MEPS 152:41-50 (1997)  -  doi:10.3354/meps152041

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

Lindqvist K, Lignell R

During the phytoplankton succession in the northern Baltic in 1988, the distribution of 14CO2 assimilated by algae into the main molecular groups [proteins, polysaccharides, lipids and low molar mass compounds (LMC)] after in situ light (6 h) and light to dark (20 h from ca 11:00 to 07:00 h) incubations at 2 m depth (just below maximum 14CO2 fixation) was studied. By early May, the high winter levels of mineral nutrients were depleted from the water column, and in middle May the spring bloom predominated by large dinoflagellates (diatoms subdominant) peaked. The proportion of 14C lipids was usually ca 15% of total 14CO2 fixation, but it showed a distinct peak of 40% in middle May. The 14C-lipid peak probably reflected nutrient stress of the algae, since nutrient (N+P) enrichment decreased this peak by 15 percentage points in 100 l enclosures. During the decline of the spring bloom, the proportion of 14C proteins increased despite low ambient mineral N concentrations. In summer, the phytoplankton community (mainly small flagellates) consistently exhibited remarkable channelling of 14CO2 into proteins (50 to 60%), which conformed to the low particulate organic C:N ratios of ca 7 (mol/mol). Summer upwellings, which introduced nutrients into the mixed layer, seemed to be accompanied by the highest proportions of 14C proteins. The proportion of 14C polysaccharides was usually ca 20%. After 6 h incubations, this proportion was significantly (on average 10 percentage points) higher than after 20 h, while the inverse was true with 14C proteins, which reflected continuous nocturnal synthesis of proteins (enzymes) at the expense of polysaccharide storage products. In conclusion, the high proportions of algal 14C proteins in summer suggest that phytoplankton is usually not physiologically N limited in our study area and provides N-sufficient food for herbivores, hence enabling high efficiency of algal C transfer to higher trophic levels.

Phytoplankton · Succession · 14C-labelled macromolecules · Proteins · Lipids · Baltic Sea

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