Scientific American - November 2018

Ancestral cyanobacteria

Synechococcus

Genome

Pelagibacter

Carbon

Shallow

Relative Abundance

High

Deep

Low

Nutrients

Genome

Prochlorococcus HL II

Ancestral cyanobacteria

Absorbs blue-green

wavelengths

Prochlorococcus HL II

Prochlorococcus HL I

Prochlorococcus LL IV

Prochlorococcus LL II/III

Prochlorococcus LL I

Light

Absorbs blue

wavelengths

New excretion

pathways

New uptake

pathway

Core metabolic

pathways

Photosystems 2 and 1

A Collaboration Is Born

Hundreds of millions of years agoîÿ ̧ ̧…ÿšDîDßx³ ̧ÿîšx­ ̧äîDUø³lD³î ̧ߐD³žä­äž³

îšxäxDîšx­ž`ß ̧UxäProchlorococcus and PelagibacterîxD­xløÇî ̧xĀǧ ̧žîxD`š ̧îšxßÜäÿDäîx

Çß ̧lø`îäÍ5šxžß` ̧§§DU ̧ßDîž ̧³­DāšDþx§Džlîšxß ̧ø³lÿ ̧ߦ… ̧ß ̧Āāx³DîšxǧD³xîÜä

̧`xD³äDlxþx§ ̧Ç­x³îîšDîßxþ ̧§øîž ̧³žąxl§ž…x ̧³DßîšÍ2x`x³îäîølžxä ̧…îšx­xîDU ̧§ž`xþ ̧§ø-

îž ̧³ ̧…îšxäxîÿ ̧ ̧ߐD³žä­äšDþxßxþxD§xlÿšDîlß ̧þxîšxžßÇDßî³xß䚞ÇD³lš ̧ÿžî`D­xî ̧UxÍ

As Prochlorococcus

evolved from ancestral

cyanobacteria, its genome

shrank and underwent

changes that altered the

organism’s metabolic

pathways for converting

solar energy into food and

eliminating waste. What

began as a photosynthetic

system for absorbing

light across the blue-green

spectrum of sunlight

shifted to a system

that absorbs mainly

blue wavelengths.

Ancestral populations once

occupied much of the sun-

light zone (bottom chart).

Over time nat ural selec tion

favored those Prochlo ro -

coccus cells that could

harvest more energy from

the sun and best ob tain

scarce nutrients. The ever

®¹àyy‡y`ïŸÿy›Dàÿyåïyàå

lowered nu trient concen-

trations near the sea

sur face, restricting cell

popula tions that required

higher concentra tions to

deeper water. This

displacement gave rise

to the partition pattern

seen among the Prochlo-

rococcus subgroups today.

To re cons t ruc t how

Prochloro coccus evolved,

researchers mapped

variations in its metabolism—

the biochemical network that

converts nutrients into the

building blocks of cells—onto

a family tree of Prochlo ro -

coccus subgroups. As

Prochloro coccus evolved to

soak up increasing amounts

of sunlight, it released

ever greater quantities of

organic carbon as waste.

Pelagibacter evolved a

compatible metabolic

network, suggesting it uses

that waste for en ergy

and releases waste prod ucts

of its own that Prochloro-

coccus uses for energy

at night.

38 Scientific American, November 2018 Illustration by Tami Tolpa