Under A Microscope: The Oceans Microbes
density of bacterial in what they thought were the cleanest ocean areas. Pristine waters were teeming with
bacteria. These exceedingly large numbers pushed the need to understand how they fit into the ecosystem and the
relationships between bacteria and host communities in natural marine environments.
Using a high-speed centrifuge and a sensitive electron microscope, scientists were shocked to discover that even
pristine marine and freshwater environments harbor astonishing numbers of aquatic viruses.
The newly discovered viral concentrations exceed by up to 10 million times those previously recorded in aquatic
environments, suggesting these minuscule microbes -- some as small as 60 nanometers -- represent a much bigger
piece of the ecological puzzle than scientists believed. Moreover, although the viruses themselves appear incapable
of infecting humans, they may create a health threat by injecting disease-causing genes into common bacteria.
Scientists at the University of Bergen in Norway subjected filtered water samples to 100,000 times the force of
gravity and analyzed the resulting sediment. Among other findings, they determined that 1 teaspoon of North
Atlantic seawater taken from 10 meters below the surface contained 75 million individual viruses. There are so
many viruses in ocean waters that researchers began asking, “What are they all doing?”
Most seem busy infecting aquatic bacteria, possibly accounting for the immense and unexplained bacterial turnover
rates in water. Every minute, grazing protozoans gobble huge numbers of aquatic bacteria, yet studies indicate
bacterial reproduction far exceeds these grazing rates. The new findings suggest that viruses, which can multiply
in bacterial cells before killing them, may account for a third or more of aquatic bacterial mortality.
The implications of this covert infection frenzy are many. For ecologists, it suggests that a surprisingly vast
majority of the energy exchange in the aquatic food web occurs among organisms small enough to pass right
through the sieves of the smallest filter-feeding animals. This could radically alter current models of aquatic
nutrient cycles, which have focused on larger plankton as the food chain's first significant link.
Marine microbiologists had known for decades that the ocean had a large density of free DNA had, which they had
previously attributed to "sloppy feeding" by protozoan grazers. The high rate of dying bacteria can now be
attributed to virus-induced bacterial rupture.
A high viral concentration in the oceans can result in unusually high rates of bacterial evolution, since viruses can
carry bits of bacterial DNA from one bacterium to another, changing their ability to infect marine species.
Ecologists hope that this could lead to the rapid emergence of bacteria capable of digesting toxic wastes after a
spill or develop enzymes that degrade things like boat bottoms, plastic cups and aluminum cans.
More worrisome to marine microbiologists is the possibility that genes providing bacteria with resistance to
antibiotics or increased bacterial virulence may rapidly spread by viral attachment to non-harmful bacterial strains.
Another worry is that laboratory engineered bacteria could make their way into ocean waters teeming with a
multitude of viruses and that these bacteria will pass their altered genes to native bacteria. So far, scientists have
looked only on land for such DNA donations and have used the negative findings to justify further releases of
The Bt corn plant is an example which is engineered to contain a gene taken from bacteria and inserted into the
DNA of corn seeds. When the corn sprouts every cell of the plant including the edible corn ear produces
insecticide intended to kill insects that try to invade the corn. DNA from these laboratory engineered bacteria will
float in runoff from farmlands eventually reaching the ocean.
Naturally evolved viruses and bacteria are already invading the coral reefs destroying large areas.
Bacteria and Viruses Are Decimating the Coral Reefs
Scientists have recently concluded that disease may be a significant factor accelerating the deterioration of coral
reefs, while others believe disease is organisms are merely opportunistic invaders taking advantage of the corals
Either way, they are causing substantial coral mortality. Close to 80% of all western Atlantic coral species are now
affected. Some corals such as Montastraea annularis complex is showing signs of 2 or 3 diseases at the same time.
Unfortunately, there is little factual knowledge about the cause, prevalence, and consequences of most disease
outbreaks, although studies suggest that these pathogens are exploiting the compromised health of corals that has
been reduced by natural and manmade environmental stressors. Extreme temperature changes and bleaching events
seem to be followed by outbreaks of disease in the coral reefs.
Harmful Coral Diseases
(A) Black-band disease is pictured on the Caribbean Siderastrea siderea, is the oldest known coral
disease and is caused by a bluegreen algae called Oscillatoria membrancea.
Caribbean. No known pathogen has yet been discovered.
(C) White plague on Caribbean Agaricia lamarcki. The bacteria that causes this disease is the Serratia
marcescens, the common bacteria associated with human and animal feces.
(D) Several yellow-band-disease infections on Caribbean Montastraea faveolata, is caused by a bacteria.
Corals Diploastraea. These are found in either the Caribbean, Florida Keys or the Indo-Pacific.
(E) White syndrome on Indo-Pacific Diploastrea heliopora is caused by a bacteria that often attacks
(F) Aspergillosis is a fugus that produces purplish lesions in sea fans and sea whips and other Caribbean
Outbreaks of these voracious predators have reached plague proportions in Australia and the Red Sea.
Diseases Outbreaks Are Increasing
Coral diseases are apparently an emergent problem, since their frequency is seemingly increasing. Between 1972
and 2005 coral diseases were reported on 39 coral genera and 148 species worldwide, with observations in 63
Most of these diseases are caused by microorganisms such as bacteria and algae.
Although Pacific reefs have a higher diversity of reef-building corals than the Atlantic and harbor 92% of the
world's coral reefs, only 14% of the global observations of coral disease were from the Indo-Pacific.
The first Caribbean epizootics were reported in 1978 and led to die-offs of two dominant structure-forming corals,
A. palmata and A. cervicornis. Between 1982 and 1984, a disease of unknown cause decimated populations of a
keystone species, the herbivorous sea urchin Diadema antillarum. This mortality triggered increases in fleshy
macroalgae and concurrent losses of coral cover, biodiversity, and habitat in many locations.
Since 1998 there has been an emergence of new and virulent diseases in the wider Caribbean, where over 30
named diseases affect 45 species of scleractinian corals, 3 hydrozoan corals, 10 octocorals, 2 zoanthids, 9
sponges, and 2 crustose coralline algae.
Thus the Caribbean has been referred to as a “hotspot” for disease, largely because of the rapid emergence, high
prevalence, wide distribution, large number of host species, and virulence of diseases in this region—and, not least,
the large body of research. Much less is known about the Indo-Pacific, but surveys there revealed new diseases,
suggesting either a rapid emergence of diseases or a new realization of their presence.
Bergh, O. 1989. High Abundance of Viruses Found in Aquatic Environments. Nature pp. 467–468.
Dubinsky, Z. Coral Reefs: An Ecosystem in Transition. 2010.
Harvell, D. 2001. Coral bleaching and disease. Hydrobiologica pp. 97–104.
Rosenberg, E. Coral Health and Disease. 2010.