Effects of aluminum (Al) on marine organisms have been attracting increasing attention in recent years. Recent studies show that Al could be beneficial to the growth of marine phytoplankton through facilitating the utilization of dissolved organic phosphorus, iron (Fe), and dinitrogen (N₂). It can also influence the decay and decompostion of biogenic matter. On the basis of this knowledge, we have proposed the Iron-Aluminum Hypothesis, which proposes that Al, through enhancing carbon fixation in the upper ocean, and facilitating the export and sequestration of the fixed carbon to ocean depths, could play an important role in the ocean carbon cycle and climate change. In this presentation, I will talk about our new results on the uptake and subcellular distribution of Al in marine phytoplankton, and the effects of Al on the carbon fixation of four marine phytoplankton species (three diatoms and one cyanobacterium) and the decay of the fixed organic carbon by using radiocarbon (¹⁴C). Our results provide strong evidence that marine diatoms can take up and internalize Al into their cells, with a rate comparable to the internalization flux of Fe to the same diatom. Once being assimilated, Al was partitioned to subcellular components in the following order: granules (69 ± 5%) > debris (17 ± 4%) > organelles (12 ± 2%) > heat-stable peptides (HSP) (~2%) > heat-denaturable proteins (HDP) (< 1%), indicating that the majority of intracellular Al was detoxified and stored in inorganic forms, and substantial Al was further transported to organelles (e.g., chloroplast). Consistent with our previous results of the beneficial effects of Al on the growth of marine phytoplankton, the presence of Al enhanced carbon fixation by the tested marine phytoplankton species (up to 9%–29% for Thalassiosira pseudonana, 15%–20% for T. oceanica, 15%–23% for T. weissflogii and 6%–13% for Trichodesmium erythraeum). More importantly, our results for the first time showed that the presence of environmentally relevant concentrations (40 nM to 200 nM) of Al could significantly reduce the decay and decomposition of the phytoplankton-produced organic carbon in seawater media (up to 21%–57% for T. pseudonana, 2%–28% for T. oceanica, 29%–58% for T. weissflogii, and 28%–57% T. erythraeum). These results provide new evidence for the Iron-Aluminum Hypothesis.