Photoacclimation of Caulerpa cylindracea: Light as a limiting factor in the invasion of native Mediterranean seagrass meadows

• Light inside P. oceanica meadows overcome phenotypic plasticity of C. cylindracea.
• Development of the alga was consistently limited inside the P. oceanica canopy.
• Light plays a major role in resistance of meadows to C. cylindracea bioinvasion.
• Other factors could also be involved in the colonization capacity of the alga.

Reduction in light availability caused by the canopy of the Mediterranean seagrass Posidonia oceanica has been suggested as a critical mechanism to resist the invasion of the exotic macroalga Caulerpa cylindracea. We experimentally evaluated the role of light as a limiting factor on the capacity of colonization and spread of this invasive seaweed in P. oceanica meadows by assessing photoacclimation responses and productivity and growth capacity of C. cylindracea in mesocosm and in situ light manipulation experiments. Despite the high photoacclimative plasticity developed by the alga, the light regime within the seagrass meadow during the study period was close to the minimum light requirements for growth, restricting the development capacity of this species. In addition, while increases in light availability resulting from canopy alteration also enhanced the productive capacity of the invasive seaweed in the field, such increase was not followed by gains in biomass production. Our results thus support the hypothesis that light availability has a major role in the underlying resistance of seagrass meadows to the invasion by C. cylindracea, but also indicate that there are additional factors related to the canopy of P. oceanica that further hinder the growth and colonization capacity of the alga.