The seagrass Posidonia oceanica (L.) Delile plays a dominant role in coastal ecosystem dynamics in the Mediterranean Sea. This species reproduces both sexually through germination and asexually through vegetative propagation of subterranean rhizomes. Though flowering and fruiting are common, seedlings rarely succeed. An extensive population of P oceanica on the coast of the Island of Ischia (Gulf of Naples, Italy) was found to be nearly clonal when analysed with 2 DNA fingerprinting techniques, minisatellites and random amplified polymorphic DNA ( R A P D ) ; the results show similarity values of 1 and >0.91, respectively. Furthermore, a comparably hlgh genetic similarity was observed between this populat~on and individuals from another lschia population and from the coast of Spain. These findlngs support clonal propagation as the dominant reproductive mode for this specles. The presence of limited numbers of genotypes in the Western Mediterranean basin coupled with a very slow rhlzome elongation rate and the sign~ficant anthropogenic stress on these populations is cause for concern over the long-term ecological stability of this keystone species.

Marine Synechococcus spp. optimize growth and photosynthesis at low light levels and show photoinhibition of photosynthesis at high levels. We exam~ned the photosynthetic response to, and recovery from, exposure to photoinhibitory light levels after growth at low photon flux densities in 2 clones of Synechococcus spp. Clones WH 7803 and WH 8018 were grown at 25 pE m-' S-', exposed to photoinhibitory light (1500 pE m-' S-') for 3 h, and then returned to the growth light level. Clone WH 7803 showed a 40 % decrease in P, , , while Clone WH 8018 showed a 30 % decrease during the exposure. This was accompanied by a proportional increase in whole-cell phycoerythrin fluorescence and by a decrease in the cellular content of P700, the photochemical reaction center of Photosystem I. On return to low light, photochemically active P700 recovered to pre-photoinhibitory levels within 1 h in Clone WH 8018 and within 2 h in Clone WH 7803. Photosynthetic rates and phycoerythrin fluorescence required 2 to 3 h and 3 to 4 h to recover in Clones WH 8018 and WH 7803, respectively. During exposure and recovery, no changes in cellular levels of chlorophyll or phycobiliproteins were observed. It is concluded that (1) the primary deleterious effect of photoinhibition of photosynthesis was reversible losses in Photosystem I activity, and (2) recovery from photoinhibition was rapid and occurred within hours.