5:15 PM - 6:45 PM
[BCG06-P05] Vertical structures of marine microbial ecosystems and roles of coupled cycles of iron and phosphorus before the Great Oxidation Event
★Invited Papers
Keywords:Great Oxidation Event, Whiffs of oxygen, atmospheric oxygen, marine microbial ecosystem
Prior to the Great Oxidation Event (GOE) in the Paleoproterozoic, 2.4-2.0 billion years ago, the oceans were essentially anoxic and rich in dissolved ferrous iron. However, it has been suggested that temporary rises in atmospheric and marine oxygen, the so-called whiffs of oxygen, may have occurred several hundred million years before the GOE (Anbar et al., 2007; Planavsky et al., 2014). If this is the case, oxygenic photosynthesis might have emerged long before the GOE, but their activity might have been suppressed until the GOE. One of the possible explanations for this suppression is that oxygenic photosynthesis was limited by phosphorus owing to adsorption of phosphate onto iron (oxyhydr-)oxides (Bjerrum and Canfield, 2002). Additionally, oxygenic and anoxygenic phototrophs may have coexisted and competed in a vertically segregated manner in the pre-GOE oceans, and this could have affected the limitation on oxygenic photosynthesis (Ozaki et al., 2019). Although Ozaki et al. (2019) suggested this possibility using a vertical one-dimensional high-resolution marine biogeochemical model, the influence of Fe-P affinity on marine microbial ecosystems has not been clearly demonstrated. Therefore, it remains unclear how ecosystems were affected by adsorption characteristics of iron oxides and the presence of iron sinks other than iron oxides, such as iron sulfides and iron phosphates.
We developed a vertical one-dimensional high-resolution marine biogeochemical cycle model to investigate the behaviors of biogeochemical cycles and microbial ecosystems in the ocean surface (0-500 m) before the GOE, and to examine how oxygenic photosynthesis could have been restricted. To investigate the coupled cycles of Fe and P in the pre-GOE marine environments and the responses of marine microbial ecosystems, we conducted a parameter study on two important parameters, the Fe/P ratio of settling iron oxide particles ([Fe/P]sorption) and the dissolved Fe/P ratio of deep water which is given at the bottom of the model as a boundary condition ([Fe/P]bottom). We show that the primary productivity of the ecosystem, especially oxygenic photosynthesis, depends critically on the combination of [Fe/P]bottom and [Fe/P]sorption. Oxygenic photosynthesis is strongly [YW1] restricted if [Fe/P]sorption < [Fe/P]bottom, regardless of the presence of photoferrotrophs in the ecosystem. We also show that, when oxygenic phototrophs coexist with photoferrotrophs, oxygenic photosynthesis is possible only if [Fe/P]sorption > [Fe/P]bottom and [Fe/P]sorption <~ 500.
We conclude that three types of changes in the surface environment could have promoted oxygenic photosynthesis: (1) an increase in phosphate in the oceans, (2) a decrease in dissolved ferrous iron in the oceans, and (3) an effective release of phosphate from iron oxides, all of which may have occurred through the late Archean and the early Proterozoic. The changes (1) and (2) correspond with decreases of [Fe/P]bottom, and (3) corresponds with increases of [Fe/P]sorption. When [Fe/P]sorption surpasses [Fe/P]bottom, oxygenic photosynthesis is considerably enhanced, and this could have contributed to the whiffs of oxygen and the subsequent GOE. The change (3) could have been triggered by increases in marine sulfate. It has been suggested that increases in sulfate may have promoted iron sulfide formation before and after the GOE (Heard et al., 2020). This could have made the phosphate removal with iron oxides less significant, in favor of oxygenic photosynthesis. Furthermore, increases in atmospheric oxygen could have elevated the oxidative weathering rate of continental crusts, further increasing marine sulfate and potentially causing a positive feedback mechanism to raise atmospheric oxygen.
References
Anbar et al. Science 317, 1903–1906 (2007).
Bjerrum, & Canfield, D. E. Nature 417, 159–162 (2002).
Heard et al. Science 370, 446–449 (2020).
Ozaki et al. Nat. Commun. 10, 3026 (2019).
Planavsky, et al. Nature Geosci. 7, 283–286 (2014).
We developed a vertical one-dimensional high-resolution marine biogeochemical cycle model to investigate the behaviors of biogeochemical cycles and microbial ecosystems in the ocean surface (0-500 m) before the GOE, and to examine how oxygenic photosynthesis could have been restricted. To investigate the coupled cycles of Fe and P in the pre-GOE marine environments and the responses of marine microbial ecosystems, we conducted a parameter study on two important parameters, the Fe/P ratio of settling iron oxide particles ([Fe/P]sorption) and the dissolved Fe/P ratio of deep water which is given at the bottom of the model as a boundary condition ([Fe/P]bottom). We show that the primary productivity of the ecosystem, especially oxygenic photosynthesis, depends critically on the combination of [Fe/P]bottom and [Fe/P]sorption. Oxygenic photosynthesis is strongly [YW1] restricted if [Fe/P]sorption < [Fe/P]bottom, regardless of the presence of photoferrotrophs in the ecosystem. We also show that, when oxygenic phototrophs coexist with photoferrotrophs, oxygenic photosynthesis is possible only if [Fe/P]sorption > [Fe/P]bottom and [Fe/P]sorption <~ 500.
We conclude that three types of changes in the surface environment could have promoted oxygenic photosynthesis: (1) an increase in phosphate in the oceans, (2) a decrease in dissolved ferrous iron in the oceans, and (3) an effective release of phosphate from iron oxides, all of which may have occurred through the late Archean and the early Proterozoic. The changes (1) and (2) correspond with decreases of [Fe/P]bottom, and (3) corresponds with increases of [Fe/P]sorption. When [Fe/P]sorption surpasses [Fe/P]bottom, oxygenic photosynthesis is considerably enhanced, and this could have contributed to the whiffs of oxygen and the subsequent GOE. The change (3) could have been triggered by increases in marine sulfate. It has been suggested that increases in sulfate may have promoted iron sulfide formation before and after the GOE (Heard et al., 2020). This could have made the phosphate removal with iron oxides less significant, in favor of oxygenic photosynthesis. Furthermore, increases in atmospheric oxygen could have elevated the oxidative weathering rate of continental crusts, further increasing marine sulfate and potentially causing a positive feedback mechanism to raise atmospheric oxygen.
References
Anbar et al. Science 317, 1903–1906 (2007).
Bjerrum, & Canfield, D. E. Nature 417, 159–162 (2002).
Heard et al. Science 370, 446–449 (2020).
Ozaki et al. Nat. Commun. 10, 3026 (2019).
Planavsky, et al. Nature Geosci. 7, 283–286 (2014).