Journal of' Gerrral Microbiology (1 985), 131, 2643-2652.
Printed in Great Britairl
2643
Chemotaxis of a Cyanobacterium on Concentration Gradients of Carbon
Dioxide, Bicarbonate and Oxygen
By G . M A L I N A N D A . E . WALSBY*
Department of Botany, University of Bristol, Woodland Road, Bristol BS8 I UG, UK
(Received 29 April 1985)
~~
Trichomes of an Oscillatoria sp. suspended in soft agar in Petri dishes showed active movement
away from areas where gaseous exchange had been prevented by placing glass coverslips on the
surface. This clearance response occurred only in the light and it was related to gradients of
inorganic carbon that formed in the agar layer. In plates incubated in the dark trichomes
accumulated in the central areas below glass coverslips but the substance eliciting this response
could not be identified. Gradients of diffusible substances were established within lawns of
cyanobacteria suspended in soft agar and it was demonstrated that trichomes of Oscillatoria sp.
moved towards CO,, HCO: and 0, in light-dependent chemotactic reactions. No chemotaxis
occurred in response to these substances in the dark. The chemotactic responses were detected
after 2 h but became increasingly distinct up to 8 h. The chemotactically active concentration of
CO, was greater than the atmospheric concentration since trichomes moved from air towards a
source of C 0 2 .Using diffusivity coefficients it was calculated that trichomes of Oscillatoria sp.
accumulated at a CO, partial pressure of 0.02bar (equivalent to 0 . 8 3 m ~ ) .With O2 as the
chemoattractant the value was 0.35 bar (14-56mM). These results are discussed with reference to
the roles of inorganic carbon and O2 in cyanobacterial metabolism and it is concluded that
chemotactic behaviour may be important in movements within the photic zone of sediment
environments.
INTRODUCTION
Filamentous cyanobacteria that move by gliding are often common in intertidal muds. In
these environments a complex system of gradients occurs within the mud profile with factors
such as light intensity and quality, oxygen concentration, sulphide concentration, pH and water
potential varying with depth and time. In these conditions tactic responses might allow a
cyanobacterium to select favourable physico-chemical conditions and avoid conditions adverse
to growth or survival. Phototaxis has been well explored in the cyanobacteria (Hader, 1979a, b ;
Nultsch & HBder, 1979) but very little is known about chemotaxis in this group.
Fechner (1915) reported negative chemotaxis of an Oscillatoria sp. in response to organic
acids. Since that time a few incidental observations have been made that suggest chemotaxis
does occur in some cyanobacteria. Castenholz (1982) reported that a chemophobic response to
sulphide occurs in the cyanobacterium Oscillatoria terebrijormis in hot springs though no
experimental details were given. Murvanidze et al. (1982) described the negatively chemotactic
behaviour of Phormidium uncinatum in response to a spatial gradient of the uncoupler TTFB
(trifluorotetrachlorobenzimidazole),which destroys proton-motive force by inducing nonphysiological H+ leaks. Whale & Walsby (1984) noted that Microcoleus chthonoplastes avoided
anaerobic conditions and this was interpreted as either a positive response to aerobic conditions
or a negative response to sulphide. None of these responses have been studied in any detail.
In this paper we present the first evidence of chemotaxis in response to COz, HCO: and O2in
a gliding cyanobacterium isolated from a salt marsh. We also discuss the possible environmental
role of this response. A preliminary report of the work was given by Malin & Walsby (1985).
0001-2655
0 1985 SGM
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2644
G . MALIN A N D A. E. WALSBY
METHODS
Organism and maintenance. The Oscillatoria sp. used in this study was isolated from the upper area of the salt
marsh near Berrow Church, Somerset, England (grid reference ST 289 524). The trichomes of this strain were
5-6 pm in diameter with a curved tip. and they moved by gliding with a velocity of 2.4 f 0.4 pm s-', as measured
by microscopic observation. The cyanobacterium was maintained in a dilute artificial seawater medium of the
following composition (g ] - I ) : NaCl (9.33), KCl (0.25), MgSO,. 7 H 2 0(2.56), MgCl, .6H20 (1*84),CaC1,. 2H20
(0.51), NaHCO, (0.007), KIHPO, (0.02), NaNO, (1.5), Na,EDTA (0.006), NaFeEDTA (0.001). The trace
element solution described by Booker 8t Walsby (1981) was also added. The pH of the final medium was controlled
by adding IOmM-HEPES and adjusting the pH to 8.0 by addition of NaOH before autoclaving at 121 "C for
20 min .
E.uperimentalmateriu1. The cyanobacteria were grown in batch culture on an orbital shaker at 120 r.p.m., 27 "C
and a photon flux density of 40pmol m-? s-I. Shaking was necessary to maintain growth in homogeneous
suspension. Concentrated suspensions of the cyanobacterium were obtained by allowing 7-d-old cultures to settle
under gravity overnight or by centrifuging at 2500 m s-? (255g) for 10 min. Osciffatoria'lawns' were prepared by
mixing measured volumes of cyanobacterial suspension with equal volumes of molten 1.2% (w/v) artificial
seawater-agar cooled to a temperature of 40 "C and pouring rapidly into Petri dishes. When required, NaHCO,
was added to the cyanobacterial suspension at the same time as the molten agar; control plates were prepared
using distilled water in place of the NaHCO, stock solution.
Chemotaxis apparatus. The lid of a Pyrex Petri dish was modified by the addition of a pair of gassing inlet/outlets
and a central glass tube I cm in diameter with a side arm (Fig. 1). Sufficient cyanobacteria-agar mixture was
poured into the Petri dish base so that when the agar gelled and the lid was fitted the base of the central tube just
penetrated the agar layer. The area below the central tube could then be subjected to continuous flow of a gas
entering via a wide-bore needle inserted through a Suba-seal stopper. The outer region of the plate was gassed with
the same or a different gas. The gas was moistened by bubbling through water and its flow was regulated with a
needle valve at 12 ml min-': The edge of the modified Petri dish was sealed with four layers of Parafilm.
E.xperimentaf conditions. Experiments in the light were done at an incident photon flux density of
36 pmol m-? s-I giving a density at the back face of 16pmol m-?s-I and a mean intensity within the
cyanobacterial layer of 24.6pmol m-: s-l, calculated by the formula of Van Liere & Walsby (1982). Dark
experiments were incubated in tins which were shown to be light-tight in tests with photographic film.
Growth estimation. Agar cores containing cyanobacteria were macerated and extracted in hot methanol. After
centrifuging at 4000 m s-? (408g) for 15 min the absorbance of the supernatant was measured at 663 and 750 nm
both before and after acidification. The chlorophyll content of each sample was then calculated using a specific
absorption coefficient for chlorophyll in methanol of 78 cm-' g-l 1 and the equation described by Golterman et al.
(1978). When necessary, samples were stored in the frozen untreated state for up to 48 h before analysis (see
Marker et al., 1980).
Penelration of light through sediment. Glass microscope slides were painted black except for a central area 1 cm in
diameter. Pairs of glass coverslips were then glued into position, one on top of the other, at each end of the glass
slide. A small amount of fine surface sediment was then placed on the slide and covered with a 22 x 50mm
coverslip which was supported on the coverslip spacers. The thickness of the mud layer, equivalent to that of two
coverslips (Fig. 2), was measured with a micrometer gauge. Mud layers prepared in this way were then placed
between a light source and a quantum sensor, on a black perspex slide holder (Fig. 2). Care was taken to prevent
extraneous light reaching the sensor by masking with black tape. Replicate readings were taken of slides before
and after making mud layers.
Dark motility. Dense suspensions of Oscillatoria sp. were allowed to settle under gravity until a large coherent
clump of trichomes formed. Excess medium was removed with a Pasteur pipette and evenly sized clumps
approximately 5 mm in diameter were divided off the large mass. These inocula were placed centrally on Petri
dishes of 0.6% (wfv) dilute artificial seawater-agar and then incubated under light or dark conditions.
RESULTS
The clearance response
After a small section of cyanobacterial lawn in solidified agar had been inspected for gliding
motility on a slide under the microscope, it was noticed that several hours later trichomes had
congregated at the perimeter of the agar under the coverslip (Fig. 3). This response occurred
under uniform lighting and did not therefore appear to result from unidirectional phototaxis.
To investigate this response in more detail glass coverslips were placed on lawns of
Oscillatoria sp. in Petri dishes. Again a majority of the cyanobacteria beneath the coverslips
moved from the central region to the periphery of the glass leaving a clearer area. This response
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2645
Chemotaxis o j cyanobacteria
(a)
E
111
*\
Fig. 1
C
/ c,s
Fig. 2
Fig. 1. Diagrammatic cross-section of a 9 cm Petri dish modified for chemotaxis experiments: A,
cyanobacteria-agar layer; T, central glass tube I cm in diameter; R,Suba-seal stopper; S, Parafilm seal;
I , and I?, inlets for gases I and 2; 0, and 01,outlets for gases 1 and 2. Arrows indicate gas flow
direct ion.
Fig. 2. Apparatus used to measured the penetration of light through mud layers of known thickness. (a)
Top view of slide holder; (h) side view with mud layer and quantum sensor in position (not drawn to
scale). E, raised edge of black perspex slide holder; H, central hole; S, screw which holds quantum
sensor in place; L, light source; M,mud layer of known thickness; C, glass coverslip; CS, coverslip
spacers glued into place; G, glass slide painted black except for 1 cm diameter central area above
quantum sensor; LS, quantum sensor taped in position to prevent entry of extraneous light; SH, black
perspex slide holder.
Fig. 3. Trichomes of Oscilluti~riusp. which had been suspended in agar and placed beneath a coverslip.
After 4 h filaments had migrated to the edge of the preparation where they formed a dense
accumulation. Bar. 300 pm.
is subsequently referred to as a ‘clearance response’. It could be detected after 2 h but was
particularly distinct after 6-8 h (Fig. 4). When a square of dialysis membrane instead of a
coverslip was placed on the agar lawn no clearance occurred (Fig. 4). However, clearance did
occur when a coverslip was placed over the square of dialysis membrane and also when the
membrane was placed over a coverslip on the lawn (Fig. 5). It was therefore concluded that the
dialysis membrane did not prevent clearance by preventing gliding motility per se.
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2646
G . MALIN A N D A . E . WALSBY
Fig. 4
Fig. 5
Fig. 4.Clearance of Oscilluroriu sp. from areas of a lawn where a glass coverslip was placed on the
surface (G) compared to the lack of clearance with dialysis membrane (D) after a 12 h incubation. The
dashed lines on this and subsequent figures show the positions of coverslips and dialysis membrane
squares.
Fig. 5 . Migration of trichomes of Oscillutoriu sp. away from areas covered with a barrier to gaseous
diffusion. Clearance occurred beneath glass (G) alone but not beneath dialysis membrane alone (D).
Clearance was also seen when dialysis membrane was placed beneath a glass coverslip (G/D) or above it
(D/G). The photograph was taken after 12 h.
Fig. 6. Effect of NaHCO, concentration on the coverslip clearance reaction after 18 h. (a) When no
NaHCO, was added clearance occurred; (b) 10 ~ M - N ~ H C Ono
, , clearance; (c) 20 mwNaHCO,, no
clearance.
These observations suggested that the coverslip, but not the membrane, prevented the
diffusion of a chemotactically active substance between the agar and the overlying atmosphere;
this indicated that the sybstance was gaseous or volatile. The movement might have been a
negative response to a gaseous product (such as photosynthetic 0 2 that
) accumulated under the
coverslip or a positive response to a gaseous substrate (such as C 0 2 )that was depleted under the
coverslip.
To test whether the observed results were caused by depletion of COz or HCO,, we looked at
the effect of adding NaHCO, to cyanobacterial lawns before placing coverslips on the surface.
Typical results are shown in Fig. 6 ; when no NaHC03 was added the trichomes vacated the
central area beneath the coverslips but with increasing NaHC03 concentration the level of
clearance was markedly reduced. Furthermore, it was unlikely that the clearance response had
been due to a build-up of gaseous O2 because with increasing NaHCO, concentration more
bubbles, presumably of O,, collected below the coverslips but these areas were not avoided by
the cyanobacteri a.
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2647
Fig. 7 . Accumulation effect observed when 0.scilluroriu sp. lawns were incubated for 12 h in the dark
with glass coverslips on the surface. Trichomes appeared to migrate from the edges to the more central
areas of' the coverslips.
Fig. 8. 0.scilluroriu sp. lawns incubated in the modified Petri dish apparatus with C 0 2 passing over the
central tube areas I cm in diameter and N , circulating over the outer areas. (u) View of whole plate. (b)
Close-up of a zone on another plate where chemotaxis occurred; bar, 1 cm. The clearance areas are
arrowed. The photographs were taken after 5 h.
Fig. 9. An 0.sc'illutoriu sp. lawn incubated in the modified Petri dish for 5 h with agar containing
125 mM-NaHC'O, in the central tube and N, circulating over the outer area. The clearance area is
arrowed.
A durk reuczim. When Oscilfutoriu lawns were incubated in the dark with glass coverslips on
the surface, trichomes within the agar-cyanobacteria layer appeared to move from a 4-5 mm
wide area at the periphery of the coverslip towards the more central region (Fig. 7). The
accumulation pattern often disappeared rapidly in the light but addition of NaHCO, to the
cyanobacterial lawns before dark incubation did not prevent its formation.
Artificial gradients
In the foregoing experiments the cyanobacteria responded to gradients produced by their own
metabolic activity. In the following experiments gradients were produced in the modified Petri
dish apparatus (Fig. 2).
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2648
G . M A L I N A N D A . E. W A L S B Y
Fig. 10. O.sci//utorirr sp. lawns which had been incubated for 24 h with agar containing ( u ) 125 mMNaHCOI or (hl 250m~-NitHC'O, in the central tube and N, circulating around the Petri dish.
Difierences can be seen in the si7es of the m i e s influenced by the availability of inorganic carbon.
Grtrtiic~nrsof C'O?. A definite response to C02/HC'O; was demonstrated in gradients created
by gassing the central area of the modified Petri dish with pure CO, at atmospheric pressure and
the outer area with pure N,. About 1-5-24) h after setting up a cyanobacterial lawn in this
apparatus a band of clearance approximately 1 mm wide was detected in the Oscillutoriu lawn
10 12 mm from the outer edge of the central tube. After 4-6 h the clearance was very distinct.
(Fig. 8). The results suggested that the trichomes might have moved from an area of insufficient
C0 ,iHC Oj to a point on the gradient where the concentration was suitable or even optimal for
growth. There also seemed to be some movement away from the central area, which had been
sparged with pure C 0 2 , and an area 2-3 mm wide beyond it. A corresponding accumulation
could be seen in a ring 4 6 mm from this central area (Fig. 8). Although only a proportion of the
cyanobacteria moved away from the central areas, microscopic examination of trichomes
remaining in this region did not reveal extensive damage or lysis. When a lower concentration of
CO: was used to set up the gradient the ring of clearing appeared in a position closer to the
central gassing tube. When the central area was sparged with CO, and air was passed over the
developed, although the
outer area a clearance response similar to that seen with C 0 2 and N-,
boundary of the clearance area was not as sharp. This suggested that the optimum chemotactic
concentration of CO, for Osci/Irrtoricr sp. was higher than the concentration in the air.
Grudients of Ntr HC'O,. Similar chemotrictic responses occurred when agar containing
NaHCO, was placed in the central well of the modified Petri dish apparatus and pure N z at
atmospheric pressure was circulated around the outer area. An example of this response can be
seen in Fig. 9 where 125 rnM-NallCO, was used during a 5 h incubation period. The position at
which the bands of accumulation or clearance occurred was time and concentration dependent.
Results from a 24 h incubation with 125 mM- or 250 mM-NaHCO, are shown in Fig. 10. With the
lower concentration the accumu1:ition zone occurred 12 mm away from the central zone whereas
with an initial NnHCO, concentration of250 mM it occurred 16 mm away from the central tube.
Additionally, a striking difference could be seen, at both NaHCO, concentrations, between the
inner area where inorganic cnrbon was available and the outer carbon-deficient area (Fig. lo).
Grcrtiicwt.\ of 0 2 Gradients
.
of 0: were also set up using the modified Petri dish apparatus. A
similar response pattern was seen but the clearance area appeared much closer to the central
tube through which pure 0, at atmospheric pressure was flowing (Fig. I I). This appeared to
reflect the higher optimum concentration for the chemotactic reaction with O-, as compared to
CO?.There also seemed to be some movement away from the central area of the plate which had
been subjected to pure 0, (Fig. 1 1 ) . A fine band of accumulation 5 mm away from the central
tube was also seen on plates incubated in the dark. It was difficult to ascertain whether
trichomes in this area had moved towards or away from the source of 0, because a distinct ring
of clearance was not seen.
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Chetnotasis o j cyanobactcriu
2649
Fig. 1 I . An Oscillntoriu sp. lawn incubated for 5 h with 0, passing over the central tube area and N 2
circulating over the outer area. The clearance area is arrowed.
Coticvntra tion thwshokh .fbr cherpiotu.i is
The probable concentration of CO, at the position o f t h e ring of clearance was calculated by
the equations of Carslaw & Jaeger (1959) for diffusion from a cylinder. From the diffusivity of
C 0 2 in water, K = 1.8 x
mm2 s-' (Princen & Mason, 1965), the time at which the ring was
first detected, t = 7200 s, and the radius of the central glass cylinder, u = 5 nim, the ratio K t / d
was calculated to be 0.52. The distance of the clearance ring from the cylinder centre,
a 1 1 mm, gives r = 16 mm, and loglo(r/u) = 0.5 1. From these two ratios, Ktja' and log,o(r/a),
can be determined the value of c/V z 0 4 2 by measurement from the graph (Fig. 41) of Carslaw
& Jaeger (1959), where I/ is the partial pressure of gas in the centre cylinder (1 bar) and 1- is the
partial pressure at distance r a n d time t , i.e. the partial pressure of C 0 2 at the clearance zone was
0.02 bar (equivalent to a concentration of 0-9 mM).
By a similar procedure it was calculated that the concentration of O2
( K = 2.1 x lOV3 mml s-* ) at the clearance ring 10 mm from the cylinder centre would have been
approximately 0.3- 0.35 bar (1 3- 16 mM).
In both cases these solutions are only approximate because ( ( I ) the diffusion coefficient of the
gases in 0.60/,, agar gel may be slightly less than in still water (see Cooper, 1963; Crowle, 1973)
and (h) the gas from the central cylinder must first diffuse down through the agar below it before
dispersing laterally.
+
Groir)thuiid p H
The maximum growth rate on agar plates, measured over a 48 h period under the light
intensity used was 0.0156 h-' (log, units). The increase in filament density by growth over 2 h
would not exceed 3q, and therefore it seemed unlikely that the apparent chemotactic reactions
observed could have been due to growth.
A check was also made to see if a pH gradient was also being produced that could be primarily
responsible for the clearance and accumulation patterns seen in gradients of CO,, NaHCO, and
02.This was done by incorporating a universal pH indicator into 0.60/, (wiv) dilute artificial
seawater-agar which coiitained 1 0 mM-HEPES buffer. In CO, gradients the pH of the central
tube and an area 2-3 mm wide beyond i t decreased from 8.0 to 6.0 as the gradient established
over a 3 h period, in the area corresponding to that where the outer chemotactic reaction
occurred the pH of the agar was u n a l t e i d . When 125 mM-NaHCOi was used the pH of the
central area increased from 8.0 to 9.0, but again was unchanged at the peripheral area of the
chemotactic accumulation. O2 did not cause any change in p H .
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2650
G . MALIN AND A . E. WALSBY
Fig. 12. Movement of Oscilltiforirr sp. away from a central inoculum on agar medium after 24 h in
light or ( h ) dark conditions. Bars, 0.5 cm.
(N)
Penetrcrtion of light through mud
Penetration of light through sectioned samples of natural sediments has been measured
previously (Doemel & Brock, 1977; Krumbein & Potts, 1978; Jrargensen et al., 1979) but depth
values for light penetration were reported to vary widely between 1 and 40 mm. The consistency
of the mud at the site where Oscillatoriu sp. was found was too fluid to allow sectioning. We
therefore measured the light penetration through layers of mud sedimented from suspension.
Results demonstrated that an initial photon flux density of 5560pmol m - ? s s l was reduced
to 0.0067 pmol m-, s-' by a mud layer only 0.34 mm deep. It proved impossible to take
measurements over a wide range of mud depths because incremental layers greater than
0.34mm thick completely blocked out the light and with thinner layers problems were
encountered with larger sediment particles and air bubbles. The maximum incident photon flux
density measured in the field was 900pmol m-2 s as compared to 5560pmol m s - ' used in
the laboratory experiments. Therefore it seems likely that a population of cyanobacteria in the
habitat where O.rcil/utoricr sp. is found could be deprived of light by a very fine layer of silt.
Dtirk nwtilit!'
After 24 h incubation there was considerable spreading of trichomes from a central inoculum
.
similar plates were incubated in the
on an agar plate incubated in the light (Fig. 1 2 ~ ) When
dark the trichomes did not spread so far, and no further movement was seen after 24 h (Fig.
12h).
DISCUSSION
We have demonstrated that an Oscillatoritr sp. moves towards C 0 2 , HCO, and 0, in lightdependent chemotactic reactions. The concentrations of C 0 2 and 0, eliciting the responses
were higher than the atmospheric concentrations of these gases.
Since cyanobacteria are photoautotrophs, movement towards suitable arid especially optimal
C 0 2 concentrations would be advantageous. Movement towards areas lacking CO, would not
be. Long-term incubation without CO, in the light may cause the breakdown of photosynthetic
pigments and thylakoid membranes (Miller & Holt, 1977) and can lead to excessive production
of superoxide ions. Osc'illntoriu sp. was also attracted to 02.I t is known that cyanobacteria
require O2 for aerobic respiration and the synthesis of certain unsaturated Fatty acids (Kenyon c t
ul., 1972; Oren c ~ fill., 1985), but it is nevertheless surprising that these organisms, which have
been shown to thrive in microaerobic conditions (Stewart & Pearson, 1970) and which produce
O2 in photosynthesis, should move to such high concentrations of this gas.
When 100% C 0 2 or O2 was used to create a gradient over a cyanobacterial lawn the patterns
of accumulation and clearance that were recorded suggested that trichomes might be moving
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C'hc~motnxisqf cyarzobacteria
265 1
away from high concentrations of these gases. High O2 levels are known to inhibit
photosynthesis due to competition between O7 and COz for ribulose bisphosphate
carboxylase/oxygenase (Lorimer, 198 I ) , and they also lead to enhanced 0 2 production. The
negative reaction to high CO, concentration could have been a p H effect since the p H in the
central area of a buffered plain agar plate decreased from 8.0 to 6.0 when C 0 7 was passed
through the central tube. The positive chemotaxis to C 0 7 was not due to pH since there was no
pH change in the area where this reaction occurred.
Dark gliding motility could be advantageous to cyanobacteria in a habitat subject to silting
because when covered with a layer of fine sediment the filaments would be deprived of the light
gradient required for a positive phototactic reaction to occur (Pentecost, 1984; Whale &
Walsby, 1984). Our measurements of light penetration through mud demonstrated that only a
minute percentage of the incident light was able to pass through a layer of sediment 340pm
deep. There are reports of cyanobacteria re-emerging at the sediment surface in the dark
(Pentecost, 1984; Whale & Walsby, 1984) and since cyanobacteria show no geotactic or
magnetotactic responses (Whale & Walsby, 1984) chemical gradients might provide the
directional information. ('hemotaxis in response to O,, which forms steep gradients in
sediments (JerIgensen, 1983; Revsbech & Ward, 1984; Jarrgensen & Revsbech, 1985), could
account for these migrations to the surface in the absence of light.
Although O.wi//[rtoriusp. can glide in the dark it does not continue to do so for very long, and
we were unable convincingly to demonstrate chemotaxis by trichomes of this species to O1 in the
dark. Halfen & Castenholz (1971) demonstrated that Oscil/utoriu princeps continues to glide in
the dark for several days by respiring a large reservoir of stored carbohydrate. 0 .princeps may
therefore be a more suitable cyanobacterium to investigate chemotaxis in the dark.
The relevance of C0,-chemotaxis in the natural environment has not been directly
investigated. Revsbech ct ul. (198 1 ) reported that the total dissolved HCO: in the upper 2.5 mm
o f a sediment in Denmark was 2-5 mM. Dense populations of phytoplankton can deplete the COz
level in eutrophic lakewater (Tailing, 1976) and fishponds (Abelovich & Shilo, 1972). Intense
microbial activity occurs at the surface of microbial mats and although the photic zone may be
only 1 mm deep the level of photosynthetic activity can be equivalent to that measured in
eutrophic lakes (see Revsbech & Ward, 1984). In conditions of localized COz depletion in
sediments, phototactic reactions would prevent movement towards the higher HCO; levels
deeper in the sediment (Jerrgensen, 1980) but chemotaxis to suitable or optimal C 0 7
concentrations may be important in lateral movement within the photic zone. Cyanobacteria
that demonstrate chemotactic behaviour might therefore be at a competitive advantage in the
n a t u ra 1 en v 1 ron m en t .
There is I need for critical investigations of the tactic responses of gliding cyanobacteria in the
field but the complexity of natural ecosystems has hindered research in this area. Simplified
laboratory models like the gradients used in this investigation may be important in providing
fund ii me n t ii 1 i 11 form at i on a bout cyan o bac t e r i a1 c he mot ax i s .
We th;ink Mr K . (icorgc. tor niaking the modified Petri dish apparatus, Mr W . Petrie for making the perspex
slide holder iind M r 1'.C'olborn f o r photography. Dr D. S. Riley kindly provided advice on diffusion equations.
This \ c o r k \c';is done u hile (;. M . \+/:is i n receipt o f ;i N E R C ' Postdoctoral Fellowship.
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