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Each spring and summer, microscopic algae become visible: huge ‘blooms’
form in the oceans and foam banks appear along the east coast of England
and the coast of the Netherlands. The growth of algal blooms is often linked
to the pollution of coastal areas by nitrates and phosphates. Sometimes
algae themselves, like the ‘red tide’ that swept south from the Baltic in
1989, are toxic. But recent evidence suggests that some algae play a vital
and subtle role in regulating the Earth’s climate.

Algae produce a sulphur compound which seems not only to be a key link
in the global sulphur cycle, but which also influences the formation of
clouds, and therefore the Earth’s temperature. Understanding how these algae
affect cloud formation in the remote oceans could be crucial to predicting
how the Earth will respond to global warming.

Climatologists know that clouds are important regulators of radiation.
Clouds reflect incoming short-wave radiation from the Sun and absorb and
re-emit long-wave radiation from the Earth’s surface. The freezing of water
vapour or the melting of ice within clouds is part of the basic energy balance
of the atmosphere. But no one knows how global warming will affect the distribution
of cloud. Clouds are fiendishly tricky to study, and even the most complex
computer models of the Earth’s climate can only treat them very crudely.

Last year, Catherine Senior and John Mitchell of the Hadley Centre for
Climate Prediction and Research, which operates under the Met Office, came
the closest yet to an accurate description of how clouds influence and are
influenced by climate change. Their model shows in detail how clouds would
respond to global warming by acting as a negative feedback mechanism, reducing
the rate at which the Earth warms up. But even this model is greatly simplified
and, like all other ‘general circulation’ models, it cannot encompass the
potential influence on cloud formation of microscopic marine algae. There
is growing evidence that these algae are important to this process.

The link between algae and climate involves dimethyl sulphide, or DMS,
the gas that gives sea air its bracing smell. It forms from the enzymatic
breakdown of a salt, dimethylsulphoniopropionate (DMSP). Marine algae produce
DMSP to keep their osmotic balance with sea water, without which water
would leave the cells of the algae, killing them. The processes by which
DMSP is released into the sea are not well understood, but most researchers
think it occurs when algae die, or are grazed by zooplankton. In the sea,
DMSP breaks down to form DMS. A fraction of this DMS, perhaps a tenth, then
enters the atmosphere. The rest is either consumed by bacteria or broken
down by sunlight to form dimethylsulphoxide.

In the early 1970s James Lovelock, the British chemist who originated
the Gaia hypothesis, suggested that DMS might provide a way of returning
sulphur washed from the land to the sea . Sulphur is a vital biochemical
element, and Lovelock was looking for an explanation of how sulphur levels
on land are maintained. In 1987 Robert Charlson of the University of Washington,
his colleague Stephen Warren, and Meinrat Andreae of Florida State University,
put forward with Lovelock a theory which suggested that the influence of
DMS goes far beyond its role in the sulphur cycle. DMS, and therefore algae,
they argued, play a vital role in regulating the Earth’s climate. DMS reacts
in the atmosphere to form three types of compound: sulphur dioxide, sulphates
and methane sulphonic acid. Water vapour can condense around particles containing
the last two to form clouds. Charlson’s idea was that in remote open regions
of the oceans – which together make up around half of the Earth’s surface
– most of the clouds form as a result of this process.

Charlson’s ideas about DMS sparked off a flurry of research, as scientists
realised that to test these ideas they needed to understand in much more
detail how sulphur travels between the atmosphere, the geosphere, the biosphere
and the hydrosphere. They already knew the basic cycle: the ultimate source
of all sulphur on the Earth’s surface is volcanic emissions, and sulphur
tied up in microorganisms and sea salt is returned to the geosphere through
sedimentation. DMS was known to be part of this cycle, but its precise
role has only recently been established.

Algae transfer between 20 and 50 million tonnes of sulphur from the
oceans to the atmosphere every year. Human activity accounts for not much
more – about 80 million tonnes. Algal volumes in the temperate oceans reach
a peak in spring and summer, and one important source of DMS is the alga
Phaeocystis poucheti, which also forms banks of foam along Britain’s east
coast and the coast of the Netherlands.

Peter Liss’s team at the University of East Anglia made the conclusive
link between algae and DMS emission after measuring concentrations of DMS
in the surface waters of the North Sea for 9 months, as part of the Natural
Environment Research Council’s North Sea Community Project. This was a multidisciplinary
project whose main aim was to develop a model of water quality in the southern
North Sea. They found that the mean concentration of DMS in the North Sea
in summer is about a hundred times that in winter. Such a seasonal variation
matched the growth patterns of the algae. The team is now looking at the
factors which control the rate of DMS production, while together with another
group at Plymouth Marine Laboratory, led by Andrew Watson, they are investigating
how DMS is transferred from the sea to the atmosphere.

Concentrations of DMS in different regions of the oceans also vary.
Generally, the nutrient-rich waters of the continental shelves have more
algae, and by implication more DMS, than the relatively barren open ocean.
However, things are not always quite this simple, because different species
of algae produce DMS in different quantities. For example, Coccolithophores,
which form huge blooms covering areas of up to 500 000 square kilometres
in the relatively nutrient-poor open oceans, produce about a hundred times
as much DMS as some other algae – such as diatoms, which thrive on the continental
shelves, among other places, but produce very little DMS.

While Liss’s team was establishing the importance of DMS as part of
the sulphur cycle, evidence was also growing that DMS has another, perhaps
more profound influence on the global system. Charlson’s original idea
was that waters heated by greenhouse warming could encourage algal production,
leading to more DMS and hence more cloud. This would lead to more solar
energy being reflected, which would in turn lower the Earth’s temperature.
Could algae act as a global ‘thermostat’, compensating for any forced change
in climate?

Global thermostat

The work of two Australian scientists helped to inspire Charlson to
develop his ‘thermostat’ theory. Keith Bigg and Greg Ayers of the Commonwealth
Science and Industrial Research Organisation in Australia were indirectly
measuring DMS concentrations in remote parts of the oceans near Antarctica.
Large amounts of DMS entering the atmosphere never reach land at all, but
are redeposited in the oceans by rainfall. Once in the atmosphere, DMS reacts
rapidly with reactive hydroxyl or nitrate radicals, which are produced by
interaction between sunlight and water vapour, ozone and nitrogen oxides.
Two things then happen. If it loses a hydrogen atom DMS will ultimately
form sulphur dioxide and sulphate aerosols. If it gains a hydroxyl group,
it forms methane sulphonic acid (MSA).

Sulphur dioxide and sulphate have many different sources, but there
is no other significant way in which MSA is produced. Bigg and Ayers used
MSA as a marker in the atmosphere to measure concentrations of DMS. Their
measurements at Samoa, Cape Grim in Tasmania and Mawson on the edge of Antarctica
showed a strong link between levels of sunlight and the concentration of
atmospheric particles. Data from Cape Grim also showed a relationship between
the number of atmospheric particles and the amount of MSA. There is no industrial
activity in these areas, so they assumed that the seasonal variation they
saw is due to the natural variability in DMS production. Because such particles
form the nuclei of cloud droplets, the implication of the finding was that
DMS influences cloud formation.

One assumption behind Charlson’s theory is that increased temperatures
will lead to increased DMS production. The idea is appealing, particularly
to those who believe pumping ever-increasing quantities of greenhouse gases
such as carbon dioxide into the atmosphere is nothing to worry about. But
more recent evidence that DMS would act in this way is not convincing.

Ice cores from the Arctic and the Antarctic provide a record of the
atmosphere’s chemical make-up going back thousands of years. In 1991 Michel
Legrand from the Laboratory of Environmental Glaciology and Geophysics near
Grenoble in France and colleagues from Russia and the US used the Vostok
ice core in Antarctica to reconstruct the atmospheric concentration of MSA
over the past 160 000 years, covering a whole glacial-interglacial cycle.
Their results suggest that concentrations of sulphate aerosols derived from
DMS and MSA are lower during warm interglacial phases and higher during
ice ages (Figure 2). This is the opposite of what would be expected if the
DMS-derived aerosols were acting as a damper on climatic change.

¿ìè¶ÌÊÓƵs have put forward various explanations as to why the experiment
does not seem to fit the theory. One is that the ecology of the southern
oceans during a glacial may favour algal species that produce more DMS.
Another is that changes in atmospheric circulation could influence the
amount of aerosol material deposited on the Antarctic land mass. Also, more
of the earth’s water is frozen during an ice age, increasing the salinity
of the oceans – so perhaps algae produced more DMSP as a response to salt
stress. One problem with using the Vostok core is that no one knows whether
it is representative of the whole Earth’s atmosphere or not. Work on ice
cores in Greenland (see next week’s issue) should help to dispel any doubts.

Charlson has suggested that to counteract dramatic global warming –
caused, for example, by a doubling of the atmospheric CO₂ level
from its pre-industrial concentration of 280 parts per million – would require
a corresponding doubling in the numbers of cloud condensation nuclei. So
even if algae do not act as a natural ‘thermostat’, could they still be
used as part of a future management strategy for global warming?

Iron management

The late John Martin of Moss Landing Marine Laboratories in California
explored this possibility. He was a chief proponent of the theory that algal
growth is limited in many areas not by a lack of conventional nutrients
such as nitrogen and phosphorus, but by iron. Iron reaches the remote oceans
via dust, blown off land masses which may explain why remoter waters rich
in nitrogen and phosphorus, such as the Antarctic seas, are not more biologically
active. Martin’s laboratory experiments at Moss Landing, carried out over
the past five years, show that when iron is added to water samples taken
from nutrient-rich regions, biological activity increases by about ten times.

These findings led Martin to suggest that it may be possible to counteract
global warming by adding iron to parts of the oceans which are rich in nutrients,
but low in biological activity. The initial proposal was that greater algal
productivity would ‘fix’ more of the excess carbon entering the atmosphere,
in the same way that planting trees does on land. Data from the Vostok ice
core supports this idea, as it shows that an increase in iron is linked
with a decrease in CO₂ levels in the atmosphere over the last
glacial-interglacial cycle.

In October, Kenneth Johnson of Moss Landing, with Liss and Watson, will
try out iron fertilisation in the ocean for the first time in experiments
devised by Martin before his death earlier this year. A patch of the Pacific
near the Galapagos Islands, perhaps a square kilometre in area, will be
fertilised with iron and the water marked. The researchers will then look
for changes in the volume and distribution of algal species, and will monitor
the emission and absorption of gases such as CO₂ and DMS.

If such an experiment were applied on a large scale to control global
warming, the whole marine ecosystem would be fundamentally altered. But
no one knows how. Would increased iron concentrations, or warmer temperatures,
favour the production of diatoms, Coccolithophores or phaeocystis? Diatoms
fix carbon, but produce little DMS. Coccolithophores produce DMS, but release
CO₂, so whether an increase in either group would counteract
global warming is doubtful. Phaeocystis absorbs carbon and produces DMS.
Because of uncertainties like these, Johnson doubts whether iron fertilisation
will ever become part of a plan for managing global warming. ‘I think the
chances of using this method to control the CO₂ in the atmosphere
are very remote,’ he says. He expects to see a shift from small to large
diatoms, on the basis of which computer models show a reduction of no more
than 2 gigatonnes in CO₂. This is small even compared with the
5 gigatonnes now released per year as a result of human activities, only
about half of which is absorbed by the biosphere, and even smaller when
compared with the predicted output of 15 gigatonnes within 50 years.

‘The reason we are carrying out these experiments is to try to understand
marine ecosystems better,’ Johnson says. ‘At the moment we don’t even understand
what regulates primary productivity in the oceans, and the more knowledge
you have the better you can manage a system when pollution occurs.’ He is
in no doubt as to where the emphasis should lie: ‘To control greenhouse
warming we need to reduce CO₂ ±è°ù´Ç»å³Ü³¦³Ù¾±´Ç²Ô.’

Meanwhile, Liss’s team will monitor the impact of iron fertilisation
on DMS emissions. They hope that such studies will help them to predict
what might happen to the climate if the marine ecosystem is affected by
global warming. Until the dynamics of algae are well understood, any attempt
to predict their effect on climate will, it seems, remain elusive.

Nolan Fell is a freelance environmental journalist and Peter Liss is
Professor of Environmental Sciences at the University of East Anglia.

* * *

1: Sulphur and the Gaia hypothesis

The interconnection and inter-dependence of all life is the theme of
James Lovelock’s Gaia hypothesis, which sees the Earth as a ‘superorganism’.
‘The entire range of living matter on Earth, from whales to viruses, and
from oaks to algae, could be regarded as constituting a single living entity,
capable of manipulating the Earth’s atmosphere to suit its overall needs
and endowed with powers far beyond those of its constituent parts.’

Lovelock was struck by the differences between the atmospheres of the
living Earth and the dead Venus. Life preserves an atmosphere in dynamic
equilibrium, one in which oxygen and methane coexist. Without life Earth
would be like Venus, dominated by CO₂ and residing in its lowest
energy state.

The influence of life on its surroundings, its ability to produce oxygen
and absorb carbon, led Lovelock to consider that ‘if the atmosphere is .
. . a device for conveying raw materials to and from the biosphere, it would
be reasonable to assume the presence of carrier compounds for elements essential
in all biological systems, for example . . . sulphur’. After a voyage across
the southern oceans in 1971, Lovelock was the first to suggest that the
carrier compound in which sulphur is returned to land is dimethyl sulphide
(DMS).

Without the return of sulphur to the land, terrestrial life would have
major problems. Without algae, antelopes and elephants would not exist.
But algae do not produce DMS for impala. The return of sulphur to land increases
the productivity of biota and the rate at which rocks weather. Both processes
ultimately provide the algae with a greater flow of nutrients. Lovelock
cites this type of symbiotic relationship as support for the Gaia hypothesis.

DMS also had an important influence on the Earth’s radiation ‘budget’.
The Gaia hypothesis would suggest that it acts as a global thermostat, but
this idea is still controversial. Lovelock has suggested that Gaia’s preferred
temperature and ours may not be the same. The interglacial phases, such
as the one which has existed for the last ten thousand years, may be Gaian
‘fevers’, and the ice ages Gaia’s more stable state. The data from the Vostok
ice core (Figure 2) does not necessarily contradict this idea and if it
is correct, algal-induced cloud cover may help to keep the Earth comfortably
cool.

* * *

2: Tracking down DMS

Estimating the flux of dimethyl sulphide (DMS) accurately is extremely
difficult. The usual method is to measure DMS concentrations in sea and
air samples taken from a boat. But as the oceans cover 70 per cent of the
earth’s surface and the DMS flux varies greatly over space and time, techniques
that show the fraction of biogenic sulphur in any given sample are extremely
important.

Atmospheric sulphur dioxide has many sources, but DMS is the only major
source of methane sulphonic acid (MSA), so this can be used as a marker
for oceanic sulphur. However, the ratio between MSA and sulphate is quite
variable, so the technique does not always provide a very clear picture.

An alternative method, developed by Nicola McArdle at the University
of East Anglia, uses the ratio between the two stable sulphur isotopes,
sulphur-32 and sulphur-34, to estimate the algal contribution of sulphur
to an atmospheric sample. Biogenic and anthropogenic sulphur have different
isotopic ‘signatures’. DMS-derived sulphur has a higher proportion of sulphur-34
than does sulphur released from the burning of fossil fuels.

This is because bacteria that lived in the swampy anaerobic environment
of a carboniferous forest absorbed sulphur across a membrane and ultimately
provided coal and oil with its sulphur content. The bacteria absorb sulphur-32
more easily, as it is lighter.

The ratio between sulphur-32 and sulphur-34 in any sample can be used
to estimate the percentage of sulphur derived from algae that derived from
fossil fuels. Using this technique, McArdle showed that atmospheric aerosols
collected off the west coast of Ireland in spring and summer have around
25 per cent of their sulphur acidity from DMS and 75 per cent from fossil
fuel combustion.