A day on Earth (i.e. the period between one sunrise and the next) lasts for 24 hours, but four billion years ago it may have been as short as six hours. Thus, the length of the day, and the length of time for which any given part of the Earth's surface is exposed to sunlight during each cycle has increased threefold over the history of the planet. The rate of photosynthesis (i.e. the rate at which oxygen is produced by Plants, Algae, and Cyanobacteria exposed to sunlight) is determined by instantaneous photon flux, and should not be affected by the length of the day, as long as the total amount of sunlight over any given period remains the same. However, the net rate of oxygen is influenced by both how much oxygen is produced, and the rate at which organic material (and the bio-available carbon it contains) is buried, and this burial rate is potentially influenced by the length of the day. Thus the net production of oxygen by benthic ecosystems will be influenced by changes in the length of the day, due to changes in the availability of metabolites, the import, export and accumulation of which can be sensitive to daylength.
In a paper published in the journal Nature Geoscience on 2 August 2021, Judith Klatt of the Microsensor Group at the Max Planck Institute for Marine Microbiology, the Department of Earth & Environmental Sciences at the University of Michigan, Arjun Chennu, also of the Microsensor Group at the Max Planck Institute for Marine Microbiology, and of Data Science and Technology at the Leibniz Centre for Tropical Marine Research, Brian Arbic, also of the Department of Earth & Environmental Sciences at the University of Michigan, Bopaiah Biddanda of the Annis Water Resources Institute at Grand Valley State University, and Gregory Dick, again of the Department of Earth & Environmental Sciences at the University of Michigan, present a model which aims to explore this interaction, and how it would influence the export of oxygen into the atmosphere.
Klatt et al. modelled benthic ecosystems as systems in which oxygen photosynthesis, anoxygenic photosynthesis, aerobic respiration, anaerobic respiration (by sulphate reduction or without sulphate reduction), and the oxidation of sulphides by non-biological means, could all occur. The simplest model, with only oxygen photosynthesis and aerobic respiration, showed the amount of oxygen exported to the water column increased with increasing day length, as oxygen will not move from the mats to the water column until it builds up to a certain concentration, which takes time at the beginning of each daylight period. As this build up time is fixed, it represents a larger proportion of the day when the day is shorter, and a smaller proportion when the day is longer. Thus, with a longer day, the amount of oxygen exported from the mats will increase. If the mats are thin, then this will not just apply to oxygen being exported into the water column, but also to oxygen being exported into the substrate, which may create a distinctive weathering pattern that could be detected. This build up of oxygen would also increase the amount of bio-available oxygen within the mats, enabling the export of more organically bound carbon from the mats, therefore influencing the overall efficiency of the mats, and increasing the amount of organic carbon being buried, which in turn influences the level of oxygen in the atmosphere. Thus, although the rate of photosynthesis is independent of the length of the day, increasing the length of the day increases the amount of organic carbon burrial, and therefore the amount of oxyen build up in the water column and atmosphere.
For a more realistic scenario, Klatt et al. also included the consumption of organic carbon by anaerobic respiration (i.e. through the sulphate reduction process). In theory, both sulphate (the oxidised form) and sulphite (the reduced form) can be reacted with any other redox pair (such as ferrous and ferric iron), but Klatt et al. concentrated on the reduction of sulphate to sulphide, as this is thought to have evolved in Microbes early in life's history, and sulphide is known to have been abundant in many Precambrian coastal sediments. Klatt et al. therefore began with a model in which anaerobic sulphate reduction occurred at a fixed rate, in order to assess the impact of day length changes on the export of sulphites from the mats. As with the export of oxygen, the export of sulphites is determined by diffusion rates, anf the extent to which the produced sulphite is consumed within the mats. The consumption of sulphites within the mats also uses oxygen, competing with aerobic respiration for the available supply. Any organic carbon that is exported from the mats must escape being used for either aerobic or anaerobic respiration, and, therefore, the rate at which this is produced relates directly to the amount of oxygen and the amount of sulphite being exported. As the day length increases, the rate of both aerobic and anaerobic respiration increases, and the rate of sulphur oxidisation decreased. Thus, the rate of aerobic respiration is less sensitive to the effect of day length when sulphate reduction is introduced to the model than it was previously, and the rate at which oxygen is exported from the mats is lowered, but not eliminated, by the inclusion of sulphite production. Modern mat-dwelling sulphate-reducing Bacteria are inhibited by the presence of oxygen; when Klatt et al. introduced this to their model, the rate at which organic carbon was buried increased with increasing daylength.
Since the redox environment on Earth is known to have changed over time (just as the day length has), Klatt et al. examined the relationship between the (daylight driven) rate of organic carbon burrial and the available oxygen in the water column. Increasing the day length was found to increase both the rate of organic carbon burial and the rate at amount of available oxygen in the water column under all circumstances. Increasing the available oxygen in the water in turn led to an increase in the rate of aerobic respiration. However, the influence of available oxygen had a more complicated affect on the rate of organic carbon burial, as organic carbon can be produced both aerobically and anaerobically, dependent on whether or not the anaerobic respiration method was sensitive to the presence of oxygen. When Klatt et al. assumed that anaerobic respiration was inhibited by the presence of oxygen, as is the case with most anaerobic respirating microbes today, then increasing the day length resulted in a higher rate of organic carbon burial.
Next Klatt et al. considered the possibility that anaerobic photosythesisers, using hydrogen sulphide as an electron donor, might be present in the mats. This resulted in a longer time-period before the mats began to export oxygen each day, during which time the anaerobic microbes were depleting the hydrogen sulphite to a level at which oxygen photosynthesis could occur. Thus, although the amount organic carbon produced by photosynthesis remains fairly constant, increasing the day length both increases the amount of oxygen being exported and lowers the rate of anaerobic respiration (due to the presence of inhibiting oxygen). Unexpectedly, this also lowers the rate of aerobic respiration, due to the presence of sulphur oxides, which compete with the respirators for the available oxygen. Thus, when modelled with a wide range of oxygen and sulphur levels in the water column, Klatt et al. found that in all cases the oxygen and sulphur model made organic carbon buial more closely tied to day length than was the case when only oxygen was considered. Thus the range of metabolic pathways available has more influence on the relationship between daylength and organic carbon burial than the redox state of the water column.
Klatt et al. were able to demonstrate that the rate of organic carbon burial, which is thought to be closely linked to the accumulation of oxygen in the atmosphere, would increase with increases in daylength under a wide range of conditions, without assuming a decreasing oxygen sink (i.e. a supply of substances on the Earth's surface which would react with oxygen, thereby preventing it from building up in the atmosphere), or an increase in the global rate of photosynthesis. It is, however, likely that the global rate of photosynthesis did change over this period, as new, more efficient metabolic pathways evolved, and redox and phosphate levels in the oceans changed due to the weathering of rocks on land during what has been turned the 'boring billion' years of the Mesoproterozoic. The accumulation of oxygen in the atmosphere is still expected to be driven largely by the global rate of production by photosynthesis, but the length of the day clearly has a major impact on the burial of organic carbon, and there is a predicted link between the extent of benthic environments in which photosynthesis can occur and the extent to which day length is able ro affect the system.
As a further test of this model, Klatt et al. measured the rate of photosynthesis and oxygen export by Cyanobacterial mats from the Middle Island Sinkhole in Michigan, USA, which are considered to be a good model for Proterozoic microbial mats under low oxygen conditions. They found that the mats only exported oxygen after they had been exposed to light for some time. White Sulphur-oxidising Bacteria grew on top of the mats during the night and early morning, and these reduced the light available for photosynthesis. Light levels did not become high enough for photosynthesis until the early afternoon, at which point the White Sulphur-oxidising Bacteria migrated downwards through the mats, possibly in response to the depletion of sulphides by the oxygen produced by the Cyanobacteria. After this, the oxygen produced needed to react with any sulphide in the water column before free oxygen began to build up, creating an additional lag of 1-8 hours. Increasing the strength of the light to the mats increased the speed at which oxygen was produced and exported into the water column, with the oxygen production remaining high once sulphides were depleted even if light levels fell.
The presence of White Sulphur-oxidising Bacteria at the top of the mats lowered light availability for the Cyanobacteria, delaying the onset of oxygen production. This implies that day length has a strong effect on overall oxygen production in communities where photosynthetic and chemosynthetic microbes are competing. Klatt et al. further tested this model by exposing samples of the mats to 'days' of different length using artificial lighting in a laboratory. They found that when the total day length was less than twelve hours then the mats produced non oxygen, instead becoming a sink for any oxygen in the water column. When the day length reached sixteen hours (predicted for the Late Archaean), then the mats became net exporters of oxygen, with the amount of oxygen produced at a day length of twenty one hours (predicted for the Late Proterozoic) doubling that produced at sixteen hours, and the amount of oxygen produced at a day length of twenty four hours being three times that of the sixteen hour day.
Similar interactions have been seen in other microbial mats at other locations, although it is unclear which of them, if any, provides the best model for life in the Precambrian. However, several different sets of conditions can be shown to produce less oxygen with a shorted day length, largely to fluctuations in the redox state within the mats over the course of the day. Many types of microbes also have the ability to alter their metabolic pathways in response to environmental conditions, which may lead to further delays in the onset of oxygen production in the presence of sulphides.
This conceptual model, in which the length of the day effects the burial rate of organic carbon, fits well with observations of modern sediments, in which it has been shown that exposure to oxygen decreases burial efficiency. As the days get longer, aerobic respiration takes over from anaerobic respiration, and the extent to which oxygen penetrates into the mat decreases. This means that layers of the mats which are not dominated by photosynthetic organisms (i.e. those lower down) actually recieve less oxygen over the course of a day. This would enhance the burial of organic carbon. The rate at which mats accumulate must also be taken into account when looking at the burial of organic carbon. Modern mats can accumulate at rates of 0.5 to 5 mm per year, with estimates of ancient mat growth having a slightly higher range, perhaps 0.5 to 15 mm per year. If lengthening days decreased the oxygen availability within the mats, and increased the burial of organic carbon, then it is likely that the accretion rate of the mats would also have increased, potentially making a more pronounced shift in productivity and carbon burial than was observed in the modern mats.
This phenomenon, in which the lengthening day caused changes in the production and export of oxygen and organic carbon from microbial mats would have gone on as long as the 'matworld' existed. Unfortunately, it is hard to quantify the rate at which this would have occurred, as we are uncertain of the rate at which the Earth's days have increased, other than that this has not proceeded at a steady rate over the history of the Earth. If we extrapolate backwards, using the current rate at which our days are lengthening and the related rate at which the Moon is moving away from the Earth, we get a scenario in which the Moon would have collided with the Earth 1.5 billion years ago; something for which there is no evidence. Some recent models have tried to take into account the influence of the position of the continents on the Earth's rotational deceleration, and have come to the conclusion that the lengthening of the days would have been at its lowest in the Middle Proterezoic. Another hypothesis suggests that there may have been periods in which the days did not lengthen at all and the Earth-Moon system remained stable due to a resonant atmospheric thermal tide. In this scenario on the length of the day would have remained stable throughout the 'boring billion' years of the Mesoproterozoic, then started to lengthen at around the time of the Neoproterozoic Oxygenation Event (between 800 and 540 million years ago), which in turn suggests there might be a link between the two events.
In order to model the impacts of a link between daylength, oxygen production, and organic carbon burial on a global scale, Klatt et al. used a recent model which suggests that the length of the days increased steadily until about 2.2 billion years ago, when they stabilized, due to a resonant stability with the Moon, with a resumption in lengthening occurring about 650 million years ago. In this model Klatt et al. combined their findings on organic carbon burial and oxygen production with other events likely to have influenced the composition of the atmosphere, notably the reducing influence on the atmosphere caused by large scale metamorphism, the production of volcanic gasses, and weathering of exposed rocks on the Earth's surface. They found that this model could account for the change in atmospheric composition associated with the Great Oxidation Event (between 2.4 and 2.0 billion years ago) without the need to invoke any change in the global rate of photosynthesis, or any other redox change in the atmosphere. This scenario starts with an 18-hour day in the Archaean, which results in zero burial of organic carbon, increasing to a 21 hour day in the mid-Proterozoic, when about 50% of the current burial rate of organic carbon is achieved, although these mats would only cover 3.7% of the area of the modern oceans (for comparison the modern shallow water zone (where photosynthesis is possible for benthic organisms) covers about 7.5% of the ocean surface. This day lengthening would allow atmospheric oxygen to reach 28% of current levels by about 550 million years ago (the beginning of the Cambrian), which is consistent with a Neoproterozoic Oxygenation Event, between 800 and 540 million years ago, and a later Palaeozoic Oxygenation Event, possibly coincident with the Ordovician Biodiversity Event, at about 400 million years ago.
Klatt et al. suggest that isotope excursions in the Precambrian rock record associated with oxygenation events are actually signals of the increased burial rate for organic carbon, which is in turn caused by increases in the global photosynthesis rate and a drop in remineralisation as the increased available oxygen caused by the increasing day length used up available oxygen-reactive minerals. It is difficult to connect isotope signatures directly to microbial mats, as limited isotope fractionation occurs within them, but the model of atmospheric oxygen enhancement and increased burial of organic carbon driven by increasing day length does match the signatures seen in the rock record. An increase in weathering in terrestrial rocks with rising oxygen levels, which would release more phosphorous into the system, resulting in a further boost in microbial productivity has been previously predicted. Klatt et al. have not included this in their models as they believe any such effect would be transitory in nature, with oxygen levels quickly returning to a steady state. The availability of nutrients could shape the rate of global photosynthesis, and therefore the possible range of oxygen pressures that could be achieved, but Klatt et al. believe the length of the day and burial rate for organic carbon would have actually driven oxygenation rates, at least until the Palaeozoic Oxygenation Event.
Klatt et al.'s models indicate that the length of the day is a consistent driver of oxygenation levels across a range of possible metabolic parameters. The possibility that the Earth and Moon could have gone through periods of resonance locking, with abrupt changes in day length as these phases are entered or escaped from, serves as a possible trigger for the distance oxygenation events seen in the rock record. In this sense, day length changes can be considered to be of similar significance to events such as the opening of major tectonic rifting zones and the formation of supercontinents, which are also thought to have had profound impacts on the composition of the atmosphere, although there is no known link between the Earth's orbital parameters and these events. Even of a simple gradual increase in day length is assumed, Klatt et al.'s model produces an increase in oxygen production and decrease in oxygen sinks that could produce relatively rapid shifts in the atmospheric oxygen level. The exact extent to which the length of days contributed to shifts in atmospheric oxygen levels is impossible to know, nut Klatt et al.'s model provides a remarkably good reconstruction of the shifts in the oxygen content of the Early Earth's atmosphere without the need to invoke other factors, which strongly suggests that the orbital dynamics of the Earth-Moon system played a role in the evolution of the Earth's atmosphere during the Proterozoic Eon.
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