Intracellular partitioning of 14CO2 in phytoplankton during a growth season in the northern Baltic

Durlng the phytoplankton succession in the northern Baltlc in 1988, the dlstnbution of "CO, asslrmlated by algae Into the maln molecular groups [proteins polysacchandes liplds and low molar mass compounds (LMC)] after in situ llght (6 h) and hght to dark (20 h from ca l 1 00 to 07 00 h) incubations at 2 m depth (Just below m a v ~ m u m I4CO2 fixation) was studled By early May, the hlgh winter levels of mineral nutrients were depleted from the water column and In m ~ d d l e May the spring bloom predom~nated by large d~noflagellates (diatoms subdominant) peaked The proportion of 'C llplds was usually ca 15% of total '"0, flxatlon, but it showed a d~stinct peak of 40"" in middle May The "C-llpid peak probably reflected nutrient stress of the algae slnce nutrient ( N + P ) enrichment decreased this peak by 15 percentage polnts In 100 1 enclosures Durlng the decllne of the spnng bloom the proportion of I4C proteins incleased despite low ambient mineral N concentratlons In summer the phytoplankton community (mainly small flagellates) consistently exhibited remarkable channelllng of '"CO2 ~ n t o protelns ( i O to 60", ,) which conformed to the low par t~cula te organic C N rat105 of ca 7 (mol/mol) Summer upwclllngs whlch Introduced nutnents Into the m ~ x e d layer, seemed to be accompanied by the highest proportions of 14C proteins The proportlon of "C polysdccharides was usually ca 2OUc1 After 6 h ~ncubations, thls proportlon was signlflcantly (on average 10 percentage polnts) higher than aftcr 20 h, while the Inverse was true w ~ t h ''C protelns, whlch reflected continuous nocturnal synthesis of proteins (enzymes) at the expense of polysaccharide storage products In conc lus~on, the high proportions of algal 14C proteins in summer suggest that phytoplankton 1s usually not phys~olog~cally N 11mited In our study area and provides N-suff~clent food for herbivores hence enabling high efflclency of algal C transfer to higher trophic levels