science sponges2008

8/13/2019 Science Sponges2008

1/323 MAY 2008 VOL 320 SCIENCE www.sciencemag.org28

A SPONGEFUL OF BACTERIA IS THE LAST

thing a dishwasher wants to think about. But

for Jrn Piel, the more microbes he finds in a

sponge, the better. Not a synthetic one, of

course, but those that adorn tropical reefs andpopulate the ocean bottom.

One of evolutions more ancient animals,

sponges at first glance seem quite simple

little more than loose consortiums of semi-

autonomous cells, stuck in one place filter-

ing food from the water column. But a closer

look reveals a surprising twist. With many

species, under the microscope you see

almost exclusively bacteria among the cells,

says Piel, an organic chemist at the Univer-

sity of Bonn in Germany. Just as microbial

ecologists are demonstrating the extent and

importance of microbes in ecosystems as

diverse as guts and glaciers (see p. 1046),

Piel and others are slowly uncovering a hid-

den microbial world inside sponges.Its a difficult job, as almost none of the

sponges inhabitants grow in the lab. But

through genetic studies, researchers are

revealing the rich diversity and unusual distri-

bution of these microbes. Some investigators

are pinning down the roles bacteria play in

sponge biology and ecology. The microbes are

teaching scientists about evolution, symbio-

sis, and the mind-boggling variety of life on

our planet. It never ceases to amaze me that a

sponge, an organism that just sits there and

pumps bucketfuls of water through its canal

has such a rich and varied, yet highly specifi

inner life, says marine microbiologist Micha

Taylor of the University of Auckland in Ne

Zealand. The research also has a practic

side: Piel and others are betting that spong

dwelling bacteria could be the source o

potentially valuable compounds for treatin

cancer, malaria, and other human diseases.

Sharper focusThe first hint of the spongespervasive inha

itants came in the 1960s and 70s, as ne

equipment allowed longer and deeper div

that gave researchers their first up-close loo

at the diversity of life on the ocean bottom. quickly became clear that something else w

living among the sponges cells. Looking

the first electron microscope images o

sponge tissue, marine biologist Jean Vacel

and his colleagues at the University of Ma

seille spotted what looked like a half-doze

different types of bacteria. Other researche

took sponges and squeezed them out ov

culture plates to see what would grow, recal

marine ecologist Robert Thacker of the Un

versity of Alabama, Birmingham, but it w

difficult to follow up on the finds. At mos

5% of sponge-dwelling species have thrivein the lab, says microbial ecologist U

Hentschel of the University of Wrzburg

Germany. And the sponges themselves a

incredibly hard to keep alive, Thacker says

Therefore, Hentschel, Thacker, and othe

have been using indirect methods to piec

together a picture of this reclusive communit

Most of the evidence comes from studies

the gene for 16S ribosomal RNA (rRNA),

piece of the genome that scientists use to ide

tify unknown microbes in the environmen

Differences in this gene can serve as a usef

measure of the kinship between two species

These genetic studies uncovered a ditinctive and extensive community, identif

ing more than 100 species of microbes th

are found in sponges but not in the surroun

ing water. This distribution indicates th

these bugs are long-term residents rath

than passersby. An individual sponge mig

host dozens of different species, and overa

the molecular analyses have found a

impressive variety: 14 bacterial phyla, tw

phyl a of ar ch ae a, an d seve ra l ty pe s o

eukaryotic microbes.

N E W S

Symbiotic ties, bioactive compounds, and mysterious distributions

of bacteria characterize these ancient invertebrates

The Inner Lives of Sponges

Hospitable habitat. The brown tube sponAgelas conifera (foreground) and the giant barspongeXestospongia muta both have complmicrobial communities living among their cells

Published by AAAS


2/3

Such diversity initially suggested multiple,

independent acquisitions of microbial sym-

bionts. But evidence is building that sponges

of different types and in different oceans host

strikingly similar microbial communities.

Hentschel and her colleagues showed in 2002

that sponges from the coast of Japan, the Red

Sea, the Mediterranean, and the Republic of

Palau in the South Pacific contained microbes

that are more closely related to each other than

to the microbes in the seawater from which the

sponges were harvested. Its astounding,

says Susanne Schmitt, a postdoc in

Hentschels lab. The different sponges the sci-

entists sampled diverged millions of years

ago, she says, but they are home to very simi-

lar, and very complex, microbial communi-

ties. In 2007, Taylor and his colleagues found

the same result when they analyzed the entire

database of 16SrRNA sequences available

from sponge-dwelling microbes collectedfrom all over the worldnearly 2000 seq-

uences in all.

But Taylor, Hentschel, and their colleagues

are still trying to work out what the results

mean. Microbes might have colonized a

sponge early in the groups evolutionary his-

tory and acquired characteristics that enabled

them to live in sponges full-time, Taylor pro-

poses. Those sponge-loving microbes could

have then spread to other spongesand other

oceans. And such a scenario could explain

what may be a new phylum calledPoribacte-

ria, after Porifera, Latin for sponge.Pori-bacteria have been found throughout the

world, albeit exclusively in sponges.

Fruitful partnership

As with much of microbial ecology, the

sponge specialists have been focused primar-

ily on taking a census. I go in just trying to

figure out whats therewhat people did

collecting insects in the forest 100 years

ago, Taylor explains. But he and his col-

leagues are now starting to take the next step,

because census data cant tel l researchers

what each side gets out of the relationship.

Ecologists want to know if the microbes andtheir hosts are obligate symbionts, unable to

survive without each other, or whether the

microbes are tolerated but dispensable

guests, says Michael Wagner, a microbial

ecologist at the University of Vienna in Aus-

tria: If we want to understand these commu-

nities, we have to know the function each

member plays.

Yet even after decades of study, scientists

are still not exactly sure what sponges and

their microbes are doing for each other. Living

in nutrient-poor but sunlit waters in the

lagoons of the Republic of Palau, sponges of

the family Dysideidae are home to blue-green

algae that probably provide their hosts with

energy and carbon. The sheer mass of the

microbes may help support the meter-high

giant barrel spongeXestospongia muta, in

which bacteria can sometimes make up

40% of a sponges volume. Microorganisms

may even help defend their hosts against

disease-causing bacteria.

But those are educated guesses rather than

proven observations. And there are a whole

lot of other things that are going on that we

just dont know about, says molecular ecolo-

gist Russell Hill of the University of Maryland

Biotechnology Institute in Baltimore.

To try to get a picture of the daily goings-

on inside a sponge, Wagner and his col-

leagues are catching sponge microbes in the

act of eating . The researchers have juststarted experiments on several species of

sponges that hostPoribacteria. NoPoribac-

teria have ever been cultured in the lab, but

the scientists are able to keep the host sponges

alive in aquaria, at least for a short time. They

use a technique that allows them to observe

the metabolic activity of individual microbes

and sponge cells. They expose the sponge to

fluorescently labeled rRNA markers, which

lets them know what species they are dealing

with, and to radioactively labeled food

amino acids, bicarbonate, and other mole-

cules. They then watch which cells take upthe labeled morsels and follow how the

morsels are processed, including

whether the sponge consumes com-

pounds excreted by the microbes.

Were asking not only Who are

you? but also What are you

eating? he says.

Whatever their function,

the microbes seem important

enough for sponges to pass on to

future generations. In the female

sponge, nurse cells, which provide

the yolk for developing eggs, also

ferry blue-green algae from thesponges outer layers to the devel-

oping oocytes located deeper in the

sponge matrix. In 2005, Kayley

Usher and her colleagues at the

University of Western Australia in

Perth even found blue-green algae

in the sperm of the sponge Chon-

drilla australiensis. A year later,

Julie Enticknap, a postdoctoral fel-

low in Hills lab, was able to culture a

sponge-dwelling alphaproteobacterium

from the larvae of a sponge collected off th

coast of Florida, another indication of possib

parent-to-offspring transmission.

But that study highlights what may be th

most baffling mystery in sponge microbio

ogy. Usually when symbionts are passe

from parent to offspring, the partne

undergo what is called cospeciation, and th

microbes develop a unique genetic signatu

and become confined to that particular hos

But that doesnt happen here, say

Hentschel. The bacteria in the larvae prove

closely related to those cultured from unr

lated sponges growing in Jamaica, Indon

sia, and the Chesapeake Bay in the Unite

States. The best explanation for the broa

distribution of this bacteriumand f

many other species found across the globe

she says, is that sponges acquire their res

dent bacteria both from their parents an

from the environment.To date, no sponge-specif ic microbe h

turned up in seawater, but scientists have

distinct disadvantage when it comes t

sampling. Although a 1-kilogram spong

can f ilter thousands of liters of seawat

a day, Hills says that if we are lucky, w

CREDIT:JOSEPHPAWLIK/UNIVERSITYOFNORTHCAROLINAATWILMINGTON

SPECIALSECTION

Medicinal potential. The stovepipe sponge Aplysi

archeriis the source of several bioactive compoun

that may come from its microbial residents.

www.sciencemag.org SCIENCE VOL 320 23 MAY 2008Published by AAAS


3/323 MAY 2008 VOL 320 SCIENCE www.sciencemag.org30

filter 200 liters, so the chances of finding

an uncommon microbe, such as the larvaes

alphaproteobacterium, are small.

If sponges are taking microbes in from the

surrounding environment, they need to be

able to tell friend from foe from food. And the

microbes need a way to protect themselves

against accidental or intentional rebuff by

their hosts. Electron microscope images

reveal that most sponge-dwelling bacteria

have either thickened cell walls or slime cap-

sules that might prevent the sponge cells from

digesting them. Once established, these resi-

dent microbes, or the sponge itself, seem to

produce chemicals that discourage interlop-

ers. Several antibiotic compounds isolated

from sponges efficiently kill bacteria found in

the water column but do not affect sponge-

dwelling organisms.

Sea-based drugsThose antibiotic compounds are driving at

least some of the interest in sponges. For bio-

prospectors looking for potential new drugs

from the sea, sponges are one of the best

sources of bioactive compounds, says Hill.

The chemical from which the antiretroviral

drug AZT was derived was first found in a

Caribbean sponge. In the lab, other com-

pounds from sponges kill cancer cells and

malaria parasites.

AIDS, cancer, and malaria are not the

sponges concern, but a powerful chemical

defense arsenal is, points out microbial ecolo-gist Julie Olson of the University of Alabama,

Tuscaloosa: Sponges cant evade predators,

and if something blocks their [water-filtering]

passages, i ts a death sentence. To protect

against unfriendly microbes, a successful

sponge probably needs a range of chemical

weapons, she says.

Initially, marine biochemists using the

grind and find approach assumed that

most of those chemicals came from the

sponge itself. But as the diversity of the

spongesresidents became clear, many began

to suspect that at least some of the com-

pounds might come from the lodgers rather

than the hosts. The quest to come up with

enough of a bioactive compound for clinical

testing is proving that these suspicions are

well-founded.

To date, few sea-based drugs have made

it to the clinic. Supply is the primary rea-

son there is no blockbuster so far, says

Piel. Its been almost impossible to purify a

compound in quantities large enough for

animal and human testing, and the chemi-cal structures are often too complex for

large-scale chemical synthesis. Halichon-

drin B, for example, is a powerful antitumor

compound in lab tests. But scientists calcu-

lated that clinical trials would require at

least 10 grams of the substance. The best

producer, a New Zealand sponge group

called Lissodendoryx, yielded 300 mil-

ligrams per metric ton of sponges. Because

the entire population of Lissodendoryx was

estimated at 280 metric tons, it was clear

that harvesting from the wild was not sus-

tainable, Piel says.Piel is trying to get around that problem.

His lab is fishing for genes involved in

making promising compounds. The goal is

to find the set of genes that codes for th

potential drugs synthesis, and if the orig

nal host wont grow in the lab, to transf

those genes to a microbe that is happy in a

artificial environment. The design

microbe would then pump out enough

the drug for testing.

The technique is pointing to bacteria a

the source of many of the compounds. S

far, every time weve found a gene cluste

we found typical bacterial genes nearb

Piel says. In 2004, the group reported th

they had pinned down the genes responsib

for producing compounds called polyk

tides in a dark-red sponge called Theonel

swinhoei that lives in coral reefs. (In the la

polyketides can kill tumor cells, and sever

types are in clinical trials.)

Based on the genes similarity to know

genes, Piel and his colleagues conclude

that the genes most likely come from a stiluncultured microbe. Whats odd, however,

that these genes are quite similar to polyketid

genes belonging to a bacterium that lives

the guts of beetles. The researchers are cu

rently working to transfer the spong

microbes genes to a lab-friendly host.

Hill and his group have focused on tryin

to harness the original bacteria producer

Part of the problem is that people have

their heads that all symbionts are difficult

grow, Hill says. But patience and hard wo

can pay off. Sometimes we get new coloni

after months of incubation.In recent work, Hills lab has homed in o

the source of a particularly promising com

pound called manzamine A. In lab test

manzamines kill malaria parasites mo

efficiently than either chloroquine

artemisinin, two of the leading antimalari

drugs. The compound was first identified

a sponge collected off the coast of Okinaw

but related compounds have since turned u

in dozens of unrelated sponge species a

over the worlda strong hint, Hill says, th

they are produced by a microbe shared by a

these species.

In as-yet-unpublished work, his grouhas isolated the bacterium that produc

manzamine A. The microbe should giv

scientists their first steady supply of th

compound, allowing them to make an

test new derivatives, Hill says. Such stu

ies led to the eventual development

AZT, Hill points out. And he is hopin

spongesor at least their microbeswi

again lend a hand in the f ight again

deadly disease.

GRETCHEN VOG

Microbial Ecology

Reclusive residents. These Actinobacteria (above)are some of the few sponge-dwelling microbes thatcan grow in culture. To find out more about thehabits of bacteria that dont grow in the lab,researchers use tagged genes and labeled nutrientsto trace their fates. Here (left), sponge cells aregreen, nitrite-oxidizing symbionts of the genusNitrospira are red, and all other bacterial symbiontsappear blue.

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