Plastic pollution may have unseen and unsuspected effects directly on marine ecosystem dynamics. Much of the low-density plastic debris derived from human activity remain on the sea-surface. Thus, they are subject to photochemical degradation, to microbial colonization and degradation, and support biofilm formation. These linked processes at the ocean’s surface may influence ocean-atmosphere dynamics by altering the flux of gases and particles at the air-sea interface with climate implications, by 1) reducing photosynthetic activity of phytoplankton and related oxygen production and 2) increasing bacterial degradation of organic material stimulated by plastics as a substrate and source of carbon. These ideas have been tested in laboratory (Micro-POSEIDOMM) and field controlled conditions (Meso-POSEIDOMM, mesocosm experiment), and the results so far suggest that marine microplastics act as localized hot spots for microbial activity and their accumulation, especially in surface waters, may interfere with the production and consumption of organic material, with implications for the air-sea exchange of gases and the export of organics across the water column.