Abstract

Aluminum is the most abundant metal element in earth crust, and it is ubiquitous in different environments. River runoff and dust deposition could bring them with Al into the ocean, where Al could potentially influence marine organisms. In contrast to substantial reports on the Al effects on biota in the acidic and neutral pH media, little is known regarding the effects of Al on organisms in the alkaline seawater. The Al speciation in seawater is quite different from that in freshwater, implying that the Al effects on marine phytoplankton could be different from those on freshwater organisms. We have started the journey focusing on the Al effects on marine phytoplankton since 2010. Based on substantial Al enrichment experiments in the field, we found that Al may be beneficial to the growth of several marine phytoplankton, including stimulating the growth of diatoms and Trichodesmium, and enhancing nitrogen fixation and chlorophyll a concentration of the entire planktonic community. We also found that the Al effects on marine phytoplankton growth were associated with nutrient status in seawater. The stimulatory growth of many marine phytoplankton species was confirmed in laboratory experiments. We found that Al could enhance the diatom utilization of dissolved organic phosphorus under phosphorus-limited condition, which could be one of the mechanisms for the Al effects on marine phytoplankton. In addition, Al could reduce the solubility of biogenic particles like diatom frustules and “Tunicata houses”, and thus increase the carbon export to the deep ocean (i.e. increase the efficiency of the biological pump). Therefore, Al may play an unrecognized important role in the global carbon cycle. We would like to propose a revised Iron Hypothesis (i.e., the Al Hypothesis) that Al, as well as iron, play an important role in the glacial-interglacial change in atmospheric CO₂ concentration. Our hypothesis may provide alternative explanation for the inconsistence between the results of the “Artificial Iron Fertilization experiments”, and the expectation of the Iron Hypothesis proposed by John Martin. That is that natural iron fertilization such as dust deposition not only brings it with iron, but also other elements including Al, which was usually ignored by the previous Artificial Iron Fertilization experiments.