Iron storage capacities and associated ferritin gene expression among marine diatoms

In large regions of the ocean, low iron availability regulates diatom growth and species composition. Diatom species often vary in their physiological response to iron enrichment, with natural and artificial iron additions in iron-limited regions of the ocean resulting in large blooms of primarily pennate diatoms. The ability of pennate diatoms to proliferate following pulse iron additions has been partly attributed to their ability to acquire and store excess intracellular iron in the iron storage protein ferritin. Recent transcriptome sequencing of diatoms indicate that some centric diatoms also possess ferritin. Using a combination of physiological and molecular techniques, we examined the iron storage capacities and associated ferritin gene expression in phylogenetically diverse centric and pennate diatoms grown under high and low iron concentrations. There were no systematic differences among ferritin-containing and non-containing diatom lineages in their ability to store iron in excess of that needed to support maximum growth rates. An exception, however, was the ferritin-containing pennate diatom Pseudo-nitzschia granii, native to iron-limited waters of the Northeast Pacific Ocean. This species exhibited an exceptionally large luxury iron storage capacity and increased ferritin gene expression at high iron concentrations, supporting a role in long-term iron storage. By contrast, two other diatoms species that exhibited minimal iron storage capacities contained two distinct ferritin genes where one ferritin gene increased in expression under iron limitation while the second showed no variation with cellular iron status. We conclude that ferritin may serve multiple functional roles that are independent of diatom phylogeny

Effect of short-term light- and UV-stress on DMSP, DMS, and DMSP lyase activity in Emiliania huxleyi

The ecological conditions and cellular mechanisms which affect the production of dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) in marine ecosystems are still enigmatic. This information is crucial for deriving accurate oceanic ecosystem models for the dynamics of these major players in the Earth’s sulfur cycle and climate. In the present study, we examined the effect of short-term increases in photosynthetically active radiation (PAR) and ultraviolet radiation (UVR) on the production of DMSP and DMS and on DMSP lyase potential activity (DLPA) in an axenic culture of the coccolithophore Emiliania huxleyi (CCMP 1742). Algal cells were subjected to a rapid shift from a low intensity of PAR (50 μE m-2 s-1; low light, LL) to a high intensity of PAR (198 μE m-2 s-1) and elevated UVR conditions (high light, HL), simulating what may occur during upward mixing in the surface mixed layer or during changes in cloud cover. During the 4.5 h exposure to HL, the intracellular DMSP normalized to cell volume increased by ca. 30%, and dissolved DMSP doubled relative to control values. However, the DLPA per unit of cell volume decreased by ~45% compared to the control value. The up-regulation of cellular DMSP concentration is consistent with an antioxidant and/or energy dissipation role for DMSP. The decrease in DLPA may indicate that the DMSP lyase enzyme plays no role in antioxidant protection in this algal species, but rather serves some other cellular function, such as grazing protection