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

AbstractRecent 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 ofTrichodesmium erythraeumIMS101 (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) ofTrichodesmium(> 2 µm,TrichodesmiumPOC), 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 ofT. erythraeumwere 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 fromT. erythraeumdecomposed faster in the Al-enriched treatments during the first decay phase when bacterial POC and DOC increased along with the decomposition ofTrichodesmiumPOC. 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 ofTrichodesmiumcolonies to the ocean bottom. Following this second addition, the decomposition rate of bothTrichodesmiumPOC 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 nitrogen fixation byTrichodesmium, and thus the supply of new nitrogen to the euphotic zone, whereas Al from sediment may decrease the decomposition rate of decayingTrichodesmiumsettled to the ocean bottom.