Photosynthesis and daily metabolic carbon balance of the invasive Caulerpa racemosa var. cylindracea (Chlorophyta: Caulerpales) along a depth gradient

The photosynthetic plasticity of the invasive green alga Caulerpa racemosa v. cylindracea (hereafter C. racemosa) has been proposed as a relevant mechanism determining its successful performance on Mediterranean benthic assemblages over broad depth gradients. In the present study, the photosynthetic performance of C. racemosa was evaluated through a carbon balance approach at three invaded sites with contrasting depths (11, 18 and 26 m) and light regimes. At each sampling depth, photosynthesis vs irradiance (P vs E) curves were performed on C. racemoso fronds and daily net productivity values were obtained by the numerical integration of P vs E models with continuous recording of irradiance measured on the sea floor. Photosynthetic responses were consistent with those typically exhibited by shade-adapted macroalgal species and other Mediterranean populations of C. racemosa: a significant reduction in maximum photosynthesis (P-max) occurring at an intermediate depth (18 m) and a higher photosynthetic efficiency (alpha) and lower dark respiration rate (R-d) at the deepest sampling depth. Mean values of daily net C balance obtained from the deeper site were only 15% lower than those obtained from the shallower site, despite the severe reduction in light availability. This daily net carbon gain was ca. 29% higher than would be expected if photosynthetic adjustments did not occur in the deeper algal population. The evidence provided by these data support the hypothesis of photoacclimation in C. racemosa as an effective mechanism to optimize algal productivity across depth gradients in the Mediterranean Sea.