XVI International Botanical Congess

Marine macroalgae dominate algal standing stocks and primary production in many coastal ecosystems. Fe is an essential trace element for all photosynthetic organisms and phytoplankton appear to be Fe-limited in oceanic and coastal upwelling regions. Here we use elemental cellular composition and growth rates to compare the N and Fe demands of phytoplankton and macroalgae globally and for coastal ecosystems. Globally, macroalgae account for 67% of algal organic C standing stocks. Compared to phytoplankton, marine macroalgae have a 3-fold higher Fe:C cellular composition and 8-fold higher Fe:N. As a result of high standing stocks and high Fe:C and Fe:N cell composition, marine macroalgae may contain 90% of algal-bound Fe in the global marine ecosystem. Our estimates of algal nutrient uptake rates for coastal shelves and estuaries indicate that the utilization of N and Fe by macroalgae exceeds that of coastal phytoplankton.