Photoacclimation of the invasive alga Caulerpa racemosa var. cylindracea to depth and daylight patterns and a putative new role for siphonaxanthin

Abstract

The introduced green alga Caulerpa racemosa var. cylindracea grows along a broad depth range (from very shallow to 60 m depth) in the Mediterranean basin. In the present work, the photoacclimation capacity of this invasive variety was investigated in summer, the season of its maximum spread. Natural populations from the Gulf of Naples (Italy) were analyzed for photoresponse on two scales of light variability: a spatial scale (at three stations along a depth gradient, from 0.3 to 20 m depth) and a temporal scale (on the shallowest meadow, from sunrise to sunset). These responses were studied through pigment analysis (with HPLC), and in situ measurements of photosynthetic parameters (with a Diving‐PAM fluorometer). Electron transport rate (ETR)–irradiance curve parameters showed acclimation along environmental gradients dominated by variation in irradiance. In the shallowest plants, the lack of a midday depression in both the maximum relative ETRs and the photosynthetic efficiency at sub‐saturating irradiance (α) pointed to a maintenance of energy conversion levels despite the protective lowering of light‐harvesting efficiency revealed by the trend in Fv/Fm. On the other side, variation of photosynthetic efficiency occurred with depth and buffered the effect of decreasing light on maximum photosynthetic rates. A previously undescribed xanthophyll cycle centred on lutein‐siphonaxanthin interconversion appeared to operate in the shallowest populations in addition to the violaxanthin/antheraxanthin/zeaxanthin cycle commonly occurring in Chlorophyta; this would further enhance phototolerance of the alga. A further role of siphonaxanthin is in the acclimation to low light of deep environments as indicated by its stronger increase from the surface to the deepest station with respect to siphonein and chlorophyll b.

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

“…However, the photosynthetic responses observed in this study are in good agreement with general metabolic strategies used by sublittoral macroalgae to optimize light use and productivity under variable light conditions (Markager and Sand-Jensen, 1992;Kirk, 1994;Pérez-Lloréns et al, 1996;Gómez et al, 1997;Lobban and Harrison, 1997;Gómez, 2001). Furthermore, our results are highly consistent with differences in photosynthetic behaviour reported by other studied Mediterranean populations of C. racemosa between shallow and deep habitats (Raniello et al, 2006). Importantly, the authors of these studies also interpreted there results as being due to photoacclimation.…”

“…deep populations in winter: Cebrian and Ballesteros, 2009). However, our knowledge of the photoacclimative capacity of Mediterranean populations of C. racemosa is sparse at best (Raniello et al, 2004(Raniello et al, , 2006. Raniello et al (2004Raniello et al ( , 2006 reported interesting data showing how C. racemosa is able to re-organize its photosynthetic pigment system in response to varying light conditions caused by depth gradients, seagrass canopies, and daily and seasonal cycles.…”

“…However, our knowledge of the photoacclimative capacity of Mediterranean populations of C. racemosa is sparse at best (Raniello et al, 2004(Raniello et al, , 2006. Raniello et al (2004Raniello et al ( , 2006 reported interesting data showing how C. racemosa is able to re-organize its photosynthetic pigment system in response to varying light conditions caused by depth gradients, seagrass canopies, and daily and seasonal cycles. Regarding depth (Raniello et al, 2006), changes in pigment composition were thought to represent algal photoacclimation responses in order to optimize light capture (increase in α) and photosynthetic performance (decrease in E k ) as light becomes limiting.…”

“…Raniello et al (2004Raniello et al ( , 2006 reported interesting data showing how C. racemosa is able to re-organize its photosynthetic pigment system in response to varying light conditions caused by depth gradients, seagrass canopies, and daily and seasonal cycles. Regarding depth (Raniello et al, 2006), changes in pigment composition were thought to represent algal photoacclimation responses in order to optimize light capture (increase in α) and photosynthetic performance (decrease in E k ) as light becomes limiting. These are common responses seen in some macroalgae species able to develop over broad depth gradients (Ramus et al, 1977;Markager and Sand-Jensen, 1992;Gómez et al, 1997;Johansson and Snoeijs, 2002).…”

Photo-physiological performance and short-term acclimation of two coexisting macrophytes (<em>Cymodocea nodosa</em> and <em>Caulerpa prolifera</em>) with depth

“…a larger photosynthetic efficiency at 20 than at 5 m depth. A similar pattern has been described for other Caulerpa species, such as Caulerpa racemosa in the Mediterranean Sea (Raniello et al 2006, BernardeauEsteller et al 2011. Not only deep-water (20 m depth) thalli of C. prolifera are more adapted to low irradiances: this alga also showed short-term acclimation under varying light regimes, i.e.…”