The green menace: non-native green seaweed detected in Western Antarctica using molecular tools

Biological invasions threaten marine ecosystems worldwide, including Antarctica. The Western Antarctic Peninsula (WAP) is especially vulnerable because it concentrates research and tourism traffic, a recurrent source of non-native species (NNS) via shipping, and is one of the fastest-warming regions on Earth, gradually easing thermal barriers that have constrained marine NNS establishment. Rising temperatures may also enhance natural rafting on floating seaweeds and plastics, promoting passive transport of NNS from sub-Antarctic regions into the WAP. Green seaweeds (Chlorophyta) are prolific opportunistic invaders, yet in the WAP the lack of molecular baselines has prevented reliable discrimination of native from non-native lineages, weakening detection and management capacity. Here we combined specimen DNA barcoding (nuclear ITS and chloroplast tuf A markers; N = 128) from the WAP and adjacent sub-Antarctic regions with environmental DNA (eDNA) metabarcoding ( tuf A) of seawater, vessel hulls, rafting seaweeds and plastic debris collected in the WAP (N = 88) to document green algal diversity and screen for incoming non-native Chlorophyta, with a particular focus on Ulva. Barcoding of adult specimens recovered a single, geographically restricted lineage, provisionally named Ulva sp. "Antarctica", with no close match to sub-Antarctic or temperate sequences, suggesting a possible endemic taxon and contrasting with earlier morphology-based records of cosmopolitan Ulva . Metabarcoding additionally detected several cosmopolitan, amphipolar and Northern Hemisphere lineages associated with multiple vectors, tentatively positioning taxa along the invasion continuum, from transient propagule arrival to possible naturalisation. Ulva sp. "Antarctica" was also detected on a sub-Antarctic vessel hull, suggesting possible bidirectional transport across the Antarctic Circumpolar Current. Although sampling was opportunistic and limited, these results provide a molecular baseline supporting early detection of non-native green seaweeds in a rapidly changing Southern Ocean.