This investigation tested the hypothesis that iron, as a micronutrient, will affect proteins in Trichodesmium and therefore affect nitrogen fixation. Changes in proteins that are a result of iron enrichment were compared to naturally occuring diel changes. Alterations in the iron protein of nitrogenase were compared to nitrogen fixation rates using the acetylene reduction technique. The observed changes in proteins were compared in Trichodesmium colonies from the Caribbean Sea and the Sargasso Sea. Trichodesmium colonies were monitored for protein and iron content over a diel period on two cruises. The changes in protein and iron content in Trichodesmium colonies were variable but at times showed a cyclic diel pattern. Changes in protein bands on SDS-PAGE showed consistent changes in the banding pattern of a low molecular weight protein that responded to iron nutrition and time of day (Elardo and Rueter 1990). These changes were similar to changes m the iron protein of nitrogenase which also responded to changes associated with iron nutrition and time of day (Elardo 1991 ). Trichodesmium appear to alter certain proteins which appear as changes in banding patterns in response to environmental factors such as nutrients, temperature and light. My research shows that the pattern of modification of the iron protein of nitrogenase differs in colonies from the Caribbean Sea compared to those from the Sargasso Sea (Elardo 1991). The Caribbean Sea population in February had a clear pattern of active and inactive forms (day vs. night) of the enzyme. The Sargasso Sea population of Trichodesmium spp. had both forms of the enzyme at all times of the day during April and May when NO3 - is present in the euphotic zone due to recent mixing. These differences between the two populations may be due to different environmental conditions since the Caribbean Sea is permanently stratified, warmer and nutrient-depleted throughout the year. The Sargasso Sea undergoes seasonal breakdown of the thermocline during winter months, resulting in an injection of nitrate from deeper water, and minimum temperatures of 18oC.