Increasing occurrence of bloom outbreaks of the marine flagellate Heterosigma have been reported over the past few decades along coastal regions around the world. Iron is an important element, often implicated in controlling bloom initiation and maintenance. As a coastal bloom former this species has an unusually high requirement for iron, yet blooms in an environment where iron may be in a form not be readily available to the cell. Ultimately, the lack of available iron may limit the extent and duration of the bloom.

An understanding of the cellular uptake mechanisms is needed to understand the importance of iron in the ecology of Heterosigma sp. We have examined a collection of toxic and non-toxic isolates of Heterosigma akishiwo (primarily from the coastal waters of British Columbia) for their ability to scavenge iron from different sources. In general, Heterosigma cells have demonstrated a high iron requirement, much higher than Prorocentrum sp., for example, yet can adjust their iron quota to match the available iron. This allows for growth over a broad range of iron levels. This observation has prompted the examination of key physiological processes to elucidate potential alterations in cellular form and/or function in response to iron limitation. Physiological responses that are characteristic of an induced iron stress, such as chlorosis, the depletion of cytochrome f pool, and a change in the ratio of soluble ferridoxin to flavodoxin were apparent. The importance of characterizing these modifications is two-fold. Firstly, they may provide evidence of cellular adaptations to variable iron conditions, and secondly, serve as an essential diagnostic tool in field studies as biomarkers of iron stress.