Eukaryotic marine parasites could have a wide impact on marine ecology. This article in Microbiology Today describes the role of these parasites in processes as diverse as species competition, carbon cycling and gene transfer:
Parasites are typically small organisms that exploit their host both as a food source and as a habitat. Although well-studied as human pathogens and organisms prejudicial to human interests, they have been persistently ignored in microbial aquatic ecology. Increased awareness of the important role of viruses in marine aquatic ecosystems in processes as diverse as species competition, carbon cycling, and gene transfers has recently changed our overall view of aquatic parasites. Recent evidence of the widespread occurrence of small eukaryotic parasites, requiring eukaryotic hosts, has highlighted the existence of another kind of pathogen which potentially has specific ecological roles.