LIGHT-DEPENDENCE OF CARBON AND SULFUR PRODUCTION BY POLAR CLONES OF THE GENUS PHAEOCYSTIS

Blooms of the marine prymnesiophyte genus Phaeocystis link the oceanic and atmospheric compartments of the carbon and sulfur cycles. Modelin g the fluxes of dimethylsulfide from the ocean to the atmosphere has b een limited due to a lack of information on functional responses to en vironmental variables. In this study, the light-dependence of extracel lular carbon production and dimethyl sulfide (DMS) production by non-a xenic polar clones of Phaeocystis spp. was examined at different growt h stages. Comparative experiments were run with non-axenic arctic clon es of the diatoms Thalassiossira nordenskioeldii and Skeletonema costa tum. A large portion of carbon incorporated by the colonial stage of P haeocystis spp. is released extracellularly, in particular in stationa ry colonies. This extracellular production can be modeled as a functio n of irradiance, as for carbon incorporation. In Phaeocyst is spp., ce llular and extracellular carbon incorporation represent different upta ke rates, indicating the formation of two distinct carbon pools. The r elease of extracellular carbon by polar Phaeocystis spp. was not a con stant fraction of total production over the irradiance range used. We observed little extracellular carbon production by cells at high irrad iance, and maximal rates were observed at intermediate irradiance. New ly incorporated carbon that accumulates in the mucilage of the colonia l stage of antarctic Phaeocystis spp. during photosynthesis was not re utilized for cellular growth during the dark period, as observed for t emperate clones. In contrast, only a minor fraction of the radiocarbon incorporated by the diatoms was released extracellularly for all grow th stages. The production of DMS was an order of magnitude higher for Phaeocystis spp. than for diatoms. The chlorophyll-specific production of DMS and DMSP (dimethylsulphoniopropionate, the precursor to DMS) b y Phaeocystis spp. showed a hyperbolic response to irradiance, while a rctic diatoms (weak or non-producers of DMS), on the other hand, did n ot show any light-dependency of DMS production. An inverse relationshi p between DMS and DMSP production in stationary clones of arctic P. po uchetii was observed, but not for the exponentially growing antarctic clone. Stationary colonies also had higher DMS and dissolved DMSP prod uction rates than exponentially growing ones, These relationships can be extrapolated to the field in areas where Phaeocystis spp, dominates .