How Ancient Ocean Chemistry Might Have Increased Complexity of Life

Ediacaran Reorganization of the Marine Phosphorus Cycle 

Thomas A. Laaksoa, Erik A. Sperling, David T. Johnstona, and Andrew H. Knoll

Summarized by Makayla Palm

What data were used? The purpose of this study was to measure if changes in the phosphorus cycle were linked to changes in the chemical composition of ocean water hundreds of millions of years ago. The phosphorus cycle is the study of the element phosphorus as it travels from deep-sea storage and rock formations into organic life, and back to the seas again. Why study phosphorus in the first place? Phosphorus is essential to life because it is an important component in DNA and RNA structure. Specifically, at the end of the Ediacaran (~625–542 million years ago or mya), there was a jump in complexity in the fossil record (i.e., life became more complex) found in the transition from the Ediacaran to the Cambrian (~542–485 mya); it may be the case that this change in phosphorus can help us understand the changes to life on Earth during this time. Previously collected phosphorite samples (rocks with a high phosphorus content) and newly found samples from the Doushantuo Formation (Ediacaran, China) were used in this study. These phosphorite samples were examined for the following: evaporite volume, strontium isotope ratios, and content of phosphate. Changes in these samples’ ratios and concentrations allow researchers to hypothesize the impacts on water and life during the Ediacaran. Originally, scientists thought the changes may have been due to increased weathering of rocks, but researchers in this study hypothesized that there may have been more to the story. 

Methods: Researchers from this study hypothesized that a change within deeper Ediacaran ocean chemistry may be the cause for the phosphorus cycle change. They tested this hypothesis by using the variables collected (e.g., isotopes) in an equation that measures the possible effects of the phosphorus evaporite remineralizing into phosphorite (typically how phosphorus is stored in the ocean) This equation measures the amount of phosphorus taken out of the storage bank by measuring the fraction of total organic phosphorus that is removed in relation to the amount of phosphorus that reverts back to its original form in the storage bank. 

Results: The changes in ocean chemistry can be found on the atomic scale, where there are electron acceptors (also known as oxidizers) and electron donors (or reducers). The ocean, having been in a state of consistent reducing reactions, may have shifted to have more oxidizers, which would have increased remineralization – specifically, phosphorus remineralization. This remineralization would explain the difference that eventually modified the Ediacaran phosphorus cycle to the modern-day phosphorus cycle. In order for phosphorus to reduce, something needs to accept its electron. In the absence of oxygen (which early Earth was lacking in for billions of years), research indicates sulfate may be a suitable candidate. Samples of sediment did not indicate a change in phosphorus content, so the hypothesis was not supported. This means that the phosphorus was likely staying within the same system and being removed. The phosphorus cycle, similar to the water cycle or carbon cycle, describes the formation, use and recycling of phosphorus from the oceans, to land, and back to the ocean. The data from this study indicate that upwelling, the mixing of nutrients from the bottom of the ocean back to the top, is the reason for increased phosphorus. Upwelling can be caused by deep water currents coming into contact with continents, where cold, nutrient rich water is propelled closer to the surface and warms. The increased upwelling makes sense in the phosphorus cycle because of the extra circulation happening, which would explain the increased presence of phosphorus without an added source of the element. 

This figure represents three different kinds of information collected over the same period of time. The top graph is a bar graph that measures the amount of phosphate evaporite that was removed and not returned to the phosphorus storage bank. The middle bar graph measures the total amount of phosphate resources stored in the form of P2O5. This graph represents the amount in millions of tons. The line graph at the bottom of the figure represents the number of strontium isotopes found within the rock samples. This graph represents inconsistent intervals of small increasing and decreasing values, showing an overall increase through time in each graph. Across all three graphs, columns highlight the appearance of phytoplankton and large animals within the fossil record. The appearance of phytoplankton is approximately 700 million years ago, and the appearance of larger animals is around 720-699 million years ago. The appearance of both is marked by horizontal black bars at the bottom; with each appearance, there is an uptick in strontium 87. More complex life is marked by more phosphate and evaporites. These bars represent the appearance of organisms in all three line graphs.
The figure represents the three different kinds of data discussed in the paper. The top demonstrates the volume ( km cubed) of phosphorite evaporite, with a general trend of increasing evaporite over time.The middle graph represents the amount of phosphate resources stored in the “storage bank” in the ocean (in millions of tons). The bottom graph represents the change in Strontium isotopes, with ebb and flow in value over time, with a general trend that after a strong dip ~ 700 million years, trends upward. Ice ages are indicated with gray vertical bars across all three graphs, indicating a change in ecosystem. The dark horizontal bars at the bottom of the figure indicate when the appearance of phytoplankton and macroscopic animals occur, which is ~ 680 million years for the phytoplankton and ~ 650 million years for the macroscopic animals. The vertical gray shading represents Ice Ages that occurred in the timeline measured on the figure. The figure as a whole points to the correlation of increased phosphorite levels and the first appearance of relatively large animals in the fossil record.

Why is this study important? This study aims to see why the change in phosphorus occurred to better understand the geologic context that precedes a big change in the fossil record. There is a large jump in complexity from Ediacaran to Cambrian organisms, and ocean chemistry (changes in phosphorus levels in this case) may have had something to do with that. The cycling of phosphorus because of upwelling, influenced by continental placement, could have been a driving reason behind these big changes, ecological and evolutionary. 

Big Picture. This study proposes a mechanism for the change in the phosphorus cycle that is observed between the phosphorus cycle today and the phosphorus cycle of the Ediacaran as we know it. Many questions still exist as to how oceans have changed through geologic time and this study provides an important piece to the puzzle. Understanding changes in ocean chemistry, too, better helps scientists understand how life evolves in response. 

Citation: Laakso, Thomas A., et al. “Ediacaran Reorganization of the Marine Phosphorus Cycle.” Proceedings of the National Academy of Sciences, vol. 117, no. 22, 2020, pp. 11961–11967., 

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