EXPLORING THE CONCEPT OF INVASIVENESS IN PLANKTONIC MICROBIAL COMMUNITIES

The concept of non-indigenous species (NIS)—also referred to as alien, exotic, non-native, or allochthonous species—traditionally applies to organisms that have been artificially introduced beyond their natural geographic range, creating a clear biogeographical discontinuity between native and introduced populations. In many well-documented cases involving macroorganisms, such as the zebra mussel in continental freshwater systems, Caulerpa taxifolia in the Mediterranean Sea, and lionfish in the Caribbean Sea, introduced species have successfully established, spread, and caused significant ecological and economic impacts. These invasions are often facilitated by release from natural predators, parasites, or competitors in the new environment. In contrast, the application of the non-indigenous concept to planktonic microbes remains controversial. Despite their immense diversity, global abundance, and ecological importance, only a limited number of planktonic microbial taxa have been classified as non-indigenous, and even these cases are frequently debated. The high dispersal capacity of microorganisms, facilitated by ocean currents, atmospheric transport, ballast water, and other vectors, challenges the notion of strict geographic boundaries. Furthermore, the cosmopolitan distribution patterns observed in many microbial taxa raise fundamental questions about whether true microbial endemism—and thus clear “non-native” status—can be reliably determined. This paper critically examines the conceptual and methodological challenges associated with defining invasive or non-indigenous planktonic microbes. We discuss the ecological, evolutionary, and biogeographical frameworks traditionally used to identify biological invasions and evaluate their applicability to microbial communities. Particular attention is given to detection limitations, taxonomic resolution, cryptic diversity, and the role of molecular tools in identifying potential introductions. We also consider whether observed microbial “introductions” represent true invasions, shifts in relative abundance of previously rare taxa, or responses to environmental change. By reassessing the criteria used to define non-indigenous status in microorganisms, this study highlights the need for refined theoretical and methodological approaches tailored to microbial ecology. Clarifying whether invasive planktonic microbes truly exist is essential for improving marine biosecurity strategies, understanding ecosystem resilience, and accurately predicting the impacts of global change on aquatic microbial communities