Cellular iron contents of plankton during the Southern Ocean Iron Experiment (SOFeX)

Adaptive strategies by Southern Ocean phytoplankton to lessen iron limitation: Uptake of organically complexed iron and reduced cellular iron requirements

“…At first glance, the higher Fe : C ratios measured for natural diatom assemblages in the Southern Ocean during the Southern Ocean Iron Experiment (SOFeX; , 6 mmol : mol) are at odds with the Fe : C ratios we report for laboratory cultures. There are a number of factors that can affect the measurement of Fe in the field as identified by Twining et al (2004). Here we focus on one potential cause for this discrepancy.…”

“…The difference arises from the algorithm of MendenDeuer and Lessard (2000) used by Twining et al (2004) to convert cell volume to cellular C concentration. Although this algorithm appears to be robust for small cells (e.g., Phaeocystis), it is not so for large cells.…”

“…In contrast, Timmermans et al (2001) have suggested that the cellular Fe demands of some Southern Ocean diatoms exceed those of both oceanic and coastal diatoms from temperate regions, with low and high Fe requirements respectively, and that cellular Fe requirements increase with cell size. However, the field measurements of Twining et al (2004) using single-cell elemental mapping reveal an inverse relationship between Fe : C and cell size. Such an inverse relationship between Fe : C and cell size is also apparent in the data sets of Sunda andHuntsman (1995, 1997) for temperate oceanic and coastal phytoplankton from non-Fe-limited regions.…”

Role of biogenic silica in the removal of iron from the Antarctic seas

“…This accumulation could lead to a permanent iron burial flux of 11 mmol m À 2 per year, which is comparable to iron input fluxes noted above. We calculate that the sequestration of iron could be up to 28 times greater if the iron-to-silicon ratio of untreated siliceous phytoplankton is more indicative of burial conditions, or if biogenic iron associated with non-siliceous cell structures is significant 23 . As biogenic iron associated with non-siliceous cell structures is most likely remineralized at a faster rate than structurally incorporated iron, this pool likely contributes little to the loss of iron from surface waters unlike the flux of recalcitrant, incorporated iron, which we propose.…”

“…After each cleaning procedure, samples were washed twice in deionized water. The samples were mounted on gold transmission electron microscopy grids according to Twining et al 13,23 and were allowed to dry. Grids were handled with Teflon-coated forceps.…”