Photosynthesis, growth and survival of the Mediterranean seagrass Posidonia oceanica in response to simulated salinity increases in a laboratory mesocosm system Lázaro Marín-Guirao a,1 , José M. Sandoval-Gil a, b,1 , Juan M. Ruíz a, * , José L. Sánchez-Lizaso b a Centro Oceanográfico de Murcia, Instituto Español de Oceanografía, C/Varadero s/n, 30740 San Pedro del Pinatar, Murcia, Spain b Dpto. de Ciencias del Mar y Biología Aplicada, Universidad de Alicante, Spain article info Article history: Received 21 June 2010 Accepted 10 January 2011 Available online 1 February 2011 Keywords: Posidonia oceanica salinity increase photosynthesis mesocosm system abstract This study aims to examine the effect of increased salinity on the photosynthetic activity of the Medi- terranean seagrass Posidonia oceanica in a laboratory mesocosm system. To do this, large rhizome fragments were transplanted in a mesocosm laboratory system and maintained at 37 (ambient salinity, control treatment), 39, 41 and 43 (hypersaline treatments) for 47 days. Pigment content, light absorption, photosynthetic characteristics (derived from P vs. E curves and fluorescence parameters), and shoot size, growth rates and net shoot change were determined at the end of the experimental period. Both net and gross photosynthetic rates of plants under hypersaline conditions were significantly reduced, with rates some 25e33% and 13e20% lower than in control plants. The pigment content (Chla, Chlb, Chlb:Chla molar ratio, total carotenoids and carotenoids:Chla ratio), leaf absorptance and maximum quantum yield of PSII (F v /F m ) of control plants showed little or no changes under hypersaline conditions, which suggests that alterations to the capacity of the photosynthetic apparatus to capture and process light were not responsible for the reduced photosynthetic rates. In contrast, dark respiration rates increased substan- tially, with mean values up to 98% higher than in control leaves. These results suggest that the respiratory demands of the osmoregulatory process are likely to be responsible for the observed decrease in photosynthetic rates, although alterations to photosynthetic carbon assimilation and reduction could also be involved. As a consequence, leaf carbon balance was considerably impaired and leaf growth rates decreased as salinity increased above the ambient (control) salinity. No significant differences were found in the percentage of net shoot change, but mean values were clearly negative at salinity levels of 41 and 43. Results presented here indicate that photosynthesis of P. oceanica is highly sensitive to hyper- saline stress and that it likely account for the decline in leaf growth and shoot survival reported in this and previous studies in response to even small increments of the ambient salinity. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction The reverse osmosis industry uses seawater to obtain freshwater for human consumption. It is an expanding industry in some countries as a result of the shortage of freshwater resources (Morton et al., 1996). The main impact of this activity on the marine coastal environment is related to the effects of increased salinity caused by hypersaline discharges (i.e. brine) on benthic organisms and communities (Einav et al., 2002; Sadhwani et al., 2005; Lattemann and Höpner, 2008). As brine and seawater have different densities, a hypersaline layer is formed, expanding over the sea bed, and this can have a negative effect on benthic communities (Fernández- Torquemada et al., 2005a, 2009). This means that we must widen our scientific knowledge of the mechanisms used by benthic organisms to acclimate or tolerate hypersaline conditions so that we can assess and predict the consequences of brine impacts on marine coastal ecosystems. Salinity is a critical environmental factor determining the abundance and distribution of seagrass communities (Montague and Ley, 1993; Adams and Bate, 1994), which are significant components of coastal and estuarine ecosystems worldwide (Green and Short, 2003). The endemic Mediterranean seagrass species Posidonia oceanica forms extensive, continuous meadows at depths of between 0.5 and 40 m. These meadows are widely recognised as having great ecological and socio-economic importance for the functioning of many coastal ecosystems (Boudouresque et al., 2009). * Corresponding author. E-mail address: juanm.ruiz@mu.ieo.es (J.M. Ruíz). 1 Tel.: þ34 968 180500; fax: þ34 968 184441. Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss 0272-7714/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2011.01.003 Estuarine, Coastal and Shelf Science 92 (2011) 286e296