Ecological implications of changes in cell size and photosynthetic capacity of marine Prymnesiophyceae induced by ultraviolet-B radiation

Abstract

A natural planktonic assemblage of oiganisms (<240 pm) was studied in inesocosm experiments for 7 d under varying conditions of ultraviolet-B radiation (UVB 280 to 320 nm) UVB excluded natural radiation and UVB enhanced at 2 ditferent levels Spec~fically the changes in Prymnesiophyceae abundance and light scatter properties attr~buted to cell size (CS) were investigated by flow cytometry during the flrst 72 h with 4 h ten~poral resolution and were thereafter examined d u n n g the 4 following days with t w ~c e dally sampllng In a d d ~t i o n the specific rate of photosynthesis (P"") of phytoplankton < 5 pm, crhate abundance (predator 15 to 35 pm) and d y n a m ~c s of nutrients were monitored Prymnesiophyceae ranged in size between 2 7 and 4 and dominated the phytoplanktonic community < 5 pm (more than 94%) Prymnesiophyceae exhibited maiked die1 vanabillty with synchronized cell division CS increased during the day and d ~m i n ~s h e d at nlghttirne, indicating cell d~vision Shortterm UVB exposules during the flrst 3 d of the experiment did not affect CS, probably d u e to vertlcal water mixlng in the mesocosms moderating harmful UVB exposure In contrast long-term UVB treatments (3 to 7 d) induced progressive CS Increases in Prymnesiophyceae as a function of increasing UVB doses The successive inhibition of PCe" of phytoplankton <5 pm was also observed as a function of increasing UVB doses The results suggest that enhanced UVB provokes the retardation of cell division and inhibltlon of PC*" whlch causes Prymnesiophyceae C S enlargement CS enlargement a n d probably a change In the food qual~ty of Prymnesiophyceae could lesult In food llm~tation for the ciliate population, although cil~ates seem to be d~rectly affected by UVB enhancement This study h ~g hlights the ecolog~cal ~m p l l c a t ~o n s of CS changes dnd p h o t o ~y n t h e t ~c capac~ty ot phytoplankton, with respect to predator-prey interactions In response to UVB enhancement