The US Food and Drug Administration is concerned that Americans consume too much salt. Yet, when my wife was pregnant with our sons, her obstetrician was concerned that she was not consuming enough salt. In other words, too much—or too little—salt in our diets can be harmful. And what is true for humans is true for every life-form. Now a new scientific discovery reveals that cyanobacteria play a vital role in maintaining a perfect salt balance in our oceans.1
Salt is a “vital poison.” Too little salt in your diet will kill you (less than 1,250 milligrams per day). Too much salt in your diet also will kill you (more than 5,750 milligrams per day). Our bodies are designed to operate within a narrow range of salt concentration. For optimal health, the salt concentration in our bodies must be just-right (227±5 grams for a 150-pound man).
Our Oceans’ Salinity History
What is true for humans is true for every life-form that exists now and has previously existed. While some life-forms can tolerate higher or lower salt concentrations, for every life-form the salt concentration must be fine-tuned.
Land-based animals can regulate their salt intake by carefully choosing what they eat. Land-based plants can sprout and thrive in soils with the appropriate salt concentration. Marine life, on the other hand, is at the mercy of the ocean’s salt concentration.
At the current time in Earth’s history the ocean’s salt concentration is equivalent to that of human blood. This salt concentration is ideal for large, active animals and permits the greatest diversity of marine life Earth has ever possessed. However, the concentration has not always been at this optimal level.
Salinity History of the Oceans
Until the beginning of the supercontinent cycle 2.4 billion years ago, the oceans’ salinity was about twice the modern value.2 Previous to 2.4 billion years ago, the absence of continents meant no sequestering of giant brine (saline) beds from evaporating seawater was occurring. With no mechanism for removing salt from the oceans, ocean salinity remained high. High ocean salinity limited marine life to salt-tolerant microbes. Since oxygen solubility in the oceans decreases strongly with increasing salinity, previous to 2.4 billion years ago oceans were anoxic (oxygenless) and therefore dominated by anaerobic (nonphotosynthetic) microbes.
The launch of the supercontinent cycle reduced ocean salinity to about 50 percent greater than the modern value. This ocean salinity level kept the oceans sufficiently starved of soluble oxygen to limit marine life to microbes. More of these microbes were aerobic than previous to 2.4 billion years ago. Nevertheless, anaerobic microbes likely still dominated marine life.
The occurrence of three major glaciation events just previous to the Avalon explosion of Earth’s first animals (575 million years ago) dramatically cooled Earth. Since oxygen solubility in the oceans increases with decreasing ocean water temperature, ocean oxygen solubility rose sufficiently to permit the existence of the first animals. These first animals, however, lacked eyes, a brain, and a digestive tract. The ocean oxygen content at that time was too low to support such body organs.
Just previous to the Cambrian explosion of Earth’s first complex animals (543 million years ago), major tectonic events sequestered enormous amounts of salt and brine in giant evaporite basins. These events brought ocean salinity down to only a few percent above the modern value, resulting in sufficient ocean oxygen solubility to permit the existence of complex animals for the first time.
As I explain and document in my book Improbable Planet, the sudden rise in soluble oxygen content in the oceans was accompanied by an equally sudden and simultaneous rise in tens of phyla of complex animals.3 This Cambrian explosion event ranks as one of the greatest challenges to naturalistic evolution.4
Since the Cambrian explosion, major asteroid collision events and their accompanying massive volcanic eruptions, plus the formation and breakup of the Pangea supercontinent, have caused ocean salinity to suddenly jump up and down by as much as 40 percent.5 These jumps played major roles in both mass extinction and mass speciation events that occurred during the past half billion years.
Role of Cyanobacteria
Until recently scientists assumed that the only factors operating to lower open ocean salinity were precipitation, melting of glaciers, and the creation of evaporite basins through the supercontinent cycle. It was also assumed that on timescales briefer than a century the only significant factor was precipitation.
A team of nine marine biologists and chemists published a paper in Geophysical Research Letters in which they show that large blooms of cyanobacteria (a phylum of bacteria that get their energy through photosynthesis) floating on the ocean surface play a vital role in reducing the salinity of ocean surface water.6 The team performed high-resolution observations on several large cyanobacteria blooms. In each case, their observations showed that in the vicinity of the large cyanobacteria blooms the blooms slightly warmed the ocean surface water and substantially lowered its salinity. As such, major cyanobacteria blooms create habitats for life-forms that can thrive under the reduced salinity conditions. Thus, it is not only at the estuaries of major rivers where marine life that requires reduced ocean salinity can thrive but also in the vicinity of major cyanobacteria blooms.
As Earth’s dominant form of life presently, phytoplankton are responsible for about half of the global net primary production.7 Cyanobacteria are one of the more prevalent phytoplankton life-forms, making up about 15 percent of all phytoplankton.8 Thus, cyanobacteria play a key role in regulating the salinity of the oceans’ surface layers. For a great diversity of both simple and advanced life to thrive in the oceans, the salinity levels at different parts of the world’s oceans must be fine-tuned. This fine-tuning requirement implies both the quantity and location of cyanobacteria must be fine-tuned. It’s a tiny creature with massive implications. Thank God for his fine-tuned design of the quantity, type, and location of cyanobacteria.