The fatty acid profiles of Euphausia superba, the Antarctic prymnesiophyte, Phaeocystis pouchetii, and a temperate diatom, Phaeodactylum tricornutum were analysed and compared. The lipid content, lipid class, fatty acid and sterol composition of E. superba fed on each cultured phytoplankton and a mixed diet of both species, were determined. No significant difference was found between total lipid levels of E. superba reared on each of these different diets. Phaeocystis pouchetii, although deficient in a number of the essential fatty acids, is apparently an adequate food source for E. superba. The proportion of polyenoic fatty acids varied within lipid classes although there was no significant difference between levels of the long chain polyunsaturate 20:5(n-3) in the total lipid of E. superba fed on these diets. This acid was found to be less than 1% of the total lipid in Phaeocystis pouchetii compared to 37% in Phaeodactylum tricornutum. This suggests that krill may possess the ability to convert exogenous shorter chain fatty acids to 20:5(n-3) and 22:6(n-3). Significant differences were detected in the isomeric ratio of 16:1(n-7c)/16:0 between krill fed the diatom compared to the prymnesiophyte diet. Significant differences were also detected in several shorter chain fatty acids and between fatty acids within their lipid classes. Such differences may have the potential to be used as biochemical signatures to provide information on food sources and possible feeding grounds of E. superba. Phaeocystis pouchetii in a very late stationary phase, although not used in this feeding study, was found to contain 11% of 22:6(n-3) for which there are few substantive sources in natural algal populations.

The presence of extracellular carbonic anhydrase (CA) in relation to medium composition was investigated using cultures of the marine diatom Phaeodactylum tricornutum Bohlin. Large-volume cultures, with low initial cell inocula were grown on ASP-2 (no dissolved inorganic carbon (DIC), 550 μM NO3−), f/2 (2·0 mM DIC, 880 μM NO3−) and modified f/2 (2·0 mM DIC, 20 μM NO3− media. Cells growing on ASP-2 showed extracellular CA in the early stages of growth, whereas extracellular CA was not detected until partial depletion of total DIC in the stationary phase for cultures on f/2 or modified f/2. Both HCO3− and CO2 were important in carbon limitation, extracellular CA being present when the free-CO2 concentration fell below 5 μM, but the HCO3− concentration needed to be below 1 mM. When carbon-replete cells were transferred to carbon-limited conditions, extracellular CA was recorded within minutes, the process being light-dependent and completely inhibited by 3,3,4-dichlorophenyl-1,1-dimethylurea (DCMU). The addition of DIC to carbon-limited cells resulted in a rapid decrease in extracellular CA activity. The membrane-impermeable inhibitor of carbonic anhydrase, dextran-bound sulphonamide (DBS) was used to inhibit extracellular CA activity in relation to photosynthetic rate in carbon-replete and carbon-limited cells. At the lowest DIC concentration (0·10 mM), for cells with maximum external CA activity, DBS gave over 80% inhibition of the photosynthetic rate, demonstrating the key role of external CA in maintaining high photosynthetic rate under conditions of carbon limitation.

It is proposed that the key factor in the regulation of extracellular CA activity is the total flux of inorganic carbon (Ci) into the cell. This determines the Ci flux into the chloroplast and when this is inadequate to support the photosynthetic rate attained by a carbon-replete chloroplast at optimum photon flux density, extracellular CA is activated. This mechanism would explain the observed interaction of CO2 and HCO3− in the regulation of extracellular CA activity.