Microplastics are a major environmental challenge, being ubiquitous and persistent as to represent a new component in all marine environments. As any biogenic particle, microplastics provide surfaces for microbial growth and biofilm production, which largely consists of carbohydrates and proteins. Biofilms influence microbial activity and modify particle buoyancy, and therefore control the fate of microplastics at sea. In a simulated ‘plastic ocean’, three mesocosms containing oligotrophic seawater were amended with polystyrene microbeads and compared to three control mesocosms. The evolution of organic matter, microbial communities and nutrient concentrations was monitored over 12 days. The results indicated that microplastics increased the production of organic carbon and its aggregation into gel particulates. The observed increase of gel-like organics has implications on the marine biological pump as well as the transport of microplastics in the ocean.

(1) Phosphorus (P) is an essential plant nutrient, and P deficiency negatively affects plant growth and development. Furthermore, P is a finite and nonrenewable resource, and there is an urgent need to recover P from some of the important waste streams in society. Newly engineered calcareous materials (sol–gel coated cat litter (CATSAN^®)) can bind P from wastewater in decentralized treatment systems and potentially enable P recycling into agricultural production by direct addition of the P saturated material. (2) The effects of the addition of two P-enriched calcareous materials as fertilizers for maize (Zea mays L.) growth were investigated in a mesocosm experiment. We compared fertilization with the P-enriched materials at rates of 6, 12, 25, 50, 100 kg P ha⁻¹ yr⁻¹ with fertilization with commercial NPK fertilizer. (3) The P fertilization by the P-enriched materials had a significant positive effect on plant height, biomass, maximum light-saturated photosynthetic rate, respiration rate, and total P content in biomass. However, plants fertilized by the commercial NPK fertilizer performed significantly better in the majority of measured parameters at identical fertilization rates. (4) The bioavailability of the P bound to the calcareous material was very low. However, the studied material has the potential to be used as part of a decentralized treatment solution to remove and subsequently recover and recycle P from wastewater.