Marine protected areas (MPAs) have been depicted as a useful tool for improving fisherymanagement and protecting biodiversity. For example, by acting as source populations, MPAsmay provide a spillover of adults/propagules, enhancing recruitment to surrounding,non-protected waters. However, such positive effect will depend on how a MPAs designmatches population biology and dispersal abilities of the target species. High levels ofintraspecific genetic diversity without pronounced structure have been found in manymarine organisms over large spatial scales (from hundreds to thousands kilometres) but donot hold for other ecologically and economically important species such as coral reeffishes, seagrasses and kelps. In particular, the limited potential for dispersal of manymacroalgae suggests the occurrence of self-sustaining, “closed” populations that contrastwith the “open” populations of many fishes and invertebrates. Consequently, the highresilience of fish/invertebrate populations cannot be generalized to the whole marinerealm. In the present work, we use genetic tools to explore the role of the MPAParc naturel marin d’Iroise in the management of the emblematic kelpLaminaria digitata. While kelps have received much more scientificattention than any other seaweed group, there is still critical baseline knowledge whichneeds to be addressed for their effective management. Our genetic survey of 12 L.digitata populations along the Brittany coast, four of them located within theboundaries of the Parc naturel marin d’Iroise, allowed us to assess theconservation value of these kelp populations. In particular, classical genetic metrics aswell as more recent clustering approaches were used to identify and characterizeManagement Units. Moreover, assignment tests were employed to determine contemporarymigration events and source/sink populations. Our results show that populations withinthis MPA have high conservation value. However, we also identify other populations ofconservation interest.

We report here the complete sequence of the mitochondrial genome of the brown alga Laminaria digitata (Hudson) J.V. Lamouroux. L. digitata mtDNA is a circular molecule of 38,007 bp (64·9% A+T), encoding 63 genes and 3 ORFs and with only 6·7% of non-coding sequences. Based on gene content and order, its overall organization is very similar to that of the mitochondrial genome of Pylaiella littoralis, another brown alga belonging to a different sublineage of the Phaeophyceae. In particular, the two genomes share unusual features, which hence could be unique to brown algae among the heterokont lineage, namely the presence of a rn5 gene, a short nad11 gene, a cox2 gene with a large in-frame insertion and α-proteobacterial-like promoter sequences. On the other hand, L. digitata lacks the sequences which are thought to have been transmitted horizontally to the P. littoralis genome, that is, the group-II introns in the rnl and cox1 genes, and it features only traces of an ancestral T7-like RNA polymerase. Distance phylogenetic trees inferred from concatenated mitochondrial genes confirm that speciation of brown algae occurred recently compared to other heterokonts.

Previous laboratory studies in species of the Laminariales have revealed that both the onset of growth in early winter and the summer drop in growth rate are controlled by the annual course of daylength synchronizing endogenous, circannual clocks within the thallus. Moreover, it is known for some laminarian species that cultivation in the laboratory in constant short days (SD) leads to arhythmic, continuous growth activity of the blade throughout the year. Such a prolonged SD treatment has now been performed for the first time in outdoor-cultivated Laminaria digitata. Field-grown sporophytes were collected in May from the sea near Helgoland (North Sea) and cultivated on the island of Sylt (North Sea) in temperature-controlled outdoor tanks (300 l) at 10 °C for 1·5 years either in a constant 8 h of light per day, controlled by an automatic blind on top of the tank, or in ambient daylengths. In constant SD, the growth rate remained at a steady-state level of approximately 0·4 cm day−1 from the end of June until mid-October, whereas growth rate in ambient daylengths declined steadily after June to half the rate in SD in October. Growth became light-limited between October and February in both treatments and, in the second year from July onwards, higher growth rates were again observed in constant SD than in ambient daylengths. Further work is required to find out whether SD treatment in summer would also prevent the summer drop in growth rate in other perennial seaweed species, e.g. commercially valuable red algae. This would potentially increase the chance of more constant biomass production throughout the year, and decrease the danger of the cultivated perennial algae being overgrown by annual epiphytes in summer.