Some effects of iron and nitrogen stress on the red tide dinoflagellate Gymnodinium sanguineum

Growth rate ( p ) , cell volume (CV), chlorophyll a quota (Qchi), and in vivo fluorescence (F) and DCMU-enhanced F (FD) were measured in Fe-limited, semi-continuous cultures of Gymnod~nium sanguineurn, which varied free fenic ion activity (pFe; i.e. -log free ferric ion activity) from 16.0 to 22.2, and in Fe-deplete batch cultures.Selected variables were also determined for cultures grown into nitrogen depletion.The majority of Fe-limited cellular characteristics changed most rapidly over the same pFe range (20.2 to 21.2), with ,U, CV and Qchl declining, while F/chl a and FD/chl a increased.The half-saturation constant for iron-limited growth (K,,) was near the maximum of values calculated for other neritic species examined previously.More importantly, however, the competitive ability of G. sanguineurn (as indicated by !k,,/KJ,, the slope of the Monod equation at lowest substrate concentrations) under conditions of severe iron stress was lowest of the 10 species considered.Fe and N depletion affected Qchl similarly, ivhile the former caused a markedly greater increase in F/chl a and FD/ chl a. Enhancement of in vivo fluorescence demonstrates the adverse effects of Fe stress on the utihzation of harvested light energy.Because of the extent to w h ~c h Fe limitation modified F/chl a and the distinction from N-mediated changes, this ratio, in conjunction with other probes of nutritional status, may be a useful indicator of Fe-stressed red tide populations.Ultrastructural observations of Fedeplete cells showed reductions in chloroplast number and some degeneration of lamellar organization, both of which provide a structural basis for changes noted in Qchl and fluorescence properties.