Photosynthesis might be limited by light, not inorganic carbon availability, in three intertidal Gelidiales species.

• The interaction between incident photon fluence density (PFD) and dissolved inorganic carbon (DIC) availability on photosynthesis is reported for three species of Gelidiales (Rhodophyta) from the Canary Islands. • Photosynthetic O2 evolution in response to both DIC concentration and PFD, and external carbonic anhydrase activity were measured in thalli of Gelidium canariensis, Gelidium arbuscula and Pterocladiella capillacea maintained in a controlled-environment room. • No detectable external carbonic anhydrase activity, high sensitivity to alkaline pH and moderate values of photosynthetic conductance for DIC indicated that the three species had a low capacity for using the external pool of HCO3 - ; therefore photosynthetic rates were dependent on CO2 availability. The seawater concentration of DIC was insufficient to saturate photosynthesis at high PFD; photosynthesis vs PFD was not affected by DIC concentration at 0.07-0.04 mol O2 mol-1 photon. ANOVA revealed that incident PFD had a greater effect than DIC availability on photosynthesis rates in G. canariensis. • Photosynthesis is probably limited by incident light rather than DIC concentration despite the apparent low affinity for HCO3 - in G. canariensis, G. arbuscula and P. capillacea in their natural habitats.

Photosynthetic acclimation to photon irradiance and its relation to chlorophyll fluorescence and carbon assimilation in the halotolerant green alga Dunaliella viridis.

This work describes the long-term acclimation of the halotolerant microalga Dunaliella viridis to different photon irradiance, ranging from darkness to 1500 mumol m(-2) s(-1). In order to assess the effects of long-term photoinhibition, changes in oxygen production rate, pigment composition, xanthophyll cycle and in vivo chlorophyll fluorescence using the saturating pulse method were measured. Growth rate was maximal at intermediate irradiance (250 and 700 mumol m(-2) s(-1)). The increase in growth irradiance from 700 to 1500 mumol m(-2) s(-1) did not lead to further significant changes in pigment composition or EPS, indicating saturation in the pigment response to high light. Changes in Photosystem II optimum quantum yield (F(v)/F(m)) evidenced photoinhibition at 700 and especially at 1500 mumol m(-2) s(-1). The relation between photosynthetic electron flow rate and photosyntetic O(2) evolution was linear for cultures in darkness shifting to curvilinear as growth irradiance increased, suggesting the interference of the energy dissipation processes in oxygen evolution. Carbon assimilation efficiencies were studied in relation to changes in growth rate, internal carbon and nitrogen composition, and organic carbon released to the external medium. All illuminated cultures showed a high capability to maintain a C:N ratio between 6 and 7. The percentage of organic carbon released to the external medium increased to its maximum under high irradiance (1500 mumol m(-2) s(-1)). These results suggest that the release of organic carbon could act as a secondary dissipation process when the xanthophyll cycle is saturated.