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Many coral reef ecosystems have undergone profound ecological changes over the past decades leading sometimes to a shift from coral to macroalgal-dominated areas. In Martinique (Caribbean region), the proliferation of macroalgae is an important phenomenon. Coral reef resilience, involving reef building species recruitment, might be modified by macroalgal presence. This work aimed at understanding reef recruitment processes in areas dominated either by macroalgae, coral or intermediate, based on scuba diving observations, manipulative experiments and laboratory studies. Particular attention was given to the physical and chemical effects of Sargassum (one of the most represented species: 100-200 g.m-2 (wet weight) in algal beds) on benthic invertebrates’ larvae recruitment. Further experiments focused on the effects of surface molecules and of the waterborne cues produced by Sargassum polyceratium on the development of marine invertebrates’ embryos. This study demonstrated that juvenile coral diversity and density vary between the considered habitat types (i.e: dominated by algae, coral or intermediate with numerous sea urchins). It was low in algal areas (0.9-1.4 recruit.m-2) as compared to coral ones (7-8 recruit.m-2) and intermediate in urchin zones (2-3.2 recruit.m-2). Moreover, species recruiting differed according to their reproductive mode. Brooders recruited more in coral areas, which suggested that they settled in the vicinity of their parent colonies. Settlement and recruitment experiments demonstrated the barrier effect of Sargassum species on settlement but no allelochemical impacts could be identified in situ. However, the laboratory based experiments demonstrated that S. polyceratium surface molecules were active against the early stages of development of Arenicola brasiliensis (annelid), Codakia orbicularis (bivalvia) and Diadema antillarum (sea urchin, a reef key stone species) (LC50 between 25 and 51 μg.mL-1). These results give insight into the coral recruitment capacities in several habitats, which is of major importance for reef managers.

Most global calcification is carried out by organisms which are also photosynthetic. In this study, the coccolithophore Emiliania huxleyi (Lohmann) Hay and Mohler and two species of hermatypic coral were used to: examine the effect of dissolved inorganic carbon (DIC) and light on photosynthesis and calcification; and determine the extent to which these two processes interact. A novel method of producing coccolith-less (non-calcifying) cells from calcifying cells of the same strain of E huxley! was developed thus allowing photosynthesis and calcification to be studied separately. The kinetics of photosynthesis in both types of cell, and of calcification in coccolith-bearing cells, were shown to be biphasic with respect to DIC concentration. The hiatus in all three cases was located at 1 mM DIC. This unusual pattern was shown to be the product of two carbon uptake mechanisms: an anion exchanger working at all DIC concentrations and an external carbonic anhydrase active only at low DIC concentrations. In contrast to the commonly-held view, this study demonstrated that calcification did not promote photosynthesis in E. huxleyi. Nevertheless, there was clearly strong biological control of calcification in this alga since DIC uptake was mediated by an anion transporter and a dehydroxylating enzyme. This work also showed that in E huxleyi, DIC addition enhanced photosynthesis at both limiting and saturating photon flux densities and that bicarbonate affected photochemical processes directly. Photosystem II activity was stimulated and non-photochemical quenching was reduced, possibly protecting the photosynthetic apparatus from damage by light. In the two corals; Porites porites and Acropora sp., strong biological control of calcium carbonate precipitation was also evident. Again, calcification did not stimulate photosynthesis. Calcification rates of Acropora sp. were monitored in the dark and although these were lower than in the light, they still increased dramatically with bicarbonate addition. This showed that high concentrations of the bicarbonate ion can compensate for the lack of light. Hence, it seems that in hermatypic corals, light-dependence of calcification may be facultative and not obligate. It is therefore clear from the results of this study that calcification and photosynthesis are not as closely coupled as has been previously thought. In neither E. huxleyi, nor in the hermatypic corals, were photosynthetic and calcification rates saturated at the present ambient DIC concentration of seawater.