Photosynthetic energy conversion under extreme conditions--I: important role of lipids as structural modulators and energy sink under N-limited growth in Antarctic sea ice diatoms

The availability of dissolved nutrients such as nitrate under extreme low temperatures is a strong determinant in the development and growth of ice diatoms. Consequently we investigated regulation of photosynthesis in a mixed culture of three diatom species, which grew in chemostats at -1 degrees C, 15 micromol photons m(-2) s(-1) under N-limitation. When nitrogen is limiting, pigment-protein complexes are one of the most affected structures under low-light conditions. The loss of integral polar thylakoid components destabilized the bilayer structure of the membrane with consequences for lipid composition and the degree of fatty acid desaturation. N-Limitation caused a decrease in monogalactosydiacylglycerol (MGDG) and a simultaneous increase in bilayer forming digalactosyldiacylglycerol (DGDG). Their ratio MGDG:DGDG decreased from 3.4 +/- 0.1 to 1.1 +/- 0.4, while 20:5 n-3 fatty acids of chloroplast related phospholipid classes such as phosphatidylglycerol (PG) increased under N-limitation. These data reveal that lipids are important components, required to sustain membrane structure under a deficiency of integral membrane bound proteins and pigments. Nonetheless, energy conversion at photosystem II is still affected by N-limitation despite this structural regulation. Photosynthetic quantum yield (F(v)/F(m)) and electron transport rates decreased under N-limitation caused by an increasing amount of electron acceptors (second stable electron acceptor = Q(B)) which had slower reoxidation kinetics. The energy surplus under these conditions is stored in triacylglycerols, the main energy sink in Antarctic sea ice diatoms under N-limitation.