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鈥淎 dark illimitable ocean without bound, without dimension, where length,
breadth, and height and time and place are lost鈥� John Milton, Paradise
Lost

ALL RESPECT to Milton, but he didn鈥檛 get it quite right. Earth鈥檚 oceans may
appear limitless from a human perspective, but look closer and you get a
different picture. They aren鈥檛 vast, structureless seas of liquid stretching
uniformly to the horizon. To most of their inhabitants, in fact, they are not
liquid at all. The oceans turn out to be鈥攚ell, jelly.

Not very thick jelly, it鈥檚 true. Certainly not thick enough to notice when
you swim in it. But for the marine world鈥檚 smallest creatures鈥攖he tiny
plankton and microbes that make up the bulk of its inhabitants鈥攕eawater is
not a uniform fluid, but a tangle of intertwined chains of sugar molecules that
trap water within their meshwork to form a gel. 鈥淎t this tiny scale, the ocean
becomes kind of cobwebby,鈥� says Alice Alldredge, a biological oceanographer at
the University of California, Santa Barbara. 鈥淚t鈥檚 filled with microscopic
strands and particles of gel.鈥�

It sounds a lot more prosaic than Milton鈥檚 poetry. But seawater鈥檚
surprisingly substantial character has given oceanographers something to wonder
about. 鈥淭his gel structure is something that oceanography has traditionally not
considered,鈥� says Farooq Azam, a microbial oceanographer at the Scripps
Institution of Oceanography in La Jolla, California. 鈥淚t鈥檚 not in the textbooks
or in the classical explanations. The gel鈥檚 existence fundamentally changes our
ideas of the microcosmos in which sea organisms live. It has added another layer
of complexity that people are only now starting to consider in the context of
whole ocean systems.鈥�

For centuries, we have had a simple notion of seawater: nothing more than a
salt solution with particles suspended in it or sinking slowly to the bottom.
Organisms were independent units that floated or swam in this fluid. But now and
then, the ocean has offered up a few hints that things might not be so simple.
Every few summers, for example, a bloom of algae (and possibly bacteria) turns
the surface of the northern Adriatic Sea to jelly, a phenomenon that the
Italians call mare sporco. 鈥淔rom Venice to Rimini, it can be a major
problem for the tourist industry,鈥� says Azam. 鈥淭he water gets sort of
驳辞辞别测.鈥�

Another clue came when Azam and others started looking closely at seawater in
the early 1990s. They found that the microbes within it weren鈥檛 randomly
dispersed, as they ought to be in a pure liquid. Instead they clustered into
tight groups. What was holding them there?

Perhaps there was a scaffolding of large molecules in the water, they
suggested. But at the time, no one had seen such a scaffolding or found any
direct evidence that it existed, and their idea created hardly a ripple.
Sceptical colleagues demanded hard proof. 鈥淕el is like the dark matter of the
sea,鈥� says Azam. 鈥淲e inferred its existence, but the structure remained hidden
from us until new ways were developed to take a very close look at it.鈥�

Now the tide seems to have turned. 鈥淚t鈥檚 all come together in the last few
years,鈥� says Dan Repeta, a marine chemist at Woods Hole Oceanographic
Institution in Massachusetts. 鈥淏iologists found their proof. Chemists found
theirs.鈥� In 1993, Alldredge began staining seawater samples with new dyes that
would reveal colloids鈥攍arge molecules stably suspended in the
liquid鈥攁nd found the water was chock-full of them. Then two years ago, a
team headed by Wei-Chun Chin at the University of Washington in Seattle saw that
much of the ocean鈥檚 pool of such molecules spontaneously assembles into a gel
matrix. Simultaneously, marine chemists elsewhere began reporting that seawater
contained high concentrations of polysaccharides, large polymers made up of
sugar molecules strung together, providing further independent evidence that the
gel might exist.

In the next year or so, Azam hopes to clinch the case by examining a living
sample of seawater with specialised microscopes. 鈥淭he technology is all in
place. It鈥檚 just a matter of putting it together the right way. When we do,
we鈥檒l be able to make direct observations,鈥� he says.

In the meantime, there are plenty of other questions to answer, like how the
gel forms at all. The basics, it turns out, are simple enough. In life and in
death, marine organisms exude many kinds of organic molecules into seawater.
Most are quickly devoured by a multitude of bacteria in the water, but some
particularly large and complex polysaccharides鈥攂ranched molecules hundreds
of sugars long鈥攁re too unwieldy for bacteria to digest. These long-lived
sugar compounds persist and cross-link to create a three-dimensional meshwork,
which traps water to form a gel.

Azam and his colleague David Smith have calculated that if the polysaccharide
molecules in a single millilitre of seawater were untangled and lined up end to
end, they would stretch 5600 kilometres. 鈥淥f course, in reality it鈥檚 all
criss-crossed, bent and folded together in a jumble,鈥� says Azam. But
polysaccharides are just the start. The meshwork in each millilitre also
includes proteins (310 kilometres), DNA (2 kilometres) and other molecules.
Together they create nutrient-rich hot spots.

For microscopic life forms, the gel offers a firm foundation for microbial
life, providing bounteous food and a diverse choice of neighbourhoods, both of
which help make richer, more diverse ecosystems. 鈥淭he existence of these
particles gives a great deal of structure to water,鈥� says Barry Sherr, an
oceanographer at the College of Oceanic and Atmospheric Sciences at Oregon State
University in Corvallis. 鈥淚t鈥檚 similar to trees in a forest. The trees provide
surface areas upon which arboreal creatures exist. In water, these tiny
particles provide more niches in which microbes can live.鈥�

鈥淚t鈥檚 a major change in thinking,鈥� adds Azam. 鈥淢icrobes aren鈥檛 just randomly
floating in seawater. They exist in microniches created not just by the
existence of the gel, but by variations millimetre to millimetre in its
structure or chemistry. It鈥檚 not just like a forest of trees. It鈥檚 like a forest
filled with different kinds of trees.鈥�

In research still to be published, Richard Long, a graduate student working
with Azam, showed just how diverse this forest is. Long scooped up a sample of
seawater off the end of the pier at Scripps. After allowing any large particles
within it to settle, he inserted a home-made array of 10 tiny glass capillary
tubes, spaced two millimetres apart along a microscope slide. Each of the tubes
extracted a single microlitre of seawater.

Using DNA fingerprinting techniques, Long found that each microlitre sample
contained at least 20 species of bacteria. But what was particularly startling
was that some of the species turned up in just a single one of the 10 samples.
鈥淭his was something we did not know we would find,鈥� says Azam. 鈥淲hat it says is
that there are differences in species composition and richness at the millimetre
scale of space in the ocean. It implies that there are microscale variations in
the structure of the system.鈥�

The gel helps lock these small-scale differences into place. But even more
important, it helps draw the ocean鈥檚 scattered microbes together into tight,
interactive neighbourhoods. To see how important this is, imagine a typical
microlitre of seawater鈥攁 1-millilitre cube鈥攕tretched to fill a
concert hall 60 metres long, 30 metres wide and 30 metres high. All the
microbial life within that huge space would be represented by one cricket bat
(that鈥檚 a single-celled alga, or phytoplankton), five basketballs (protozoans),
1000 peas (bacteria), and 10,000 pinheads (viruses). Scatter those randomly
through the hall鈥檚 volume and there鈥檚 not much chance of them bumping into one
another.

Fortunately, the real world is far from random. 鈥淢arine bacteria experience a
world that is not uniform, but patchy,鈥� says Azam. 鈥淭here are areas of
biological activity surrounded by areas of inactivity.鈥� This patchiness exists
both horizontally and vertically, on an oceanic scale right down to the
microscopic. The gel provides a focus around which neighbourhood economies can
develop at the finest level.

But why does the gel form? As yet, no one really knows, but phytoplankton
seem to be at the centre of things. These tiny organisms use energy from
sunlight to convert carbon into sugars, some of which they release into the
water as polysaccharides. Bacteria feed on these sticky sugars, which may also
act as a kind of flypaper, snagging passing organic debris rich in nitrogen and
phosphorus. Or the sugars could provide a protective coating against invasive
viruses.

In any case the polysaccharides, which carry a negative charge, interact with
positively charged ions of calcium, magnesium and sodium in the water, aligning
themselves into microfibrils called transparent exopolymer particles, or TEPs.
In Alldredge鈥檚 seawater samples there were between 28,000 and 5 million TEP
particles per millilitre. The TEPs, she says, act as a sort of biological glue,
binding to each other and to matter such as microscopic faecal pellets from
plankton, drifting strands of DNA, algal cells and other detritus to form larger
sheets, webs, discs and balls with the consistency and behaviour of gel.

Bacteria flock to these invisible TEPs. Some use them as shelter from
predators, but many chew down on this ready-made meal, wielding enzymes that
break them down into a form they can absorb, all the while belching out
phosphates and ammonia as waste. This, in turn, is just what the phytoplankton
require. Ammonia provides nitrogen; add the phosphate and you鈥檝e got the perfect
fertiliser.

鈥淏acteria bring several things to their relationship with phytoplankton,鈥�
says Azam. 鈥淭hey help produce high, local concentrations of ammonia and
phosphate. And the iron in the bacteria, released when they鈥檙e eaten by
protozoans, can produce a pulse of missing nutrients.鈥� The result is a microbial
circle of life: phytoplankton exude sugars that bacteria eat, producing wastes
that the phytoplankton use to grow, resulting in more phytoplankton producing
more sugars for more bacteria.

Of course, reality is a bit messier than this tidy scenario. Bacteria seem to
munch selectively on the feast, consuming a bit here, a bit there, notes Azam.
Much is either ignored or tossed aside, and becomes part of the gel. And TEPs
don鈥檛 appeal to every palate. 鈥淭hey鈥檙e like candy. They don鈥檛 have any protein,鈥�
says Alldredge. 鈥淭hat鈥檚 not good for animals who need a more balanced diet.鈥�
Krill and the tiny crustaceans called copepods will eat TEPs if they鈥檙e hungry
enough, she says, 鈥渂ut it would be like us trying to live off Mars bars鈥�.

Both Azam and Alldredge are quick to caution that these descriptions are
somewhat speculative. 鈥淭here鈥檚 a lot we don鈥檛 yet know or understand,鈥� says
Azam. For example, scientists don鈥檛 know exactly what conditions cause the gel
to form, or how environmental effects, such as global warming, influence the
gel鈥檚 structure.

They are keen to learn more, not least because knowing how the ocean gel
works will help us understand larger, more complicated subjects like the Earth鈥檚
carbon cycle and its role in global warming. Oceans may help to regulate the
amount of carbon dioxide in the atmosphere by absorbing it at the surface,
converting it to other compounds, and then stashing it away in deep water or
sea-floor sediment for thousands of years. And in recent years, it has become
clear that much of that absorption happens when phytoplankton and bacteria
photosynthesise, using the CO₂ to make sugars and, ultimately,
everything else in their cells. The fate of this 鈥渇ixed鈥� carbon鈥攚hether it
is eventually respired back into the atmosphere or becomes buried in deep-sea
sediments鈥攄epends greatly on the action of microorganisms.

No one knows yet exactly how microbes control carbon fluxes, says Azam, but
it seems a fair bet that their diversity has a big influence, and that this in
turn is linked to seawater鈥檚 gel-like properties. 鈥淭he importance of the gel
structure is that it creates a world that is not uniform, in which there isn鈥檛
ultimately a single winner,鈥� says Azam. 鈥淭he gel structure sustains a high
diversity of microbes, and high biodiversity means many more types of mouths
working on carbon in the water.鈥�

Exactly what kinds of mouths is the sort of detail Azam and others are now
investigating. The gelatinous world of the invisibly small is a frontier
scientists didn鈥檛 know existed until the 1990s. 鈥淭he lives of marine microbes
aren鈥檛 like anything we can imagine,鈥� says Sherr. In the world of marine
microoganisms, he says, 鈥渕atter behaves in ways that defy common sense.鈥�