Plastics are ubiquitous in the oceans and constitute suitable matrices for bacterial attachment and growth. Understanding biofouling mechanisms is a key issue to assessing the ecological impacts and fate of plastics in marine environment. In this study, we investigated the different steps of plastic colonization of polyolefin-based plastics, on the first one hand, including conventional low-density polyethylene (PE), additivated PE with pro-oxidant (OXO), and artificially aged OXO (AA-OXO); and of a polyester, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), on the other hand. We combined measurements of physical surface properties of polymers (hydrophobicity and roughness) with microbiological characterization of the biofilm (cell counts, taxonomic composition, and heterotrophic activity) using a wide range of techniques, with some of them used for the first time on plastics. Our experimental setup using aquariums with natural circulating seawater during 6 weeks allowed us to characterize the successive phases of primo-colonization, growing, and maturation of the biofilms. We highlighted different trends between polymer types with distinct surface properties and composition, the biodegradable AA-OXO and PHBV presenting higher colonization by active and specific bacteria compared to non-biodegradable polymers (PE and OXO). Succession of bacterial population occurred during the three colonization phases, with hydrocarbonoclastic bacteria being highly abundant on all plastic types. This study brings original data that provide new insights on the colonization of non-biodegradable and biodegradable polymers by marine microorganisms.

The influence of humic-like substances was studied on various crop plants (cucumber, maize, pelargonium, and wheat, for example) cultivated under hydroponic conditions. The humic-like product, called SHB®, was extracted from poplar sawdust by a thermical and mechanical process and applied diluted into nutrient solutions of soilless culture at concentrations ranging between 100 to 400 mg SHB® L⁻¹. The application of the humic-like substance induced a biostimulating effect on plant growth of each species studied. Treated plants present a faster development and reach reproductive stage three to five days earlier than control. SHB® also provoked a better efficiency of plant water uptake and they improved the mineral nutrition. It is assumed that most of direct effects on plant metabolisms are related to the low molecular size fractions (LMS) because it is the only part of SHB® which can penetrate into root tissues. But, it is also suggested that high molecular size fractions (HMS) could participate in plant water saving by slowing down its passage in roots.