Résumé: Marine organic aggregates are sites of high of viral accumulation; however, still little is known about their colonization processes and interactions with their local bacterial hosts. By taking advantage of a novel approach (paramagnetic functionalized microsphere method) to create and incubate artificial macroaggregates, we examined the small-scale movements of viruses and bacteria between such marine snow particles and the surrounding water. The examination of the codynamics of both free-living and attached viral and bacterial abundance, over 12 hours of incubation in virus-free water, suggests that aggregates are rather comparable to viral factories than to viral traps where a significant part of the virions production might be locally diverted to the water column. Also, the near-zero proportion of lysogenized cells measured in aggregates after mitomycin-C induction seems to indicate that lysogeny is not a prominent viral reproduction pathway in organic aggregates where most viruses might rather be virulent. Finally, we hypothesize that, contrary to bacteria, which can use both strong attachment and detachment from aggregates (two-way motion of bacteria), the adsorption of planktonic viruses appears to be numerically negligible compared to their massive export from the aggregates into the water column (one-way motion of viruses).

Résumé: Factors shaping the distribution of mesophotic octocorals (30-200 m depth) remain poorly understood, potentially leaving overlooked coral areas, particularly near their bathymetric and geographic distributional limits. Yet, detailed knowledge about habitat requirements is crucial for conservation of sensitive gorgonians. Here we use Ecological Niche Modelling (ENM) relating thirteen environmental predictors and a highly comprehensive presence dataset, enhanced by SCUBA diving surveys, to investigate the suitable habitat of an important structuring species, Paramuricea clavata, throughout its distribution (Mediterranean and adjacent Atlantic). Models showed that temperature (11.5-25.5 degrees C) and slope are the most important predictors carving the niche of P. clavata. Prediction throughout the full distribution (TSS 0.9) included known locations of P. clavata alongside with previously unknown or unreported sites along the coast of Portugal and Africa, including seamounts. These predictions increase the understanding of the potential distribution for the northern Mediterranean and indicate suitable hard bottom areas down to > 150 m depth. Poorly sampled habitats with predicted presence along Algeria, Alboran Sea and adjacent Atlantic coasts encourage further investigation. We propose that surveys of target areas from the predicted distribution map, together with local expert knowledge, may lead to discoveries of new P. clavata sites and identify priority conservation areas.