Nurdles found on the beach, that have had time to adsorb Persistent Organic Pollutants from surrounding water, are shown to be more toxic than 'virgin' pellets fresh from factories. However, even these raw pellets lead were shown to be harmful to the growing mussel embryos, thought to be due to chemical additives leaching from the pellets into the surrounding sea water. These chemicals are mixed with a polymer (the main component of the plastic pellet) to give it certain favourable characteristics. This research suggests that microplastics could be harmful to marine life, even if it is not directly ingested.
It is well known that microplastics, and pellets in particular, are very efficient at absorbing certain chemicals from surrounding seawater onto their surface (see Plastics - A Toxic Combination, and www.pelletwatch.org ). However, it is harder to confirm whether these toxins, once attached to plastic, are a threat to marine life. This has been a question that several studies have examined in the last few years, with mixed results. One study, for example, uses a computer model to predict that the toxins would be unlikely to be re-released into an animal once it has been eaten. It is likely that the exact pathway these chemicals take will depend on the individual animal, the type of chemical and the way the animals interact with the plastic.