Our (Pre­vi­ously) Pur­ple Planet

by Gillian Belk

Fig­ure 1. La­guna Rosa, Sali­nas de Tor­re­vieja, Al­i­cante, Spain. Source

A fly­ing saucer hur­tles into our so­lar sys­tem, swings past Mer­cury and Venus, and comes to rest hov­er­ing over Earth, 3.7 bil­lion years ago, deep dur­ing the Archean. The aliens peer down and pon­der a planet as for­eign to them as it would be to us. The at­mos­phere lacks oxy­gen but is full of methane and car­bon diox­ide. The sun is 30% dim­mer than it is to­day, and a run-away green­house gas ef­fect is the only thing pre­vent­ing our planet from freez­ing over. The small con­ti­nents teem with ac­tive vol­canos, and a vast, seem­ingly un­bro­ken ocean cov­ers the planet. Earth ap­pears life­less. Dead. The be­ings hov­er­ing above are about to leave, their mis­sion to find life in for­eign so­lar sys­tems an ap­par­ent bust, when some­one in their crew shouts, "The wa­ter is teem­ing with… pur­ple!"

Wait. Pur­ple? Deep in our evo­lu­tion­ary his­tory pur­ple mi­crobes might have dom­i­nated the sur­face of the planet, ab­sorb­ing sun­light and con­vert­ing it into en­ergy in our world's first at­tempt at so­lar power. This un­con­ven­tional way to look at an­cient Earth is a the­ory known as the Pur­ple Earth Hy­poth­e­sis, the brain­child of Shi­la­ditya Das­Sarma from the Uni­ver­sity of Mary­land. His hy­poth­e­sis ar­gues that the se­cret of an­cient Earth rests in today's pur­ple ar­chaea, known as Halobac­te­ria (un­for­tu­nately, the poor ar­chaeon was named be­fore Carl Woese dis­cov­ered Ar­chaea as the third do­main of life).

In the Spirit of Com­pe­ti­tion

Fig­ure 2. Biosyn­thetic path­ways for pho­topig­ments. Path­ways lead­ing to reti­nal (pur­ple) and chloro­phyll (green) branch­ing from cen­tral me­tab­o­lism (red) are shown. Source

To­day, we find Halobac­te­ria thriv­ing in salt lakes all over the world from Israel's Dead Sea, to Utah's Great Salt Lake, to the high salt lakes in the An­des. When these unique mi­crobes bloom, they turn the wa­ter pur­ple. This color is due to reti­nal, an an­cient chro­mophore em­bed­ded in their mem­branes.

Reti­nal may be the world's Min­i­mal Vi­able Prod­uct for pho­to­syn­the­sis, and its mech­a­nism is sim­ple, yet ef­fec­tive. Reti­nal is part of the trans­mem­brane pro­tein bac­te­ri­orhodopsin. When a pho­ton strikes it, reti­nal changes shape. This con­for­ma­tional change pushes a pro­ton out­side the cell, gen­er­at­ing a pro­ton mo­tive force that helps drive ATP syn­the­sis by ATP Syn­thase. (To take a closer look at the process, see this YouTube video.)

Fig­ure 3. Cou­pling be­tween the Sun's en­ergy, bac­te­ri­orhodopsin and ATP syn­thase. Source

What strikes me about reti­nal is how it turns the mi­crobe pur­ple and not green – the color that dom­i­nates our planet to­day.

A quick light-physics les­son. When we shine sun­light through a glass prism the light is split into the vis­i­ble spec­trum, just like a rain­bow. Be­fore learn­ing about the Pur­ple Earth Hy­poth­e­sis, I as­sumed the Sun dis­trib­uted en­ergy equally be­tween the dif­fer­ent col­ors of the vis­i­ble spec­trum. Turns out I was wrong! The Sun emits more en­ergy in the green wave­lengths and less in the red/blue wave­lengths. Reti­nal takes ad­van­tage of this abun­dance of green light as it ab­sorbs the Sun's green and yel­low wave­lengths and re­flects pur­ple.

Fig­ure 4. Pho­totrophic pig­ment ab­sorp­tion as a func­tion of wave­length. Bac­te­ri­orhodopsin (BR, dark pur­ple line) is the only pig­ment that ab­sorbs in the green-yel­low range. Source

Chloro­phyll , con­ve­niently, ab­sorbs blue-red light. It is the­o­rized that chloro­phyll evolved out of ne­ces­sity to har­ness the Sun's en­ergy but could only work us­ing light of the wave­lengths left over from the dom­i­nat­ing pur­ple mi­crobes. Let us pic­ture a bac­te­r­ial com­mu­nity that is bro­ken down into dif­fer­ent strata. Each stra­tum is de­ter­mined by re­source avail­abil­ity, and dif­fer­ent species find their own unique niche in­side the ecosys­tem. In today's mi­cro­bial com­mu­ni­ties these lay­ers are of­ten de­fined by oxy­gen avail­abil­ity. Mi­crobes that thrive in oxy­gen-rich en­vi­ron­ments are at the top of the com­mu­nity while mi­crobes that are sen­si­tive to oxy­gen are found in deep lay­ers, the anaer­o­bic zones.

Dur­ing the Archean, bac­te­r­ial com­mu­ni­ties likely also lived in strata dif­fer­en­ti­ated by light avail­abil­ity. The pur­ple mi­crobes would live at the top, ab­sorb­ing the Sun's green light and emit­ting pur­ple. Un­der­neath this layer of pur­ple mi­crobes, other com­mu­nity mem­bers would have to make do with the left­over wave­lengths of light. Over time, they evolved to use a mol­e­cule that could ab­sorb the blue-red light and re­flected green. Thus, chloro­phyll came to be!

How Chloro­phyll Won

So why isn't our planet a med­ley of pur­ple and green life to­day? Well... chloro­phyll-con­tain­ing mi­crobes may have poi­soned their pur­ple com­peti­tors by pro­duc­ing oxy­gen. Oxy­gen gas is a waste prod­uct of chloro­phyll-based pho­to­syn­the­sis, and as chloro­phyll-uti­liz­ing mi­crobes pro­lif­er­ated, oxy­gen gas built up in the at­mos­phere to the point where it be­gan to kill the sur­round­ing oxy­gen-sen­si­tive mi­crobes. Over mil­lions of years, that mi­cro­bial waste prod­uct caused one the largest mass ex­tinc­tions on Earth, known as the Great Ox­i­da­tion Event (a story for an­other time!). Anaer­o­bic mi­crobes were forced to find cover in oxy­gen-free en­vi­ron­ments or face ex­tinc­tion.

An­other school of thought pro­poses that reti­nal-con­tain­ing mi­crobes may have died long be­fore oxy­gen ac­cu­mu­lated in our at­mos­phere. Chloro­phyll is more ef­fi­cient than reti­nal at har­ness­ing sun­light and con­vert­ing it into en­ergy. Thus, green mi­crobes may have sim­ply out­com­peted the reti­nal-based ones well be­fore they poi­soned them with oxy­gen.

So how come some pur­ple mi­crobes sur­vived? Halobac­te­ria and other pur­ple mi­crobes around to­day may have eked out an ex­is­tence for a few bil­lion years by pick­ing up some oxy­gen tol­er­ance genes from their neigh­bors through hor­i­zon­tal gene trans­fer. This al­lowed them to live in the new oxy­gen-laden en­vi­ron­ment. 

Halobacteria's unique habi­tat may be the rea­son they are still around to­day. The salt lakes Halobac­te­ria call home are in­hos­pitable to most other forms of life. In a lit­tle ironic twist, these high salin­i­ties are so toxic, most cells will ex­plode on con­tact as the wa­ter in­side them rushes out into the salty sur­round­ings. These ex­treme en­vi­ron­ments may have shel­tered Halobac­te­ria from com­pe­ti­tion So, pur­ple mi­crobes re­mained!

Where are the New Pur­ple Mi­crobes?

Fig­ure 5. Why aren't forests pur­ple? Or pur­ple and green? Source

This leaves me with a ques­tion, if there is an abun­dance of en­ergy in the green side of the spec­trum, why haven't more mi­crobes evolved to take ad­van­tage of this abun­dance of light? We don't re­ally know. It is un­der­stood that plants and cyanobac­te­ria are green be­cause of chloro­phyll but there is still a de­bate on why chloro­phyll is green in the first place. Some sci­en­tists be­lieve that pho­to­syn­thetic or­gan­isms to­day, from cyanobac­te­ria to plants, avoid ab­sorb­ing green light be­cause this ex­tra en­ergy can do dam­age to cells. David Des Mara­ias, a geo­chemist at NASA echoes this idea, and points out var­i­ous evo­lu­tion­ary processes that or­gan­isms pos­sess to pro­tect them­selves from the Sun's in­ten­sity. For ex­am­ple, in the oceans, cyanobac­te­ria and al­gae live just be­low the sur­face, where there is enough light to power their pho­to­syn­thetic en­gines, but where they are pro­tected from the harm caused by Sun's full en­ergy. Plants, on the other hand, pro­tect them­selves from the Sun's in­ten­sity by con­vert­ing the ex­tra en­ergy not used to make sug­ars into heat.

An­other school of thought is that mi­crobes (and then plants) kept their ver­sion of pho­to­syn­the­sis sim­ply be­cause it worked. Ab­sorb­ing light in the blue-red parts of the spec­trum pro­vided plenty of en­ergy to power the cell, so there was no need to in­vest in ab­sorb­ing en­ergy of the other col­ors of the spec­trum; if it ain't broke, don't fix it.

Pur­ple Fu­ture

Fig­ure 6. How many pur­ple plan­ets are we look­ing at when we gaze up at the night sky? Source: Melanie Belk

As as­tro­bi­ol­o­gists tilt their fancy tele­scopes into the night sky to look for life on other plan­ets, per­haps it will change their de­f­i­n­i­tion of what a hos­pitable planet looks like. Cur­rently we seek out plan­ets that re­sem­ble Earth at this point in its his­tory. Ones with oxy­gen sig­na­tures and signs of chloro­phyll through what is known as a "veg­e­ta­tion red edge," or the abil­ity for green veg­e­ta­tion to counter the in­frared ra­di­a­tion of in­ter­stel­lar space. But as we con­tinue to look back in time, we find that for most of Earth's his­tory the planet looked vastly dif­fer­ent than to­day. If we seek to find plan­ets with in­tel­li­gent life, well, I am not quite sure how we do that. Per­haps the Voy­ager space­craft will bring such life to us. But if we are look­ing for life in gen­eral, this the­ory can help broaden our search for hos­pitable plan­ets. One day I hope these pur­ple mi­crobes will cause one as­tro­bi­ol­o­gist peer­ing through a gi­ant tele­scope to turn and shout to an­other "Look! This planet is teem­ing with pur­ple!"

 

Gillian is an As­so­ciate Sci­en­tist at Cepheid, a med­ical de­vice com­pany in the San Fran­cisco Bay Area. She has a mas­ters from the Dut­ton lab at the Uni­ver­sity of Cal­i­for­nia, San Diego. In her spare time she en­joys mar­veling at the mi­cro­bial world.