Photosynthetic energy conversion under extreme conditions—II: the significance of lipids under light limited growth in Antarctic sea ice diatoms

Abstract

Low photosynthetic active radiation is a strong determinant in the development and growth of sea ice algae. The algae appear to have universal mechanisms to overcome light limitation. One important process, which is induced under light limitation, is the desaturation of chloroplast membrane lipids. In order to discover whether this process is universally valid in sea ice diatoms, we investigated three species coexisting in chemostats illuminated with 15 and 2 mmol photons m À2 s À1 at À1 C. Growth under 2 mmol photons m À2 s À1 caused a 50% increase in monogalactosyldiacylglycerols (MGDG) thylakoid membrane related 20:5 n-3 fatty acids. This fatty acid supports the fluidity of the thylakoid membrane and therefore the velocity of electron flow, which is indicated by increasing rate constants for the electron transport between Q A (first stable electron acceptor) and bound Q B (second stable electron acceptor) (11.16 AE 1.34 to 23.24 AE 1.35 relative units). Two mmol photons m À2 s À1 furthermore resulted in higher amounts of non-lipid bilayer forming MGDG in relation to other bilayer forming lipids, especially digalactosydiacylglycerol (DGDG). The ratio of MGDG:DGDG increased from 3.4 AE0.3 to 5.7 AE0.3. The existence of bilayer thylakoid membranes with high proportions of non. bilayer forming lipids is only possible when sufficient thylakoid pigment-protein complexes are present. If more thylakoid pigment-protein complexes are present in membranes, as found under extreme light limitation, less bilayer forming lipids such as DGDG are required to stabilize the bilayer structure. Differences in protein contents between both light intensities were not found. Consequently pigment contents which nearly doubled under 2 mmol photons m À2 s À1 must be responsible in balancing the potential stability loss resulting from an increase in MGDG:DGDG ratio. #

Phytoplankton sterol contents vary with temperature, phosphorus and silicate supply: a study on three freshwater species

“…This might be due to the mixotrophic lifestyle of this alga (Tranvik et al, 1989) and associated differences in the adaptation response to changing environmental conditions as compared with exclusively phototrophic species. In order to sustain high photosynthetic rates, phototrophic organisms have to maintain a certain fluidity of chloroplast membranes, which ensures electron flow between the electron acceptors of photosystem II (Horva´th et al, 1987;Mock & Kroon, 2002). Although thylakoid membranes presumably do not contain sterols (Havaux, 1998), chloroplast envelope membranes contain low but significant amounts of sterols (Douce & Joyard, 1990).…”

Snapshot prediction of carbon productivity, carbon and protein content in a Southern Ocean diatom using FTIR spectroscopy

“…The ability to rapidly acclimate via phenotypic plasticity is essential for cells to satisfy physiological requirements and maximize fitness in variable environments. Antarctic diatoms have been shown to adjust their photophysiology to maximize photosynthesis for growth (Ralph et al, 2007; and vary their macromolecular pools to changing environmental conditions, increasing lipid concentrations for maintaining membrane structure and for energy storage (Mock and Kroon, 2002) or altering protein concentrations in response to osmoregulation and cryoprotection (Krell et al, 2008). Although speciesspecific data are scarce, increasing evidence suggests that phenotypic responses are highly variable within and between taxonomic groups Sackett et al, 2013).…”

“…In variable environments, such as those in the Southern Ocean, the caloric value of food can be of critical importance, in particular in winter when sunlight and food are scarce. For example, lipids, which are the most energy-rich macromolecule, are known to vary under different environmental conditions (Mock and Kroon, 2002); therefore, changes in macromolecular composition and energy partitioning in the cell will determine the nutritional value of the food and productivity of the entire food web (Diekmann et al, 2009). Quantifying a species' capacity for phenotypic variation can inform predictions of its ability to survive environmental change (Charmantier et al, 2008) and may provide insight into which species could dominate under future environmental conditions in the Southern Ocean, such as a reduction in sea-ice thickness, duration and extent.…”

Proteomic Analysis of a Sea-Ice Diatom: Salinity Acclimation Provides New Insight into the Dimethylsulfoniopropionate Production Pathway

“…An increase in polyunsaturated fatty acids (PUFAs) is important for maintaining membrane fluidity at freezing temperatures (MorganKiss et al, 2006). Furthermore, increases in PUFA composition within the thylakoid membrane enables increased electron flow under light limitation (Mock and Kroon, 2002b), compensates for pigment and protein loss under nitrogen limitation (Mock and Kroon, 2002a), and has been shown to confer salinity tolerance in the freshwater cyanobacterium, Synechocystis (Allakhverdiev et al, 1999). Polar diatoms also produce ice-binding proteins that serve as cryoprotectants via their ability to prevent ice recrystallization (Janech et al, 2006).…”