Aluminum-induced changes in the net carbon fixation and carbon decomposition of a nitrogen-fixing cyanobacterium Trichodesmium erythraeum

Recent studies suggest aluminum (Al) likely plays a role in the ocean carbon cycle by altering the biological carbon fixation and carbon decomposition of marine diatoms. However, it remains speculative whether Al has similar effects on other ecologically important phytoplankton groups such as the globally important nitrogen-fixing cyanobacterium, Trichodesmium. Here we report the influence of Al on carbon fixation and decomposition in non-axenic cultures of Trichodesmium erythraeum IMS101 (CCMP 1985). By using radiocarbon, and adding oceanic relevant amounts of dissolved Al (yielding concentrations of 40 and 200 nM) along with non-Al-amended controls, we investigated the changes in particulate organic carbon (POC) of Trichodesmium (> 2 μm, Trichodesmium POC), and free-living bacteria (0.2–2 μm, bacterial POC), and dissolved organic carbon (< 0.2 μm, DOC) over a 116-day growth period. The results showed that the rates of increase of POC in the declining growth phase of T. erythraeum were significantly higher (by 11–14%) in the Al-enriched treatments than in the control, and this Al-enhanced carbon fixation is consistent with previous observations on marine diatoms. On the other hand, unlike diatoms, the POC from T. erythraeum decomposed faster in the Al-enriched treatments during the first decay phase when bacterial POC and DOC increased along with the decomposition of Trichodesmium POC. Further addition of the same amounts of Al (again calculated to increase the Al concentration by 40 and 200 nM) was performed on day 71. This treatment was designed to mimic Al supply from sediment after the settling of Trichodesmium colonies to the ocean bottom. Following this second addition, the decomposition rate of both Trichodesmium POC and DOC slowed down by 20–27% and 31–62%, respectively, during the second decay phase, when DOC and bacterial POC decreased. The study suggests that Al fertilization in the surface ocean via dust deposition may increase the net carbon fixation and associated nitrogen fixation by Trichodesmium, and thus the supply of new nitrogen to the euphotic zone, whereas Al from sediment may decrease the decomposition rate of decaying Trichodesmium settled to the ocean bottom.