Seasonal and tidal variability of environmental carbon related physico-chemical variables and inorganic C acquisition in Gracilariopsis longissima and Enteromorpha intestinalis from Los Toruños salt marsh (Cádiz Bay, Spain)

Abstract

The seasonal and tidal variability of inorganic C acquisition mechanisms, photosynthesis, internal composition and growth were studied in two co-occurring macroalgae in Los Toruños salt marsh (Cádiz Bay), Gracilariopsis longissima and Enteromorpha intestinalis. This variability was monitored together with physico-chemical variables affecting carbon availability, photosynthesis, and growth. The environmental variables, such as light, temperature, pH, salinity, oxygen, alkalinity, dissolved inorganic carbon (DIC) and CO 2, displayed not only an expected seasonal cycle but also a daily (tidal) variability, with abrupt and rapid changes influenced by biological activities, physical variables, tidal state and tidal timing. In contrast to environmental variables, photosynthesis, pigments and C:N composition were affected by seasonal changes but not by tidal regimes, as organisms integrated these short-term fluctuations in physico-chemical variables. Photosynthesis, pigments and internal N composition were maximal in autumn and minimal in summer for both species. Growth showed a seasonal trend, displaying a summer drop with negative values. This response can be the result of extreme values of environmental variables (temperature, light, pH, nutrients, and the shortage of DIC) in summer, in comparison with higher growth rates in September onwards. The use of inhibitors of carbon acquisition in situ at normal DIC concentrations (2.2. mM) revealed species-dependent differences. While the external carbonic anhydrase (CA) activity showed a constitutive character in G. longissima, it showed little effect in E. intestinalis, which relies on internal CA activity. The 4, 4′-diisothiocyanatostilbene-2,2′-disulfonate (DIDS)-sensitive bicarbonate transport in G. longissima was effective in winter. In contrast, DIDS stimulated photosynthesis in summer, and relieved AZ inhibition. This response could suggest a stimulation of a H + extrusion mediated-CO 2 transport in periods of low CO 2 availability.