits mature from May to June (Buia and Mazzella 1991). The fruit of P. oceanica is ovoid, with a fleshy and spongy pericarp bearing a single seed. Once separated from the plant, the green-coloured fruit floats freely on the sea surface for a few days until dehiscence occurs, and the negative buoy- ancy of the seed leads it to sink. It is then dragged along the bottom until it becomes fixed (Tomlinson 1982). The fruit dehisces by three longitudinal openings which originate from the base or point of fruit attachment (Fig. 1b). The non-dormant seed of P. oceanica is divided into three areas: apical, central and basal (Fig. 1a). Within the fruit the seed is positioned with its radical end at the fruit base, and the apical end protected until the seed is completely released (Belzunce et al. 2005). The seed is characterized by a massive central axis, composed principally of an Communicated by P. Ralph. D. Celdra ´n Á A. Marı ´n (&) Dpto. Ecologı ´a e Hidrologı ´a, Universidad de Murcia, 30100 Murcia, Spain e-mail: arnaldo@um.es 123 Mar Biol (2011) 158:853–858 DOI 10.1007/s00227-010-1612-4