| Biological invasions can lead to significant ecological and economic impacts, posing a major threat to local biodiversity, habitats and ecosystem services. Species traits have been used extensively in invasion science, providing common metrics across taxa that enable comparisons based on functional responses. Despite the susceptibility of aquatic ecosystems to invasive species, particularly to invasive seaweeds, much of the focus on traits in invasion science is based on terrestrial plants. A key component to be explored are carbon concentration mechanisms (CCMs), as it provides biological species with the ability to directly use bicarbonate ion (HCO3 -), which is the dominant dissolved inorganic carbon (DIC) species in seawater. There are two main categories of CCM: one primarily based on the HCO3- dehydration to CO2 by an extracellularly carbonic anhydrase (CA), and the other involving an anion exchange protein that transports HCO3 – directly into the alga. The perennial rhodophyte Gracilaria vermiculophylla (Ohmi) Papenfuss is a well-studied species as a result of its relevance in aquaculture, its invasiveness along European and North American coasts, and the phenotypic shifts reported for some traits in its non-native populations. While considered a model alga and the existence of several studies in relation to the CCMs within the Gracilariaceae family, no efforts have been directed toward this species. Based on previous unpublished testing at the GEOMAR institute indicating higher DIC uptake by non-native algae, a series of experiments were conducted in order to ascertain presence of a CCM in G. vermiculophylla, describe its nature, and determine if it is a key component for the aforementioned higher yield of non-native algae. Differences in the photosynthetic apparatus were also tested as an alternate hypothesis. In order to test these hypotheses four experiments were designed (i-iv). (i) The CCM was determined by quantifying direct DIC evolution of the medium during a pH-drift experiment under the exposition to the inhibitors acetazolamide (AZ), 6-ethoxyzolamide (EZ) and 4,4′-diisothiocyanatostilbene- 2,2′-disulfonate (DIDS), which respectively disrupt the extracellularly acting CA, both the extra- and intracellular CA and the anion exchange protein found in algae CCMs. (ii) From these inhibitors, AZ was selected in order to compare native and non-native individuals in regards to their responses during a pH-drift experiment, (iii) as well as the content of CA with a fluorometric method. (iv) The photosynthetic apparatus was analysed with PAM fluorometry. The results indicate that G. vermiculophylla relies on both extra- 0and intracellularly-acting CA as a CCM. However, the activity of a DIDS-sensitive mechanism was not confirmed for a pH close to that of standard seawater. Thus, we suggest that the alga relies on CA transformation of environmental HCO3 – to CO2, which enters the alga by diffusion. Within the cell, intracellular CA transforms HCO3 – to CO2, enhancing RUBISCO’s yield. The diversity in CCMs within the Gracilariaceae family, as observed in this study, suggests variations in carbon utilisation strategies among closely related taxa. No differences were observed in DIC utilisation or the photosynthetic apparatus of G. vermiculophylla individuals cultured for five years. Hence, it is hypothesised that in spite of the difference in the photosynthetic response (measured as DIC utilisation and efficiency of the photosystem II) between native and non-native algae that underwent a short acclimatisation, the mechanisms that drive this difference may be subject to variations regarding the ecophysiological needs of the alga. Furthermore, the differences in short-term and non-acclimatised non-native and native algae may be partially linked to a differential CA content, as hinted by this study. KEY WORDS: Carbonic anhydrase, Carbon assimilation, Chlorophyll fluorescence, Electron transport, Inorganic carbon uptake, NIS (non-indigenous species), PAM, pH-drift. |