Ecology of Emiliania huxleyi
What is the ecological niche of Ehux? More specifically, which
environmental factors (biological, chemical and/or physical) favour this
phytoplankton species? The eventual aim of work on this topic is to be able
to understand why blooms occur in particular places and at particular times.
For instance, we would like to be able to say that Ehux bloomed in
place X at time Y because silicate had just been stripped from the water, or
perhaps that Ehux bloomed in place X at time Y because the water had
suddenly got cooler. These are the sorts of explanations of E.
huxleyi blooms that we would like to obtain.
Picture courtesy of
(adapted from Tyrrell & Taylor, 1996)
A literature search of reports of natural blooms of Ehux (and the
associated measured water conditions) reveals that they all occur in highly
stratified water where the mixed layer depth is usually ~10-20m, and is
almost always <=30m (Nanninga & Tyrrell, 1996). This suggests that high
light intensities may be influential in causing blooms to form, as does the
usual timing of the blooms, in mid-summer when surface irradiances are high.
Additional evidence for this hypothesis comes from several years of mesocosm
experiments in the Norwegian fjords where Ehux is a common part of
the phytoplankton succession. An analysis of the distribution of
Ehux population sizes against various environmental parameters (Egge
& Heimdal, 1994) points to high incident surface irradiance as the most
influential parameter (click
to see the plot). Laboratory and field PI-curves in which Ehux has
been acclimatised to and tested at high light intensities show a lack of
photo-inhibition, even at the highest light intensities likely to be
encountered in nature (Balch et al, 1992; Nanninga & Tyrrell, 1996),
and this may be a reason underlying their apparent success at high light.
Mesocosm experiments in the Norwegian fjords (Egge & Heimdal, 1994), and
modelling of the results of these experiments (Aksnes et al, 1994),
have suggested that Ehux may have a competitive advantage over other
phytoplankton when inorganic phosphate is limiting, but nitrate is more
abundant. The modelling work also concluded that the observed growth of
Ehux could best be explained by assuming that Ehux is better
able to utilise dissolved organic phosphates than other phytoplankton
(Aksnes et al, 1994). An experimental study by Kuenzler & Perras
(1965) showed that there is a greater activity of the enzyme alkaline
phosphotase in Ehux than in most other phytoplankton, and that this
enzyme is used to assimilate dissolved organic phosphates (as opposed to
dissolved inorganic phosphate). In-situ observations of natural
blooms have implicated low phosphate as a possible factor leading to blooms
(e.g. Townsend et al, 1994), and competition experiments
(multi-species chemostat experiments) have obtained greater numbers of
Ehux, and greater percentages out of the total, at high N/P ratios
rather than at low N/P ratios (Riegman et al, 1992). A
of Ehux in the NE Atlantic (Tyrrell & Taylor, 1996) obtained the most
realistic results (model blooms of Ehux) when Ehux in the
model was made to be adapted to low phosphate and high light.
The availability of CO2 in the water has been suggested as playing a role
in determining the latitudes and times of year at which Ehux can
become dominant. Coccolithophores such as Ehux are thought to
possess a competitive advantage at low concentrations of CO2 due to the
process of calcification (Dong et al, 1993; Brownlee et al,
1994; Nimer, Brownlee & Merrett, 1994), which is believed to synthesise CO2
intracellularly from HCO3 according to the reaction
2HCO3 + Ca2 --> CaCO3 + CO2 + H2O
Since HCO3 is always in plentiful supply in sea-water, those organisms such
as Ehux which can use it to generate CO2 within the cell may be less
vulnerable when external CO2 is in short supply. However, this hypothesis
has been disputed on the basis that the occurrence of Ehux in the NE
Atlantic does not correspond with places and times of low CO2 (Tyrrell &
Low silicate has been advanced as a possible explanation of the
biogeographical distribution of Ehux (Brown & Yoder, 1994), and
mesocosm studies in the Norwegian fjords have shown that diatoms always
bloom (usually to the exclusion of everything else) when silicate is present
in concentrations of >= 2 uM kg-1, but diatoms are less likely to bloom when
silicate concentrations are lower than that (Egge & Aksnes, 1992).
As a top-down rather than a bottom-up forcing, the respective numbers of
different species of grazers at a location will influence the viability of
that location for different species of phytoplankton. The major grazers of
Ehux in the northeast Atlantic, as measured in 1991, were
microzooplankton (P.H. Burkhill, pers. commun., 1995; Holligan et al,
1993a) (even though larger zooplankton may be more influential in
determining carbon fluxes associated with Ehux (Harris, 1994)) and
therefore variations in the density of microzooplankton could be important
in determining where blooms of Ehux can form.
Some other hypotheses that have been put forward in the past are an
influence due to `seeding effects' (the advection of cells from elsewhere to
provide the starting population that is required before a bloom can take
place) (e.g. Birkenes & Braarud, 1952), an advantage at low irradiance and
low nitrate/ammonium (Eppley et al, 1969), and a high tolerance for
low iron concentrations (Brand, 1991). Ehux is also known to have a
requirement for thiamine (vitamin B1), which is not present in the water in
the absence of biological activity. Blooms of Ehux may therefore
be impossible without preconditioning of the water by other blooms, for
instance diatoms blooms (Rothschild & Squire, 1996).
In a review of the `physiological ecology of marine coccolithophores',
(Brand, 1994), it was noted that Ehux is likely to be
r-selected rather than K-selected, since it has a high maximum
growth rate (up to 2.8 doublings per day (Brand & Guillard, 1981)). It was
also suggested that Ehux is usually found in cold waters, and in
waters with low nutrient concentrations. Another review paper (Young,
1994a) suggested that there are three common features to the occurrence of
placolith-bearing coccolithophores such as Ehux: firstly, they
predominate in areas of upwelling; secondly, they are usually bloom-forming;
and thirdly, they are normally dominant in coastal and shallow-sea
assemblages. Hurlburt (1990) classifies Ehux, together with diatoms
and Gephyrocapsa oceanica, as "unevenly occurring, narrow-niched
species", and additionally as fast-growing, r-selected species.
In a review of the influence of environmental factors underlying blooms of
Ehux in the North Sea (Holligan et al, 1993b) it is suggested
that "In general, blooms of Ehux follow those of diatoms in waters
that have been recently depleted in inorganic nutrients and are becoming
more stable in terms of vertical mixing (e.g. following the relaxation of
upwelling, or establishment of the seasonal thermocline)." It has also been
noted that "Along the south-western Norwegian coast E. huxleyi blooms
which colour the sea milky-green, annually follow the spring diatom bloom, a
situation which is well known and familiar to the people living in the
coastal area." (Heimdal et al, 1994).
A major aim of future ecological studies on Ehux will have to be to
explain it's peculiar biogeography: why are the blooms clustered around the
northern Atlantic area? Two possible explanations involve bottom-up
forcings, with the N Atlantic experiencing unusually high N:P and N:Si
ratios. These hypotheses also need to tested in other areas such as the
large blooms on the Patagonian Shelf.
While carrying out these studies, we should appreciate how lucky we are with
Ehux. We cannot even ask these sorts of questions about other
species. Other species do not shed coccoliths to advertise their presence
to satellites, and so their global distributions are not accurately known.
Links to other work on the ecology of Emiliania huxleyi:
Satellite-derived global distribution of Ehux.
Modelling study of
ecology of Ehux in the northeast Atlantic.
studies on the ecology of Ehux.
ecological ramifications of an Ehux bloom in the eastern Bering
Toby Tyrrell : T.Tyrrell@noc.soton.ac.uk