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Miracle of Man: The Problem of Phosphorus

Photo: Phosfate mine, Republic of Nauru, by Lorrie Graham/AusAID, CC BY 2.0 , via Wikimedia Commons.

Michael Denton’s series of books about our Privileged Species, culminating with The Miracle of Man, brings together an astonishing collection of natural “coincidences” that make human life possible. These finely tuned parameters, from the nature of water to the composition of earth’s atmosphere and crust, to the metals that inhabit key enzymes, and many more, leave readers with little choice but to conclude with Denton that the scientific evidence converges on a prior fitness for life on earth. Unlike astrobiologists who are content to discuss mere habitability, Denton proposes that these details appear designed beforehand for complex beings like us, with bodies and brains equipped to invent and use technology. 

Yet among the chemical elements he considers in detail in his books, there is one he passed over comparatively lightly: the element phosphorus (P). 

An Essential Element

Phosphorus, element 15 in the periodic table, is essential for energy (ATP, adenosine triphosphate), the genetic code with its sugar-phosphate backbone, cell membranes, hormones, bones and teeth, and much more. After describing the exquisite fitness of phosphates for energy in cells in The Miracle of the Cell, Denton quotes Edward O’Farrell Walsh who summed up this element’s importance: “It is no exaggeration to say ‘without phosphorus: no life.’”

And yet the bioavailability of P sets up a problem. While it is the 11th most abundant element in the crust, it tends to be concentrated in certain isolated places, far from the plants and animals that need it. This is obvious from the fact that we need to buy fertilizers for our gardens and farms. Phosphate is the middle number in the familiar triad of elemental ratios in store-bought fertilizers such as “21-10-3” which stands for amounts of N, P, and K in the product. Nitrogen can be obtained from the atmosphere, and potassium (K), while also not ubiquitous, is more abundant than phosphorus. 

Worse, most inorganic phosphorus (Pi) is locked up in insoluble rocks like apatite and phosphorite. (Elemental P is highly reactive; Pi is almost always found in phosphates, PO4, which are what life uses.) Although small concentrations can be found globally, 70 percent of the commercially available phosphate deposits are in Morocco and China. Readers may recall hearing at the outset of Russia’s invasion of the Ukraine that 28 percent of the world’s fertilizer is exported by those two countries, threatening global food shortages.

Humans have known for millennia that fertilizing plants increases their yield. Once phosphorus was identified as an essential element in fertilizer, people went looking for it. Bat guano served the purpose in the 19th and 20th centuries, but demand for phosphorus has risen sharply since the mid 1950s, and guano is not sufficiently abundant for the global demand. The world has 8 billion hungry people to feed, not counting all the other species in the biosphere needing phosphate.

An Instructive Primer

Last month, Current Biology published an instructive primer by Yves Poirier, Aime Jaskolowski, and Joaquín Clúa on “Phosphate acquisition and metabolism in plants.” Figure 1 shows a historic phosphate mine in Peru that was depleted in 1874 by exports to Europe. Another photo shows a currently active mine in Morocco. A bleak outlook follows:

Current economically exploitable P-rich deposits could be exhausted within 50–100 years, with the mining of sub-optimal phosphate rock potentially extending production for an additional 200–300 years. Regardless of the various estimates, P-rich deposits are finite resources, and their limited availability will eventually become a key issue for long-term food security. [Emphasis added.]

With these disturbing facts in mind, how can we fit the phosphorus problem into Denton’s hypothesis of prior fitness for complex life? Spoiler alert: the fact that plants and animals flourish so well around the globe suggests that there are answers. Here’s a hint: look at organic phosphate (Po). Phosphorus may be a limiting factor in rocks, but it is plentiful in life and can be recycled.

Considering our current dependence on phosphorus fertilizers for food production and the geopolitical aspects associated with current resources, it will be important to develop technologies enabling the maintenance of high crop yield with reduced fertilizer input. This will require an in-depth knowledge on the various pathways that enable plants to acquire phosphorus from the soil and maximize its economical use for growth and reproduction.

Design to the Rescue

PAE (Pi Acquisition Efficiency) refers to the suite of strategies plants use for acquiring inorganic phosphate. Once they have it, they also optimize its use in cells.

How do plants optimize Pi use? Plants can adapt to Pi-deficient conditions by improving PAE and optimizing the internal use of Pi and Pi-derived metabolites to maintain growth, i.e. increase the ratio of biomass or yield generated per molecule of acquired Pi (Pi use efficiency (PUE)….

The authors describe sets of design principles (they call them adaptations) for optimizing phosphate. Here are some of a plant’s strategies for phosphate acquisition (PAE) in times of phosphorus deficiency:

  • Grow longer root hairs to explore more soil.
  • Stimulate commensal fungal mycorrhizae to explore hundreds of times the volume of soil available to the root hairs. This is accomplished by upregulation of strigolactone enzymes.
  • Acidify the soil with carboxylates to promote release of Pi from minerals.
  • Shorten the primary root and decrease gravitropy to explore the upper layers of soil where Pi tends to be more plentiful.
  • Stimulate uptake of Pi in the roots with upregulation of the PHT1 gene family of Pi/H+ co-transporters and optimize their placement in the roots.
  • Release nucleases and phosphatases in the soil to break down leaf litter and other dead plant material and recycle its Po content in DNA, RNA, and proteins. 

Here are some of the plant’s strategies for efficient use of phosphates in the cell (PUE) in times of phosphorus deficiency:

  • Substitute Pi-rich phospholipids with galacto- and sulfolipids.
  • Scavenge Pi from DNA, ribosomal RNA, and other Po-rich molecules.
  • Redistribute Pi between tissues depending on need.
  • Activate molecular process that do not require ATP.

These strategies are all mediated by molecular machines arranged in cooperative systems that monitor and regulate P levels. Many of these are named in the article for those who take interest in the details; I counted at least two dozen enzyme families named in the paper that participate in phosphorus homeostasis, and that is most likely an incomplete inventory.

Of particular interest are the signal transduction systems involved in P homeostasis — some of which need P to work. Phosphorylation, for instance, attaches phosphate groups to genes and proteins to switch them on and off. P regulation, therefore, requires P itself — another chicken-and-egg puzzle for those speculating about the origin of life. ATP gives another example. Cells need ATP and GTP to operate some of the phosphate-intake enzymes, which would not work without phosphate already working in those energy sources. And the ATP synthase rotary engine that manufactures ATP uses phosphate in its components and the components of its upstream machinery. That’s why the metabolic system in mitochondria is known as “oxidative phosphorylation.”

Animals possess numerous strategies to maintain optimal phosphorus levels as well. They, too, have organs and systems highly dependent on P for their genetic code, energy, membranes, signaling — the whole toolkit plants have, and more. Vertebrates need P for bones and teeth (hydroxyapatite). Animals return the favor to plants through their urine and manure — recycling phosphorus naturally long before humans invented agriculture. Leaf litter and decay of plant material also recycles P to the soil. It doesn’t all have to come from Morocco and China.

For humans, the richest sources of bioavailable Po are dairy, red meat, poultry, seafood, legumes, and nuts (Harvard Nutrition Source). In modern times, additives of inorganic phosphorus, used for preservatives, contribute a non-trivial amount of Pi to the human diet, which is readily absorbed. P toxicity is rare because the body is very effective at removing excess P through the kidneys. 85 percent of our phosphorus is stored in bones and teeth. These stores serve as a backup reservoir for P in times of phosphorus deficiency.

Industrial transport of phosphorus via fertilizers can cause leaching of the soil and eutrophication of water sources, causing algal blooms that can be toxic to fish and other animals. Such phosphorus sinks into seafloor sediments out of reach for land plants. Planning for sustainable human agriculture, therefore, must consider the long-term consequences of phosphate mining and the political implications of depending on limited outcrops of apatite and other primary sources of Pi. But has this always been a concern? 

Here’s a question on the side: does the practice of leaving cropland fallow help conserve phosphorus? Every seven years, according to Leviticus 25, the Hebrews were commanded to give the land a “sabbath rest.” Did this serve to replenish bioavailable phosphate? Here’s a paper about a research project in Japan (2018) that indicated improved phosphorus availability for a rice farm that used fallow farming methods. This topic deserves more study. 

The Phosphorus Cycle

Nothing is ever lost, of course, as the First Law reminds us. Because P is a chemical element, it is not destroyed — but it can become less available. Phosphorus, like every other essential element, cycles through the biosphere. Unlike carbon, oxygen, and water, though, P never enters a gas phase at ambient temperatures, and so its cycle is much slower. Plate tectonics can deliver it back to the surface again, but a much quicker method is volcanoes. Indeed, evolutionists at the University of Washington think volcanoes were essential to supply P for the first life, and that erosion of volcanic rocks supplied this essential element for all subsequent life.

Scientific materialists who deny any prior fitness of the planet for life must surely wonder how earth got its original supply of phosphorus and the other requisite elements. They must believe that the solar nebula had the right concentration of each element, and that they all ended up in the shallow crust to be available for land organisms. The evolutionary timeline, with its rapid colonization of the oceans and rich biomes like tropical rainforests, coral reefs, and montane forests, does not picture a biosphere starved for phosphorus. New Scientist opines that even mighty dinosaurs were cold-adapted, some of them having thrived in the arctic. The active predatory dinosaur pictured at the top of the article is not showing symptoms of hypophosphatemia (phosphorus deficiency). Think how much phosphorus that one dinosaur was using. If a man’s cells can manufacture his body weight in ATP in a day, how many ATP were needed for that theropod, or for an Ultrasaurus, not counting P requirements for their bones and teeth?

The success of the biosphere through time provides ample circumstantial evidence of adequate availability of phosphorus from the beginning, despite P’s uniqueness as a limiting factor for life in solid form. Studying the phosphorus cycle in detail — from astronomy through geology through biology — would be a good research project for design-favoring scientists. It would eliminate a potential exception to Denton’s “prior fitness” argument. Most likely, with the circumstantial evidence at hand, it could become one of its strongest examples.