EXTRAORDINARY ORGANISMS—The cyanobacterium, Trichodesium thiebautii, form filaments (bottom image) that contain many individual disk-shaped cells, each about 15 micrometers (10-6 meters) wide. Hundreds of T. thiebautii filaments join to create a macroscopic colony about 2.0 millimeters (10-4 meters) in diameter (top image). (Photos by John Waterbury, WHOI)
BARBELL BACTERIUM—Cyanobacteria have mechanisms that allow two antagonistic physiological processes to coexist in the same organism: oxygen-producing photosynthesis and dinitrogen fixation, which is inhibited by oxygen. In Richelia (above), the two processes are separated by space: Dinitrogen fixation occurs only in the bulbous, specialized cells (heterocysts) at the end of a 60-micrometer-long, filamentous cyanobacterium. (Photo by John Waterbury, WHOI)
The cyanobacterium Trichodesium erythraeum forms filaments (top) made up of many cylindrical cells, each about 9 micrometers (10-6 meters) wide. Hundreds of filaments form a raft-shaped colony of Trichodesium erythraeum several millimeters (10-4 meters) long (bottom). The raft is colored red because the cyanobacteria contain the red light-harvesting pigment, phycoerythrin. In calm weather, buoyant colonies rise to the surface in massive blooms that can cover thousands of square kilometers. These blooms gave the Red Sea its name. (Photos by John Waterbury, WHOI)
THE INSIDE STORY—Richelia are cyanobacteria that live symbiotically inside single-celled marine plants called diatoms. The cyanobacteria have specialized dinitrogen-fixing cells that provide nitrogen to their hosts. Top: a light micrograph of the diatom Hemiaulus sp. Bottom: an epifluorescence light micrograph of the same cells, showing the red chloroplasts of the diatom and the orange fluorescence of the barbell-shaped endosymbiotic Richelia. (Photo by Dave Caron, Woods Hole Oceanographic Institution)
By John Waterbury, Associate Scientist Biology Department Woods Hole Oceanographic Institution
When people think of bacteria, they usually think of
germs—disease-causing agents that threaten human health. In reality,
they make life on Earth possible.
One group of bacteria—the
cyanobacteria—has completely transformed Earth’s environment through
their long history. Three billion years ago, ancestors of cyanobacteria
infused Earth’s ancient atmosphere with the byproduct of their
photosynthesis—oxygen—changing the chemistry of the planet and setting
the stage for entirely new oxygen-breathing life forms to evolve.
Without the cyanobacteria, the life we see around us, including humans,
simply wouldn’t be here.
Before 1970, cyanobacteria were known
to occur widely in fresh water and terrestrial habitats, but they were
thought to be relatively unimportant in the modern oceans. This
perception changed dramatically in the late 1970s and 1980s with the
discovery of photosynthetic picoplankton by scientists at the Woods
Hole Oceanographic Institution and the Massachusetts Institute of
Technology.
Tiny members of this group of newly discovered cyanobacteria, Synechococcus and Prochlorococcus,
turn out to be the most abundant organisms on the planet today. They
are at the base of the ocean’s food chain, making air, light, and water
into food for other life. Today, exploiting new biotechnological
techniques, we are exploring their genes and uncovering the secrets of
these extraordinary organisms.
An unexpected glow In 1977, I was on the Atlantis II
in the Arabian Sea with WHOI microbiologist Stanley Watson, measuring
bacterial abundance and biomass. We were using a new technique
employing epifluorescence microscopy: Fluorescent dyes specifically
labeled nucleic acids, making bacterial cells fluoresce green when
excited with blue light.
But, to our great surprise, some
samples contained cells that glowed a brilliant orange—before any dye
was added. The color was produced by the natural fluorescence of
phycoerythrin, the primary light-harvesting pigment in many
cyanobacteria. This was our first introduction to Synechococcus.
To
examine this new cyanobacterium, we attempted to culture it on that
cruise, using media developed during my Ph.D. studies. But the cells
died within 24 hours. It would take almost a year to develop media in
which Synechococcus could successfully be isolated and grown in the laboratory.
We knew right away that Synechococcus
was something important by the impressive numbers of them that we found
in seawater samples. Since 1977, they have been found everywhere in the
world’s oceans when the water temperature is warmer than 5°C (41°F) at
concentrations from a few cells to more than 500,000 cells per
milliliter (about 1/5 of a teaspoon), depending on the season and
nutrients. This amazing abundance makes them a source of food for
microscopic protozoans, the next organisms up in a food chain that ends
in fish and mammals.
Cycles of life
Bacteria take up the elements essential to life—especially carbon and
nitrogen—and incorporate them into molecules that higher
bacteria-consuming organisms use for growth. Bacteria also can reverse
the transformation, returning elements to the environment, completing
sequences of reactions known as nutrient cycles. Without the continuous
cycling of these elements, all biochemical life processes would lead to
a dead end.
Cyanobacteria are vital to two
primary nutrient cycles in the ocean. In the carbon cycle, they
photosynthetically “fix” carbon from air into organic matter at the
base of the food chain, simultaneously releasing oxygen. Many are also
important in the nitrogen cycle—a complex series of reactions and
transformations, including one known as nitrogen fixation, which
converts nitrogen from air and incorporates it into cellular compounds.
The key is cyanobacteria’s ability to use molecular nitrogen (N2, or dinitrogen) as a source of nitrogen for their cells.
Cyanobacteria
live anywhere there is light and moisture: in the open oceans, in
pristine or polluted lakes and streams, in soils, hot and cold deserts,
hot springs, brine pools, and salt ponds. In symbiotic relationships
with algae and plants, they provide nitrogen to their hosts in exchange
for a site to live on.
In many instances,
cyanobacteria are visible to the naked eye. In coastal oceans,
cyanobacteria form dark blue-green mats covering rocks and mollusk
shells in tidal pools. Along upper limestone shores, they form black
crusts that erode rocks.
In salt marshes
throughout the world, several types of cyanobacteria play a key
ecological role in binding sediments by forming dense layered mats. In
the tropics, these mats, called stromatolites, become very thick;
cyanobacteria inside them look almost indistinguishable from those in
3-billion-year-old fossil stromatolites. This is evidence that
cyanobacteria inhabited the seas when the Earth was still young.
How oxygen got in the atmosphere
Three billion years ago, Earth’s atmosphere contained little oxygen.
But ancestral cyanobacteria thriving in the early oxygen-free oceans
evolved a biochemical mechanism for photosynthesis, which used light to
generate cellular energy by splitting water molecules, and producing
oxygen in the process.
For a billion years,
growing and multiplying in the sea, they slowly raised the oxygen level
in the atmosphere to 20 percent, the level that supports
oxygen-breathing life. Cyanobacteria alone, directly or indirectly, are
responsible for all of the oxygen in our air.
In
every case, the green plants we are most familiar with, from
unicellular algae to trees, owe their photosynthetic abilities to small
chlorophyll-containing bodies within their cells known as
chloroplasts—which look a lot like cyanobacteria. In fact, most
microbiologists believe that chloroplasts are derived from
cyanobacteria—or, more precisely, that ancestral cyanobacteria entered
larger cells and became symbiotic in them, making them photosynthetic,
and creating plants.
An ancient process
Both plants and cyanobacteria use carbon dioxide in air to synthesize
cell carbon. But only bacteria can fix dinitrogen as a sole source of
nitrogen in cells. Microbiologists believe this ancient process evolved
very early, while Earth’s atmosphere was still without oxygen, because
the necessary enzyme, nitrogenase, is inactivated by oxygen.
Cyanobacteria
have mechanisms that allow oxygen-producing photosynthesis and
dinitrogen fixation—two antagonistic physiological processes—to coexist
in the same organism. In some, the two processes are separated by time:
Photosynthesis happens during daylight and dinitrogen fixation at
night. In more complex species, the two processes are separated by
space, with dinitrogen fixation occurring only in specialized cells
(heterocysts) within filaments.
Trichodesmium,
a filamentous cyanobacterium, plays an important ecological role by
replenishing nitrogen in the central oceanic gyres—areas of widely
circulating currents in the middle of oceans—where nutrients like
nitrogen, required by other marine microorganisms for growth, would
otherwise be low. In calm weather, their buoyant red-colored colonies
rise to the surface, resulting in massive blooms that can cover
thousands of square kilometers. These blooms gave the Red Sea its name.
Cultural breakthroughs Trichodesmium
quickly disintegrates when collected at sea and has been notoriously
difficult to culture in the laboratory. In 1990, my lab at WHOI
established conditions that made culturing routine and reliable by
using very rigorous cleanliness. It turns out that instead of failing
to add something these cyanobacteria required, we were inadvertently
poisoning them with trace contaminants in our chemicals and on our
glassware.
We can now grow four of the five species of Trichodesmium
in the lab and use molecular genetic methods to study them. (See The
Deeps of Time in the Depths of the Ocean)
In collaboration with the U.S. Department of Energy’s Joint Genome
Institute, we have sequenced the entire genome of one Trichodesmium
species. These advances give scientists at WHOI and elsewhere the
ability to uncover the genetic reasons for Trichodesmium’s success.
We can also culture Synechococcus,
and using molecular methods, scientists have found 12 distinct
groupings, or clades, of marine Synechococcus, each
approximately equal to a species. Scientists at the DOE’s Joint Genome
Institute have already sequenced the genome of one type, and others
will soon follow.
Scientists are examining the factors that control Synechococcus’s
growth and distribution to understand more about their role in the
ocean, especially in the food chain. Others are examining how Synechococcus coexists with a diverse and abundant group of cyanophages.
Microbial libraries Even as we studied Synechococcus,
new surprises awaited. In 1985 Robert Olson of WHOI and Sallie Chisholm
of MIT discovered a second group of even smaller photosynthetic
picoplankton in the Sargasso Sea, in the central North Atlantic Ocean.
Olson took to sea, for the first time, an instrument that could count
bacterial cells using fluorescence, the Flow Cytometer. The instrument
led to the discovery of cyanobacteria ranging in size from 0.7 to 1.0
micrometers called Prochlorococcus.
It
is our great fortune that these cyanobacteria can also be cultured in
the lab. Scientists at MIT have assembled a collection of strains (cell
lines) for Prochlorococcus collected from various places, while WHOI maintains collections for Synechococcus, Trichodesmium, and Crocosphaera,
another recently discovered cyanobacterium. As a sort of lending
library of cells, these two sites provide cultures for microbiologists
all over the world to study.
Oceanographers measuring Prochlorococcus
at sea have found it to be staggeringly abundant in central oceanic
gyres, where it can reach concentrations in excess of 100,000 cells in
a milliliter of seawater. It may represent fully half the total
photosynthetic production in these waters. Rough calculations, based on
the surface area of the oceans and the abundance and distribution of Synechococcus and Prochlorococcus, suggest that these are the two most abundant organisms on Planet Earth.
Cyanobacteria continue to surprise Discoveries about cyanobacteria continue. We recently isolated Crocosphaera,
a new genus of dinitrogen-fixing cyanobacteria, from the tropical
Atlantic and Pacific Oceans. Surprisingly these 2-to-4-micrometer
cells, which might otherwise occur in vast areas of the ocean, are
relegated to the tropics by a quirk in their physiology: They cannot
grow at temperatures colder than 24°C (75°F)!
Scientists have also found Richelia,
cyanobacteria with specialized cells for fixing dinitrogen that live
inside single-celled marine plants, including some diatoms. (See Revealing the Ocean’s Invisible Abundance) With Richelia
fixing dinitrogen for them, the diatoms form extensive blooms. Such
symbiotic relationships between phytoplankton and dinitrogen-fixing
cyanobacteria, once they can be successfully cultured, may be shown to
play a significant role in the carbon and nitrogen cycles of the oceans.
Clearly,
cyanobacteria, which have been so central to life on Earth, will
continue to provide many new surprises, as scientists learn more about
them as they explore the world’s oceans.
Telltale fluorescence
Many biological compounds, including photosynthetic pigments such as the chlorophylls and phycobiliproteins, fluoresce naturally when excited with light. This natural fluorescence played a key role in the discovery of the marine photosynthetic picoplankton.
In 1977, we were using epifluorescence light microscopy to count bacteria in seawater aided by fluorescent dyes that stained bacterial nucleic acids. Synechococcus was discovered when quite by chance we examined unstained samples and were immediately struck by the numerous small cells that fluoresced bright orange (photo at right, by John Waterbury). The brilliant orange color results from the natural fluorescence of phycoerythrin, one of the phycobiliproteins abundant in cyanobacteria.
In 1985, WHOI scientist Rob Olson was the first to take a new instrument, the Flow Cytometer, to sea. It exploits fluorescence to study individual cells. With it, he and Sallie Chisholm of MIT detected very small cells with natural fluorescence of their chlorophylls. This unique “signature” led to the discovery of Prochlorococcus, which turn out to be among the most abundant organisms of Earth.
Eureka moments and cul-de-sacs
In 1975, Ralph Lewin from Scripps Institution of Oceanography found something scientists never knew existed—Prochloron,
a symbiotic cyanobacterium living in sea squirts in Palau. It was a
legitimate “Eureka moment,” signifying the discovery of a previously
unknown kind of organism known as prochlorophytes. But it also offered
the tantalizing possibility of an even more momentous, heart-thumping
discovery: how the first plant on Earth evolved.
Cyanobacteria
inhabited the Earth billions of years ago, and scientists believe that
ancestral cyanobacteria started symbiotic relationships with larger
cells and provided them with the ability to photosynthesize.
Eventually, these cyanobacteria evolved into chloroplasts, the
photosynthetic factories inside all plant cells.
Prochlorophytes, like other cyanobacteria, contain chlorophyll a,
a pigment important in photosynthesis. But unlike other cyanobacteria,
which contain phycobiliproteins to absorb solar energy for
photosynthesis, prochlorophytes contain chlorophyll b as their light-harvesting pigment.
So do all green plants.
Microbiologists
speculated excitedly that prochlorophytes were on the same evolutionary
pathway that led directly to chloroplasts in modern green plants.
But the theory didn’t hold. Phylogenetic studies showed that the three known prochlorophytes (Prochloron, Prochlorothrix, and Prochlorococcus)
evolved separately from within the cyanobacteria, and none was on the
same line of descent leading to higher-plant chloroplasts. Although
chloroplasts also arose from cyanobacteria, their modern cyanobacterial
relatives have yet to be found.
The
study of cyanobacteria demonstrates the strength of scientific inquiry.
Scientists follow paths that lead sometimes to unexpected discoveries
and sometimes to nowhere. But every line of investigation adds to our
knowledge.
