Download Giant Marine Cyanobacteria Produce Exciting Potential

Giant Marine Cyanobacteria Produce
Exciting Potential Pharmaceuticals
Cyanobacteria produce an array of exceptionally potent toxins,
some with promising anticancer or other anti-disease activities
William H. Gerwick, R. Cameron Coates, Niclas Engene, Lena Gerwick,
Rashel V. Grindberg, Adam C. Jones, and Carla M. Sorrels
ur standard procedure for collecting specimens of tropical marine
cyanobacteria as potential sources
for novel pharmaceuticals requires
some of us to dive about 15 meters
below the surface and 1 meter above the tropical
sea floor, scanning the underwater landscape for
giant colonies of marine cyanobacteria. In such
settings, benthic cyanobacteria grow as nearly
pure strains—forming tufts, mucilages, and
mats that adhere to coral, algae, and marine
invertebrate substrates (Fig. 1).
Because cyanobacterial colonies grow in such
profusion, we can collect them by hand in large
enough quantities to investigate their chemical,
pharmacological, and genetic properties. This
approach contrasts with how microbiologists
O
Summary
• Giant cyanobacteria that grow in marine environments produce an array of natural products,
some of them potent toxins with promising
anticancer or other anti-disease activities.
• Filamentous marine cyanobacteria may yield
promising natural products and, of these, Subsection III cyanobacteria are the source of
nearly half the 800 reported compounds.
• Such filamentous forms integrate features from
the polyketide synthase and nonribosomal peptide synthetase pathways.
• Marine invertebrate species are remarkable for
how extensively they associate with marine microbiota, which produce many of the natural
products that chemists isolate from those invertebrate sources.
typically come to know particular microorganisms, often depending on elaborate manipulations and culture procedures to acquire adequate materials to study. In our case, however,
the substantial capacities of some cyanobacteria
for photosynthesis and nitrogen fixation enable
them to reach localized population sizes that are
inconceivable for so many other species of the
microbial world. Of course, this physical stature
also makes them subject to predation by fish,
mollusks, and a variety of mesograzers. To defend themselves in this predator-rich environment, cyanobacteria produce an array of exceptionally potent toxins— often the very materials
that are of such keen interest to us.
Building in part on the pioneering efforts of
the late Richard Moore during the 1970 –1990s
at the University of Hawaii, we are continuing to determine structures and information
about the biosynthesis and activities of natural products from marine cyanobacteria,
working at Scripps Institution of Oceanography after moving from Oregon State University. These insights are helping us to
better appreciate the role that marine invertebrate-associated cyanobacteria play in
producing natural products in marine environments.
Giant marine cyanobacteria, producing
secondary metabolites with unusual structures and potent activities against cancer
and other diseases, thus are an exciting area
for microbial research. The genes and biochemical pathways responsible for making
these metabolites are equally worthy of
study and will help to address a series of
puzzling questions regarding their natural
William H. Gerwick
is Professor of
Oceanography and
Pharmaceutical Sciences, R. Cameron
Coates is Senior
Research Assistant,
Niclas Engene is
doctoral student,
Lena G. Gerwick is
Research Scientist,
Rashel V. Grindberg
is doctoral candidate, Adam C.
Jones is doctoral
candidate, and
Carla M. Sorrels is
doctoral candidate
at the Center for
Marine Biotechnology and Biomedicine, Scripps Institution of
Oceanography, and
the Skaggs School
of Pharmacy and
Pharmaceutical Sciences University of
California San Diego, La Jolla, Calif.
Volume 3, Number 6, 2008 / Microbe Y 277
times requires further analysis. In several cases, seemingly invertebratederived molecules instead trace to
biosynthetic pathways that are associated
with individual species of microorganisms or sets of microorganisms, including
heterotrophic bacteria, cyanobacteria,
and fungi. Furthermore, when one predicts the ultimate source of recently evaluated clinical trial agents based on chemical class and likely biosynthetic pathway
for making them, many such agents (15
of 20) are products of single microbial
genetic capacities. Thus, the potential of
the sea to yield useful secondary metabolites rests not so much with its diverse
macro-life forms, but rather, with its incredible microbiota.
For the nearly 800 compounds reported from marine cyanobacteria (Fig.
2), the more advanced species such as
the filamentous forms tend to yield
promising natural products. Of these,
Subsection III cyanobacteria, according
(A) Collection of filaments of a marine cyanobacterium into plastic collection bags using
to bacteriological systems of phylogSCUBA off the coast of Papua New Guinea. (B) Appearing as electric orange gelatinous
eny, or the order Oscillatoriales by phyorbs, colonies of the marine cyanobacterium Schizothrix sp. from reefs near New Britain,
Papua New Guinea. (C) Cultured strain of Lyngbya majuscula 3L in 10 L of BG11 medium,
cological delineation, are the source of
and (D) cultured L. majuscula 3L at high magnification showing that the filaments are
nearly half the reported compounds.
composed of coin-shaped cells (6.3 ␮m long by 60 ␮m diameter) and surrounded by a
From the 17 genera in Subsection III, or
polysaccharide sheath.
9 genera currently listed in the Oscillatoriales, Lyngbya dominates, with
nearly 300 distinct substances coming
chemistry, such as how microorganisms convert
from this single genus. There are 12 marine
methyl to trichloromethyl groups; how acetyspecies in this genus, and 236 compounds are
lenic bromide groups are made; and what reacascribed to L. majuscula and a further 11 to L.
tions form pendant carbon atoms at the C-1
bouillonii.
positions of polyketides, some of which appear
In contrast, other species of Lyngbya contribas cyclopropyl rings and others as vinyl chloride
uted only a few metabolites; however, species
arrangements.
identification is rudimentary, making it likely
that many sources are not identified to the speMarine Settings a Key Source of
cies level because environmental factors affect
Potentially Therapeutic Natural Products
morphological features such as growth form,
coloration, and cell size measurements, renderMarine bacteria have contributed at least 15
ing them taxonomically unreliable.
natural products that are being evaluated in
We recently began identifying cyanobacterecently concluded or in continuing clinical triria based on 16S rDNA internal transcribed
als, especially as anticancer therapeutics. In
spacer (ITS) sequences and other housekeepterms of the sources of the bacteria from which
ing genes. This approach is revealing much
those promising materials were collected, marine invertebrates such as sponges, tunicates,
greater genetic diversity within this group
than is generally appreciated. Many new speand mollusks account for three-fourths of these
cies and perhaps genera await taxonomic dispromising molecules.
covery—especially for isolates from marine
However, rigorously determining the real
environments because the preponderance of
metabolic sources of these metabolites someFIGURE 1
278 Y Microbe / Volume 3, Number 6, 2008
Gerwick: Water Is Loved for Leisure and as a Source for Natural Products
William Gerwick was about eight
years old when he first saw an
iridescent thread of seaweed
shimmering below the surface in a
tidal pool in Pacifica, a coastal
town south of San Francisco,
Calif. He was with his father on
one of their outings that kindled
his early interest in the ocean.
“We would go to some of the tide
pools outside San Francisco and
poke around, and then have
breakfast at the local fishermens’
coffee shop,” he recalls.
The memory of that sliver of
shining seaweed—and a desire to
understand why it glowed under
water, but not outside it— eventually led to his first scientific report,
“Structural, chemical and ecological studies on iridescence in Iridaea (Rhodophyta),” published
in the Journal of Phycology in
1977. It was based upon an undergraduate research project that
he conducted at the University of
California, Davis (UCD), with
Norma Lang, now professor
emerita. “This particular type of
seaweed has a multilayered cuticle
on its outer surface that works
like a very sophisticated soap
bubble,” Gerwick says. “It reflects certain wavelengths of light
with brilliant intensity.” Born and
raised in Oakland, Calif., Gerwick graduated from UCD in
1976 with a B.S. in biochemistry,
received his Ph.D. in oceanography from Scripps in 1981, and
later did research in pharmacy at
the University of Connecticut.
Gerwick, 53, a professor of medicinal and natural products
chemistry at the Center for Marine Biotechnology and Biomedicine, Scripps Institution of Oceanography and Skaggs School of
Pharmacy and Pharmaceutical
Sciences at the University of Cali-
fornia, San Diego, continues to
call himself “a water person.” In
his professional capacity, he and his
students search the seas for natural
products that might become medicines. More generally, he adds, “I
love to be in the water and doing
things in the water, and my happiest place to be at the end of a long
day is in the hot tub.
“We go out into the field and
collect samples,” he says, describing his research and work. “I
travel with my students and we go
somewhere for two or three
weeks, oftentimes scuba diving 3
or 4 times a day, surveying species
and collecting algae of interest to
us. We can’t predict what will
contain a compound of interest,
so we collect small amounts of
everything and bring them back
to San Diego to test them for biological activities. We’ve had some
promising compounds move to
preclinical stages.”
The research “fascinates and
intrigues me, and satisfies my personal curiosity,” Gerwick says.
Moreover, he likes teaching. “Students . . . after all. . . outlive us.”
Sometimes the research also
scares him, particularly when it
entails close encounters with
sharks. “We were diving off the
Pacific coast of Mexico, myself
and a graduate student at the
time, Valerie Paul (currently Head
Scientist of the Smithsonian Marine Laboratory in Ft. Pierce, Fla).
We jumped in the water, went
down about 40 or 50 feet, and
came upon a fairly large shark. He
was only seven feet long, but of
considerable girth. It was a grey
reef shark—very territorial. This
one came swimming directly toward us.
“Usually sharks will swim by
you, and then come back,” he
continues. “But this one was behaving very aggressively, swimming in tighter and tighter. I guess
he was coming in for a test nibble.
I’d been diving a few months earlier in the Galapagos, where I’d
seen lots of sharks and where it is
the custom to dive with a poker. I
had one with me. I gave it a pretty
good poke in the nose. I hit it
pretty hard, and it took off. I
turned around and saw that Valerie had brought an underwater
movie camera and had been filming the entire thing.” Instead of
retreating to the safety of their
boat, he adds, “We continued to
collect algae, finished our dive,
and had a good story when we
came up.”
Gerwick is married with two
children: a son, 22, who is a graduate student in particle physics at
the University of Edinburgh in
Scotland, and a daughter, 24,
who is a science writer for the
communications office of the Oregon Health Sciences University
in Portland. His wife, Lena, has a
background in medicine and a
Ph.D. in immunology, based on
research studying fish and their
immune systems. With their children grown, the couple now
shares their home with Oliver, a
Jack Russell terrier.
They also share labs. “Upon
our move to Scripps/UCSD nearly
three years ago, my wife and I
merged our laboratory efforts
such that we have several shared
grants and research projects, as
well as separate projects,” he
says. “Our labs and offices are
contiguous, and we spend a lot of
time together at work and home.”
Marlene Cimons
Marlene Cimons is a freelance writer
in Bethesda, Md.
Volume 3, Number 6, 2008 / Microbe Y 279
terize biosynthetic gene clusters responsible for making specific natural
products in cyanobacterial species.
Complementing this approach is one
that relies on analysis of the growing
number of cyanobacterial genome sequences that are or soon will be available. For instance, by early 2008,
sequencing of 29 cyanobacterial genomes was complete, while analyses of
another 46 such genomes were under
way. The initial natural products focus
is on genes encoding polyketide synthase (PKS) and nonribosomal peptide
synthetase (NRPS) pathways.
Together, this genetic information
reveals that the more advanced filamentous forms of cyanobacteria are
rich in modular biosynthetic pathways
that integrate features from both the
PKS and NRPS families. In some cases,
a single cyanobacterial genome contains multiple pathways, making cyanobacteria resemble some streptomycetes and myxobacteria in this respect.
With only a small fraction of the
compounds putatively encoded by
these biosynthetic pathways isolated
or characterized, cyanobacteria are an
opportunity-rich frontier for identifying natural products with potential
medical utility. To address those opportunities, researchers are developing
a growing number of innovative methods with which to detect, isolate, and
determine chemical structures and bioPie charts of a) the secondary metabolites reported from the various groups of marine
logical properties of those molecular
cyanobacteria (Types I to V with botanical order equivalents provided), and b) the
metabolites of the Type III cyanobacteria (Oscillatoriales) separated by genus (data from
products. For example, we devised a
the MarinLit® database).
strategy for using bioinformatics to
guide isotope-labeled, precursor-feeding experiments that we call the genoisolates whose taxonomies were characterized
misotopic approach.
in this way came from freshwater environMeanwhile, developing knowledge of regulaments.
tory gene sequences and their protein transcriptional modulators also looms as critical for unProbing the Biosynthetic Genes that
covering the full natural products capacity of
Yield Valuable Natural Products
FIGURE 2
We and others are investigating how secondary
metabolite biosynthetic genes are distributed in
cyanobacteria. For example, members of our
group along with David Sherman and his collaborators at the University of Michigan (UM),
Ann Arbor, are beginning to isolate and charac-
280 Y Microbe / Volume 3, Number 6, 2008
Figure 3 (facing page). (A) Natural products of marine
cyanobacteria discussed in text with their principal biological properties in human health oriented assays. In panels
B-D, the genes, encoded proteins, and deduced catalytic
domains in the (B) curacin A biosynthetic gene cluster, (C)
hectochlorin biosynthetic gene cluster, and (D) lyngbyatoxin biosynthetic gene cluster.
FIGURE 3
Volume 3, Number 6, 2008 / Microbe Y 281
tive when tested against a cancerous
mammalian cell line, and this finding
led us to isolate a lipid that we named
curacin A (Fig. 3).
Subsequently, we determined that
curacin A inhibits tubulin polymerization through interactions at the colchicine-binding site. While various scientists developed several routes for
synthesizing it, we began exploring
how it is made biochemically. Our initial approach involved feeding stable
isotope-containing precursors to L.
majuscula in culture, followed by nuclear magnetic resonance (NMR) analysis of labeled curacin A. These efforts
led us to collaborate closely with David
Sherman and his colleagues at UM
and, subsequently, to isolate the gene
cluster that encodes enzymes responsible for synthesizing curacin A.
We are continuing to characterize
that gene cassette as well as the enzymes that catalyze formation of the
cyclopropyl ring in curacin A (Fig. 3).
This remarkable set of biochemical reactions uses three acetate molecules
(plus coenzyme A) to produce a single
enzyme-bound molecule of 3-hydroxyImaging of the sponge-cyanobacterial cell assemblage Dysidea herbacea with its cyanobacterial symbiont Oscillatoria spongelae by light microscopy (A) and epifluorescence
3-methylglutaryl acyl carrier protein
microscopy (B) showing short rods of cyanobacterial cells surrounded by irregular sponge
(ACP), which is the branched 6-carbon
cells. (C) Control epifluorescence micrograph showing location of cyanobacterial cells. (D)
precursor for terpenes. A series of adThe same micrograph as in panel C probed with the sponge-derived halogenase gene
probe (CARD-FISH) showing that this unique halogenase, responsible for creating
ditional reactions converts this precurtrichloromethyl groups as in barbamide, is exclusively located in the cyanobacterial cells.
sor into a 5-carbon cyclopropyl-con(Reprinted from P. M. Flatt, J. T. Gautschi, R. W. Thacker, M. Musafija-Girt, P. Crews, and
taining alkyl chain. Because several of
W. H. Gerwick, Marine Biol. 147:761-774, 2005, with kind permission of Springer Science
and Business Media.)
these reactions involve novel biochemical transformations, we are continuing a
detailed mechanistic investigation that
these photosynthetic bacteria. For example, we
includes X-ray crystallographic analysis of several
recently began using methods similar to those
of these enzymes.
developed by Rolf Müller of Saarland University
Similar gene cassettes are found in other main Saarbrücken, Germany, to study regulatory
rine cyanobacterial natural product biosynthetic
pathways in myxobacteria. Our efforts are startpathways. However, they yield different strucing to reveal some previously unrecognized regtures such as the vinyl chloride function in the
ulatory mechanisms in cyanobacteria.
natural product called jamaicamide A. Hence,
we are investigating how highly comparable
Cyanobacteria from Caribbean Yield
gene pathways and enzymes lead to such diverPotent Anticancer Candidates
gent functional groups.
We launched a program in 1993, surveying maAdditional Promising Natural Products
rine algae and cyanobacteria in Curaçao in the
from Marine Cyanobacteria
southern Caribbean for bioactive natural products. The extract of one shallow-water marine
An organic extract of a small tuft of L. majuscyanobacterium, L. majuscula, was highly accula that was collected in 1996 near the eastern
FIGURE 4
282 Y Microbe / Volume 3, Number 6, 2008
shore of Jamaica yielded powerful anticancer
cell activity. Because the sample extracts were
insufficient for chemical studies, viable material
from a single filament was clonally expanded,
and it yielded two exceptionally interesting secondary metabolite classes, designated the jamaicamides and hectochlorin (Fig. 3). Hectochlorin
proved to be the agent responsible for that initial, powerful cancer cell toxicity, whereas the
jamaicamides displayed moderate neurotoxic
properties.
Jamaicamide A incorporates three distinct
functional groups whose biosynthetic origins
remain poorly understood—namely, a vinyl
chloride appendage on a polyketide, a terminal
acetylenic bromide, and a pyrrolidone ring.
While we deduced the fundamental building
blocks of jamaicamide A from a series of experiments involving the use of stable isotope-labeled precursors, figuring out the biosynthetic
chemistry that leads to its assembly required us
to take a molecular genetics-based approach.
Thus, we developed a cloning strategy that
yielded a 58-kb gene cluster, which encodes the
proteins that catalyze jamaicamide biosynthesis.
We are continuing to investigate the steps that
form the distinctive vinyl chloride functionality
of this terpene variant.
We also are investigating hectochlorin biosynthesis, focusing on the origin of the gemdichloro functionality within its polyketide and
on the basis for the two different regiochemical
incorporation patterns of 2,3-dihydroxyisovaleric acid (Fig. 3). We find high sequence homology of hctB with barB1 and barB2, which are
two marine examples of radical halogenases
that we discovered within the barbamide biosynthetic gene cluster that specifically halogenate a methyl group. The hctB gene encodes a
radical halogenase that catalyzes chlorination of
a carbon atom other than a methyl group. We
are collaborating with Christopher Walsh at
Harvard University and his collaborators, seeking to explain the structural basis for this alternate regiochemical selectivity.
Drifting Tufts of Cyanobacteria Can
Produce Biologically Active Agents
Sometimes waves and currents dislodge cyanobacterial filaments. Those filaments, in turn,
excrete a slimy mucilage that entraps bubbles of
photosynthetically derived oxygen, floating the
filaments to the surface where they continue to
grow with other plankton. Sometimes these
floating assemblages drift as large cyanobacterial tufts, and are common in places such as
Hawaii and northeastern Australia.
Some dislodged and drifting strains of cyanobacteria produce extremely potent dermatotoxins that cause harm in near-shore environments, especially when they are being used by
local swimmers or water-loving tourists. In Hawaii, this phenomenon gives rise to sporadic
bouts of swimmer’s itch, whose immediate
cause is an alkaloid, that Richard Moore of the
University of Hawaii described in 1979 and
named lyngbyatoxin A (Fig. 3). It contains a
number of noteworthy structural features, including a lactam ring, a reduced phenylalanine
carboxyl group, and a reverse-prenylated indole. Some strains of L. majuscula produce another type of dermatotoxin, called aplysiatoxin.
Because of the potential public health benefits
from understanding better how these environmental toxins are produced—and also out of
pure curiosity—we began characterizing biosynthetic gene clusters in toxin-producing cyanobacteria collected near Hawaii. Before beginning, we recognized that, early in the process,
the NRPS involved in producing lyngbyatoxin
likely forms a peptide bond between valine and
tryptophan. Moreover, a biochemical reduction
reaction likely releases that dipeptide from the
NRPS complex. This hypothesis was recently
confirmed by Walsh and his collaborators at
Harvard, who characterized the reductive mechanism for releasing the lyngbyatoxin dipeptide
from its NRPS complex.
With those biochemical steps in mind, we
devised a cloning strategy to isolate the biosynthetic gene cluster. From a bioinformatic analysis of the cluster, we postulate that a cytochrome
P450 activates the phenyl ring of tryptophan
such that the lactam ring of lyngbyatoxin can be
formed. Next, a reverse prenylation at the opposite side of this ring occurs, possibly first at the
indole nitrogen atom, and then through a
Claisen-type rearrangement, moving to the final
C-7 location.
Many Natural Products Derive from
Invertebrate-Associated Microorganisms
Microorganisms produce many of the natural
products that chemists isolate from marine in-
Volume 3, Number 6, 2008 / Microbe Y 283
vertebrate sources. More generally, marine invertebrate species are remarkable for how extensively they associate with marine microbiota.
For instance, some sponges consist of 40% bacteria by weight! These and other marine invertebrates harbor diverse microorganisms, sometimes benefitting through rich suites of microbially
produced chemicals that protect these sponges
or other hosts against predators or pathogens.
Meanwhile, however, microbiologists have
had little success trying to cultivate these invertebrate-associated microorganisms. Hence, little
is yet known about precisely which microorganisms are producing the secondary metabolites.
To date, most insights come from non-culturebased methods, including cell separations, imaging with immunogold reagents, direct imaging
that depends on spectral features of the molecule
under study, catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH) to
localize biosynthetic genes within particular cell
types, and mass spectrometric imaging (MSI)
(Fig. 4).
Although MSI rapidly and directly determines
where natural products are located in complex
assemblages, even this type of information
sometimes can prove misleading. Thus, for example, one cell type might produce and then
secrete a particular compound that is then selectively absorbed by a second cell type. This sce-
nario apparently explains earlier confusion over
the origins of cyclic peptides that are associated
with the tunicate Lissoclinum patella. Researchers thought that this tunicate produces cyclic
peptides in its own tissues. However, subsequent studies involving gene cloning, expression, and genomic sequencing indicate that symbiotic cyanobacteria actually produce these
peptides. It is possible that through the process
of metabolite excretion from this microbial
source that they can accumulate and be detected
in the tunicate tissues.
These giant marine cyanobacteria are indeed
an exciting area for microbial research. Their
structurally novel secondary metabolites arise
from unique biochemical pathways and machinery that are equally worthy of study. Complementing this structural and biosynthetic novelty, the activities of some of these metabolites
are exerted through new mechanisms that reveal
innovative pathways with pharmacological potential. From the phylogenetic relationship of
these organisms, the evolution of their natural
product pathways, and the structures and activities of their metabolites, these organisms have
much to teach us, much to provide in terms of
useful products and bio-chemical tools, and ultimately, much to inspire us with through integration of natural products chemistry and microbiology.
SUGGESTED READING
Corre, C., and G. L. Challis. 2007. Heavy tools for genome mining. Chem. Biol. 14:7–9.
Donia, M. S., B. J. Hathaway, S. Sudek, M. G. Haygood, M. J. Rosovitz, J. Ravel, and E. W. Schmidt. 2006. Natural
combinatorial peptide libraries in cyanobacterial symbionts of marine ascidians. Nature Chem. Biol. 2:729 –735.
Edwards, D. J., and W. H. Gerwick. 2004. Lyngbyatoxin biosynthesis: sequence of biosynthetic gene cluster and identification
of a novel aromatic prenyltransferase. J. Am. Chem. Soc. 126:11432–11433.
Flatt, P. M., J. T. Gautschi, R. W. Thacker, M. Musafija-Girt, P. Crews, and W. H. Gerwick. 2005. Identification of the
cellular site of polychlorinated peptide biosynthesis in the marine sponge Dysidea (Lamellodysidea) herbacea and symbiotic
cyanobacterium Oscillatoria spongeliae by CARD-FISH analysis. Marine Biol. 147:761–774.
Hildebrand, M., L. E. Waggoner, G. E. Lim, K. H. Sharp, C. P. Ridley, and M. G. Haygood. 2004. Approaches to identify,
clone, and express symbiont bioactive metabolite genes. Natural Product Rep. 21:122–142.
Newman, D. J., and G. M. Cragg. 2007. Natural products as sources of new drugs over the last 25 years. J. Natural Products
70:461– 477.
Rachid, S., K. Gerth, I. Kochems, and R. Müller. 2007. Deciphering regulatory mechanisms for secondary metabolite
production in the myxobacterium Sorangium cellulosum So ce56. Mol. Microbiol. 63:1783–1796.
Ramaswamy, A. V., C. M. Sorrels, and W. H. Gerwick. 2007. Cloning and biochemical characterization of the hectochlorin
biosynthetic gene cluster from the marine cyanobacterium Lyngbya majuscula. J. Natural Products 70:1977–1986.
Simmons, T. L., R. C. Coates, B. R. Clark, N. Engene, D. Gonzalez, E. Esquenazi, P. C. Dorrestein, and W. H. Gerwick. 2008.
Biosynthetic origin of natural products isolated from marine microorganism-invertebrate assemblages. Proc. Natl. Acad. Sci.
USA 105:4587– 4594.
Simmons, T. L., and W. H. Gerwick. 2008. Anticancer drugs of marine origin, p. 431– 452. In Oceans and human health.
Elsevier, Academic Press, Burlington, Mass.
Van Wagoner, R., A. K. Drummond, and J. L. C. Wright. 2007. Biogenetic diversity of cyanobacterial metabolites. Adv. Appl.
Microbiol. 61:89 –217.
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