Evidence for active vertical migration by two dinoflagellates experiencing iron, nitrogen, and phosphorus limitation

Abstract

Vertical migration and subsequent assimilation of iron (Fe) and nitrate at depth by the dinoflagellates Akashiwo sanguinea and Prorocentrum micans were investigated using laboratory vertically stratified water columns. Active migration was observed in four separate experiments with varying degrees of nitrogen (N), phosphorus, and Fe stress. Stable isotopes of Fe ( 57 Fe‐FeCl 3 ) and N ( 15 N‐NaNO 3 ) were assimilated from the bottom 0.4 m of the column during migration, with significant enrichment of particulate samples collected from the surface compared to the initial A. sanguinea culture or natural water from Monterey Bay containing P. micans . In all four experiments, there was significant drawdown of nitrate from the column, with depth‐integrated drawdown of greater than 50 mmol m −2 in the P. micans experiment over 72 h. These experiments provide evidence for dinoflagellate assimilation of N and Fe at depth. Harmful algal blooms of vertically migrating dinoflagellates such as A. sanguinea and P. micans are often associated with macro‐ and micronutrient depletion in surface waters; assimilation of iron as well as macronutrients at depth could be an important ecological advantage in environments where light and nutrients are spatially separated.