Some aspects of iron limitation in a marine diatom

Batch cultures of the marine diatom, Thalassiosira pseudonana were grown in the defined medium Aquil without EDTA and with varying concentrations of added iron (Fe). The response of cultures to Fe depletion was more rapid and dramatic when in vivo fluorescence (a correlate of cellular chlorophyll a) was used as a measure of biomass instead of cell density. In vivo fluorescence and in vivo fluorescence per unit cell density were found to be more sensitive and reliable measures of Fe limitation than cell density alone. Because the physiology of nutrient-limited cells changes rapidly in batch culture, a chemostat was designed and constructed to grow axenic cultures of T. pseudonana under constant degrees of Fe limitation. Fe-limited chemostat cultures grown in Aquil without EDTA were also partly limited by silicate. Despite growing to higher cell densities, Fe-replete batch cultures were not silicate limited. It thus appears that Fe-stress contributes to increased silicate quotas in this organism. Doubling the silicate concentration of the medium alleviated silicate limitation and cells responded to Fe limitation by continuing cell division for a time while in vivo fluorescence remained constant. Measured quotas of cellular Fe were 370 attomoles⋅cell⁻¹ (1 attomole is 1 X 10⁻⁸ moles) for an Fe-replete culture and ranged from 50 to 100 attomoles⋅cell⁻¹ for Fe-deplete batch and chemostat cultures. Fe associated with the cell surface was not detected in cells grown in a batch culture which received no added Fe or in an Fe-limited chemostat culture. As a comparison, the dinoflagellate, Protogonyaulax tamarensis Clone D-255 grown in Fe-limited batch culture was found to have an Fe quota of 116 femtomoles⋅cell⁻¹ which, however, was in close agreement with T. pseudonana when Fe quotas were calculated on a per unit volume basis. A method for the measurement of Fe uptake by ⁵⁵Fe .labelling was developed which estimated an uptake rate of 90B attomoles⋅cell⁻¹⋅hr⁻¹ over a 10 min exposure to 450 nM Fe. Many of the methods developed and tested in this study should prove valuable in future study of the iron requirement of phytoplankton and their adaptations to low-Fe stress.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat