Dynamics of oxygen production / consumption in Dunaliella salina, Thalassiosira weissfiogii and Heterocapsa triquetra circulating within a simulated upper mixed layer

Abstract

Oxygen production / consumption dynamics in three phytoplankton species (20-25 µm in effective diameter), Dunaliella salina (Chlorophyceae), Thalassiosira weissfiogii (Bacillariophyceae) and Heterocapsa triquetra (Dinophyceae), was experimentally determined when cells circulated within a simulated upper mixed layer (UML). Samples were exposed to three radiation treatments receiving: a) full solar radiation (PAB, 280-700 nm), b) PAR+UV-A (PA, 320-700 nm), and c) only PAR (P, 400-700 nm). Two pathways were simulated (as if the cells started to circulate from the surface or from the bottom of the UML): 1) downward circulation (i.e., from 100% to 9% irradiance and back to 100%), and 2) upward circulation (i.e., from 9% to 100% irradiance and back to 9%). There were no significant differences among radiation treatments (p < 0.05) and photosynthetic inhibition was only due to PAR. We found important inter-specific differences in O2 rates when cells circulated within the simulated UML, D. salina was affected by both high and low irradiances whereas T. weissfiogii was only inhibited by high irradiances. On the other hand, H. triquetra showed the least variability and it benefited by fluctuating radiation regimes. We also determined differences in the depth integrated O2 production when species performed a complete rotation within the simulated UML, with the highest values in H. triquetra and the lowest in D. salina. Our findings suggest that the different pathways of the cells circulating in the water column should be considered at the time to assess primary productivity in areas exposed to changing meteorological conditions throughout the year, and hence with variable UMLs