Earth History – Microbial Numbers and Impact

by Roberto
 

Fig. 1. Earth. Source

More often than I care to admit, my attention is captured in circling thoughts about the many harrowing events currently unfolding on Earth and how they might affect me. No need to list them here, no doubt you get a sense of what I mean. In such times and for whatever reason, I find solace by putting my life in perspective. And one of my favorite ways to approach this is to consider life at scales that are orders of magnitude away from my direct experience. Since all the life we know of is on Earth, here are a few of Earth's basic numbers: diameter 1.2 x 10⁷ meters, mass 6 x 10²⁴ kilograms, age 4.5 x 10⁹ years. And, as far as living cells, how many?

 

When so many of us read "Prokaryotes: The unseen majority," about a quarter of a century ago, our field first became enamored with the concept that there are in the order of 10³⁰ prokaryotic cells in the biosphere. In that paper there were several other gems that we tend not to quote as often but which I find equally fascinating. The vast majority of these cells (indeed, about 10³⁰) are present in the subsurface, where they seem to follow the slow life as their numbers turn over every thousand years or so. Comprising just about 10% of the total are the marine prokaryotes, coming in at some 10²⁹ cells but turning over quite a bit faster, every week or thereabouts. The authors estimated that Earth's overall cellular production rate is about "10³⁰ cells/yr and is highest in the open ocean."  What have these cells been doing during Earth's history?

 

Fig. 2. The fraction of global productivity of archaea and anoxygenic bacteria (including chemolithotrophs), cyanobacteria, algae, and land plants. Adapted from Source.

A recent paper presents some calculations that begin to address this question and come up with fascinating answers. One of their estimates, that perhaps could have been derived from the numbers I note above, is that since the origin of life on Earth ~10⁴⁰ cells have inhabited the planet. So now microbiologists have another unimaginably large number to start conversations. This new paper focuses, quite rightly so, on numbers surrounding the biological process that lies at the very base of the planetary food chain. The technical term, which I suspect will be familiar to most STC readers, is "primary production." Simply put, it's the synthesis of organic compounds from carbon dioxide. This is carried out primarily through photosynthesis (but don't forget about chemosynthesis). While on present-day Earth most of photosynthesis is carried out by land plants and algae, that is really a rather "recent" development, about 500 million-years-old. For close to two billion years the cyanobacteria dominated in terms of primary productivity (Fig. 2). What has been the sum total of that synthesis of organic compounds from carbon dioxide? Their result astonished me. The authors estimate that the cumulative amount of carbon fixed through primary production totals ~10¹¹ to ~10¹² gigatons. How much is that? That is ~100 times the Earth's entire carbon stock! How's that for microbial impacts on the planet! As if that were not enough, the authors go on to speculate as to what this means in terms of where we are in the natural history of primary productivity. Based on current models of stellar evolution they project that solar brightening will lead to dramatic carbon dioxide reduction in the future; all primary productivity will cease within two billion years. They conclude: "If one assumes that future primary productivity is similar in makeup to the modern, it can be inferred that we are likely living close to the 50% mark of the total carbon that will ever be fixed by Earth's biosphere. This inference further implies that it is unlikely that Earth's biosphere will ever grow beyond a time-integrated 10⁴¹ cells across the planet's entire habitable lifetime."  Wow! But OK, enough solace in large numbers. Back to real time, I've got to finish writing this post in the next few minutes.

 

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