Petrol-based plastic debris reaching the ocean releases dissolved organic carbon (DOC) and it can also leach fluorescent dissolved organic matter (FDOM). DOC is available to microbial uptake but the FDOM bioavailability has been scarcely studied. Although the most common plastic found in the ocean is petrol-based, the use of biodegradable plastic has increased substantially over the last years. Here we analysed the DOC and FDOM released by three petrol-based (LDPE, PS and EPS) and one biodegradable plastic (PLA) in seawater and examined their bioavailability. We found that the three petro-based plastics released FDOM in the protein-like region while the biodegradable PLA did not. FDOM released by EPS was available to microbial uptake while in the LDPE and PS treatments an increase of FDOM was observed, likely because the FDOM production by bacteria was higher than the consumption in those treatments. Biodegradable PLA leached similar amounts of DOC with comparable microbial bioavailability than petrol-based plastics indicating that, in seawater, it was not degraded faster than petro-based plastics. Especial care should be taken with biodegradable plastic since not all the types degrade in the ocean.