198
STORAGE OF OXYGEN IN THE BLADDERS OF THE
SEAWEED ASCOPHYLLUM NODOSUM AND THEIR
ADAPTATION TO HYDROSTATIC PRESSURE
BY G. C. C. DAMANT
(Received 6 October 1936)
(With One Plate and Five Text-figures)
THE writer's interest in the facts described below arose out of an experience in
salvage work when a sunken submarine had to be marked by buoys moored so as
to float on the surface at low water only and to become submerged as the tide rose.
When submerged they were naturally exposed to an external pressure corresponding
to the hydrostatic head of water above them, and this was often too great for the
strength of lightly constructed steel buoys which, in consequence, crumpled up,
sank to the bottom and were lost. The difficulty was overcome by charging the buoys
with air under a pressure of 10 lb. per sq. in. which, while insufficient to burst them
when at the surface, afforded adequate counter-pressure to support their shell
plating when submerged. As the floats or bladders of many seaweeds are subjected
to similar alternations of hydrostatic pressure as the tide rises and falls it seemed
interesting to enquire whether they have evolved any special defence and it was
soon noticed that the bladders of AscophyUum nodosum usually contained gas at a
pressure of some 2-4 lb. per sq. in. above atmospheric pressure. This seaweed
consists of narrow branching fronds which, at intervals of a few inches, are inflated
into bladders which are, roughly speaking, of the same shape and size as acorns
though often much larger. It grows so densely on rocks that neighbouring individuals must be in competition for sunlight, and an obvious function of the bladders
is to float up the plants so as to obtain as much of it as possible. The habitat is
between tide-marks, so that for one part of the day the plants are submerged, for
another floating, and for the third and generally longest portion stranded high and
dry. At high tide (which we will suppose to occur a.m.) the fronds stick up like
standing corn (Text-fig. 1 A) and what sunlight reaches them through the water,
often very muddy, is mainly captured by the outer ranks. At half-tide, when direct
sunlight can reach them, most of the fronds are arranged in a thick horizontal layer
(Text-fig. 1B) with the bladders of the uppermost tier half out of water and basking
in the sun while those of the underlying fronds are heavily shaded. At this phase the
efficiency of the bladders becomes of prime importance for, under the sifting action
of the wavelets, those with the highest buoyancy shoulder themselves to the top of
the floating raft of vegetation and, unless the sea is very rough, will remain there
until the whole mass is stranded by the falling tide (Text-fig. 1C), when the upper
Oxygen in the Bladders of the Seaweed
199
layer can look forward to some 6 hours of daylight while the undermost, buried
under a mat of seaweed some inches thick, is in darkness. This jostling interlude is
the critical period of the plant's tidal day.
It will be shown later that hydrostatic pressure may cause such loss of gas from
some bladders as to render them flabby and less eflicient as floats than their better
adapted neighbours which, in consequence, will override them at every tide and
allow them small chance of recovery. It will be further shown that the plants
protect themselves from this misfortune, physiologically, by storing a reserve of gas
under pressure and, anatomically, by a specific thickening of the bladder walls.
Al high tide,
submerged for
\ hours
At balf-lide, partly
(lolling at the surface
for 2 hours
A
B
i
C
Text-fig. 1. Postures of a clump of Atcophyllum nodotum at different states of tide. The times given
are those applicable to seaweed growing at half-tide level at Cowes.
GAS PRESSURE WITHIN THE BLADDERS
On palpating the bladders of a healthy plant they feel tense as though the contained gas were under pressure, and this impression is confirmed if one is punctured
under water when gas bubbles rush out in a characteristic way. The pressure can
be exactly measured by the device illustrated in Text-fig. 2.
A hypodermic needle (A) is fixed with sealing wax into the end of a narrow glass tube
(C) connected through rubber tubing (D) with a levelling reservoir (E) and the whole
system filled with mercury up to the base of the needle. A glass cup (B) is fitted to the top
of (C) and is filled with water till the top of the hypodermic needle (which projects into it)
is under water. A bladder (A) is cut from the seaweed so that a few millimetres of frond
are left projecting from each end, if cut off too closely the bladder will leak. The bladder is
then pushed down on to the point of the needle and impaled in such a way that the needle
pierces along the axis of the frond till it enters the cavity of the bladder when, if the contained gas is under pressure, some of it will pass through the hollow needle and force down
the mercury in the glass tube. The reservoir is now raised till the mercury returns to its
former level at the base of the needle, when the height of the top of the mercury in the
reservoir above this point will give the original gas pressure inside the bladder in milli1-2
200
G. C. C. DAMANT
metres of mercury. The object of the water in the glass cup is to demonstrate any leakage
of gas along the needle track, and if bubbles are seen to escape at the moment of impalement the operation has failed and another attempt must be made with a fresh bladder. To
prevent the hypodermic needle cutting out a plug of tissue in the manner of a cork borer
and so blocking itself I find it desirable to plug the piercing tip with hard wax or lead and
to file a hole for admission of gas in the side of the needle just behind the plug.
Text-fig. 2. Arrangement for measuring the gaa pressure inside seaweed bladders.
It is convenient to express the gas pressure inside the bladder in terms of water
head so that it may readily be compared with the external hydrostatic pressure
which it helps the bladder to resist, and this has been done on the basis that I ft.
of salt water head is equivalent to 23 mm. of mercury.
The pressure found in A. nodosum bladders varies seasonally and from day to
day, as it depends on the amount of sunlight caught by the plant under observation;
Table I is a synopsis of the mean pressures found in batches of ten bladders- collected at random from the sea wall at East Cowes at different times of year.
From these and other observations it appears that for about 9 months of the year
the bladders contain gas under appreciable pressure, the maximum being reached
in April when the plant's reproductive activity is at its height. In the dark months
of October, November and December the pressure is zero or slightly negative, a
number of the bladders become somewhat flattened, and the whole plant appears
to be in a dormant condition. Now the tidal range at Cowes and the distribution in
depth of A. nodosum growing there are such that the average hydrostatic pressure
on the bladders at high tide is about 4 ft. of water head and the maximum 8 ft., so
Oxygen in the Bladders of the Seaweed
201
that for most of the year the gas pressure within the bladders would afford fair
counter-pressure and support. It will be shown later that the bladder gas consists,
roughly speaking, of air enriched by additional oxygen secreted by the plant, and
Table I. Gas pressures found in the bladders of Ascophyllum nodosum at different
times of year. Pressures are expressed in feet head of salt water and all are
positive except those prefixed by a minus sign.
Month
Jan.
Feb.
April
May
July
Sep.
Nov.
Mean
pressure
ft.
Highest
pressure
observed
ft.
Lowest
pressure
observed
ft.
2-O
3"8
6-6
i-o
3-1
6-i
Vs
S'2
3'2
i-o
— O-2
IO-Q
S-8
S'2
3-0
i'3
3'9
i'3
i-o
-o-8
that its pressure is due to the latter component which constitutes a reserve of gas
which can be drawn on for metabolic purposes and also serves to maintain buoyancy
at a maximum, in spite of the occasional losses of bladder gas which occur from
causes set out in a subsequent section.
THE COMPOSITION OF BLADDER GAS
Zeller & Neikerk (1915) have summarized the literature on the gaseous contents
of seaweed bladders; Willie (1889) found that various species contained from 20 to
37 per cent of oxygen, but maintained that COa was always completely absent.
Lucas, working on Australian seaweeds, also failed to detect COa and concluded
that the high percentage of oxygen was the result of the oxygen dissolved in the sea
"osmosing" into the bladders; Zeller & Neikerk (1915), however, found COa in
Nereocystis from Puget Sound in quantities ranging from 2-5 per cent by night to
0-29 per cent by day, and concluded that the pneumatocyst is "not only afloatbut
a reservoir in the gas exchange of the metabolic process". Langdon (1916), also
working on Nereocystis, disagreed with some of Zeller & Neikerk's findings and
made the surprising discovery that the floats of this weed, having a capacity running
up to 4 litres, contain carbon monoxide in sufficient concentration to kill a canary
in 15 sec. and a guinea-pig in 10 min. In twelve analyses he found CO ranging from
1-i to 5 per cent, oxygen from 16 to 23 per cent and a little COa but, as it appears
that the gas was collected over water, the determination of the last would not be
very accurate.
In the case of A. nodosum when the COa content was needed I have collected the gas
from a number of bladders over mercury and pooled it so as to provide enough gas for
duplicate analyses with the Haldane apparatus, but for analysing the gas in a single bladder
have used the Krogh micro-apparatus which though giving the oxygen within 1 per cent
is not capable of determining the small amount of CO2 present.
202
G. C. C. DAMANT
On a summer day at 9.0 a.m. the pooled contents of a number of bladders gave
oxygen 26-8 per cent and C0 2 0-2 per cent. The tide then rose and covered the
plants. At 4.30 p.m., when they were again accessible, fresh samples were taken
which showed that the oxygen had risen to 29-8 per cent, the C0 2 remaining at
0-2 per cent at 8.30 p.m., when the rising tide was once more about to reach the
weed on which the evening sun had been shining as it hung moist 011 ihe sea wall,
the oxygen had mounted to 31-9 per cent with C0 2 0-2 per cent. During the night
the oxygen fell so that bladders gathered at 6.30 next morning contained oxygen
27-2 per cent and C0 2 0-25 per cent. From these and similar observations it can be
Text-fig. 3. Percentage of oxygen in successive bladders of Ajcophyllum nodotum. A is an undisturbed plant. B another plant which has been cut from the rock and re-attached upside down. In
each case the uppermost bladders contain most oxygen though in A they are the youngest and in B
the oldest on the plant.
said that the percentage of oxygen rises during sunlight and falls during darkness,
and that secretion of oxygen can take place both when the plant is submerged at
high tide and when it is stranded at low tide. In view of Langdon's discovery a
careful search was made for carbon monoxide, but no trace of this or any other
combustible gas could be detected and the residual gas is taken to be nitrogen. In
October, November and December when, as already stated, the bladders maintain
no internal gas pressure they contain what is^ practically air, the oxygen ranging
from 19 to 22 per cent. If artificially cut off from all daylight the plants use up all
the oxygen in their bladders and the C0 2 content may rise to 5 per cent, but in
Nature I have rarely found the oxygen below 19 per cent, and the highest figure
found has been 37 per cent, which is fairly common. The oxygen content of
Oxygen in the Bladders of the Seaweed
203
different bladders on the same plant may differ widely according to variations of
sunlight and shadow, and in long fronds the distal bladders contain more than the
proximal because they are the first to reach daylight as the tide falls and the last
to be submerged as it rises. Also while submerged, being uppermost, they get most
illumination. This is illustrated in Text-fig. 3 A, showing the gradient of oxygen
found in the bladders from top to bottom of a frond about 3 ft. long. To establish
that this gradient depends on the relative positions of the bladders and not to the
distal ones being younger or more efficient than the proximal, a similar frond was
cut off and tied down by the tip to its own stump so that it floated upside down. On
analysis a few days later the original gradient was found to have been reversed, the
older bladders, now uppermost, containing more oxygen than the younger (Text-
CONNEXION BETWEEN OXYGEN PERCENTAGE
AND INTERNAL GAS PRESSURE
It is found that when the percentage of oxygen in the bladders corresponds with
that in ordinary air, so also does the internal gas pressure correspond with that of
the atmosphere, and no excess pressure can be detected by the manometer but that
when bladders contain a higher percentage of oxygen than does air they also have a
correspondingly higher gas pressure. The following examples are taken from a
series in which the total gas pressure and oxygen percentage were measured in
individual bladders freshly uncovered by the tide at different times of year. If
gathered after long exposure to the air or allowed to dry in the laboratory, shrinkage
of the bladder wall may raise the pressure anomalously.
The third column in the table is important as showing that the positive gas pressure is
derived from secreted oxygen; it was calculated as follows. Taking the last example in the
Table II. Relation of total gas pressure to percentage of oxygen
in a series of bladders of Ascophyllum nodosum
Positive gas pressure
taken by manometer Oxygen percentage
and expressed in
by analysis
feet head of
salt water
n
20
22
as
27
30
32
3S
0 0 M M M N 0
00000000000000
o-o
0-9
30
36
S'3
Partial pressure of
nitrogen calculated
from the preceding
columns and expressed as a
percentage of
1 atmosphere
of pressure
table we see that the positive gas pressure was 7-8 ft. (head of salt water). Now 33ft.head
is equivalent to 1 atmosphere of pressure, so the absolute pressure in the bladder was 40-8 ft.
The gas contained 35 per cent of oxygen and (disregarding the trace of C0 2 likely to be
204
G. C. C. DAMANT
present) we may say that there was 65 per cent of nitrogen. The partial pressure exerted
by this nitrogen was therefore 65 per cent of 40-8 ft., which is 26-5 ft., which is 80 per cent
of 33 ft. or 80 per cent of 1 atmosphere of pressure. The other bladders in the table give
similar results showing that, whatever the total pressure within a bladder, the nitrogen
component is only exerting the same partial pressure as the nitrogen in the atmosphere or
that naturally dissolved in the sea water bathing the plant and presumably is a passive
element which has entered the bladder by diffusion.
The bladders do not stretch appreciably so that, once they are full of gas,
secretion of additional gas while raising the internal pressure does not increase their
volume or add to their efficiency as floats; on the other hand, so long as some internal pressure remains, gas may be lost from the bladder without diminishing its
buoyancy. Of course if so much gas is lost as to bring down the internal gas pressure to atmospheric any further loss will diminish the volume of the bladder and
the support it gives to the frond.
LOSS OF GAS FROM THE BLADDERS
During darkness the plant draws on the reserve of oxygen stored in its bladders
and may use up so much as to reduce the internal pressure to zero, but such physiological loss is easily made up during the day by any plant which gets its fair share
of light. A more serious loss of gas may arise if, through an abnormally high tide,
the plant is exposed to a greater hydrostatic pressure than that to which it is
adapted. This can be imitated by weighting a bunch of the weed and lowering it
into deep water for a few hours when, on pulling it to the surface again, the bladders
will be found to have become flattened or deeply dimpled (PI. I, fig. 2) owing to
loss of the gas which has diffused out through their somewhat permeable walls.
When the tide rises over a bladder or, as in this case, it is lowered into the sea, the
increasing hydrostatic pressure is at first countered by the internal gas pressure, but,
with increasing depth, the two opposed pressures become equal and the bladder is
then in equilibrium with no strain on its walls. Beyond this point the hydrostatic
pressure can still increase by several feet without distorting the bladders whose
tough walls and smooth oval shape enable them to resist considerable external
pressure and to shield their contained gas from it. This gas therefore remains at its
original pressure but, with still deeper submersion, a point will be reached where
the bladder collapses with formation of a dimple. When this occurs the contained
gas will be compressed by the deformation and will take up the pressure corresponding to the hydrostatic head above the bladder. For instance, if a bladder
becomes submerged to a depth of 33 ft. (corresponding to a pressure of 2 atmospheres absolute) and fails to resist, the gases in it will assume that pressure. For
the sake of simplicity we may disregard the exact proportions of nitrogen and oxygen
and call the mixture air. Now the air dissolved in the surrounding sea water is
normally at a pressure of about 1 atmosphere absolute, thus the bladder gas (which
resembles air in composition) will tend to diffuse out into solution in the sea under
a pressure of about 1 atmosphere. In this way losses of gas take place which will increase the dimpling and be more than the plant is able to replace with the amount of
Oxygen in the Bladders of the Seaweed
205
sunlight it can catch in its crippled condition, with the result that succeeding high
tides squeeze out more and more gas till the bladders are emptied. Obviously the
plants which have stored a large reserve of gas are least liable to this disaster but,
unfortunately, those which grow at the greatest depth and so are exposed to the
highest hydrostatic pressure are the very plants which, from the nature of things,
get least sunlight and least opportunity of making and storing oxygen. We have
seen (Text-fig. 3) how the oxygen decreases from bladder to bladder going downwards on the same plant and similarly the gas pressures found in plants growing low
in the tidal zone are less than those in their neighbours at a higher level. In spite of
this disharmony the seaweed seems to protect itself pretty well at places with a
moderate tidal range, and at Cowes (with a 9^ ft. tidal range) one rarely finds a
dimpled specimen but it seemed desirable to find out how it fared in spots with a
much greater rise and fall of tide.
ADAPTATION TO HYDROSTATIC PRESSURE AT
PLACES WITH LARGE TIDAL RANGE
Commander J. Whitla Gracey, R.N.R., Haven Master of Bristol, was able to tell
me that Ascophyllum nodosum grew abundantly at Portishead (with a 40 ft. tidal range),
and most kindly procured samples for me, while I am indebted to his department
for the levels and hydrographic data in Text-fig. 4, which illustrates the conditions
at a steep bluff of rock named Battery Point. Seaweeds of all kinds overlap there,
but the zone of A. nodosum is quite distinct. The axis of the diagram is the "halftide level" ascertained by meaning the heights of all the high waters and low waters
for the year. Similarly the "mean high water" is the mean of the year's high tides,
and "high-water equinoctial springs" is the highest to which the tide ever rises
except under the influence of great storms. We see that at the average high water
the plants living at the upper margin of their zone are only submerged to a depth
of 5-1 ft., but those at the lower margin to a depth of 20 ft., which corresponds to
a hydrostatic pressure of 9 lb. per sq. in., while at equinoctial tides they may have
to resist 12 lb. per sq. in. At every low tide all the plants are high and dry.
Measurement of the gas pressure in different plants showed that it was in no
wise adapted to counter the hydrostatic pressures they had to resist, for plants from
the upper margin had an internal pressure corresponding to 11 ft. head of water
(more than would ever be above them) while plants from the lower margin had only
3-7 ft. of pressure which could not give them much support against 20 ft. of hydrostatic pressure; but further examination showed that these seaweeds are protected
in another way which is graded to suit the depth at which they grow, for when
bladders of plants from the lower margin of the zone are slit open their walls are
found to be nearly three times as thick as those from plants of the upper margin.
The standard method of measuring thickness for constructing Table III was to pass
a small cork borer right through each specimen at its widest point in a direction normal to
the plane of the frond so as to cut out a disc from each side, both discs were gauged in a
suitable micrometer and, if they differed at all in thickness, the mean was taken.
206
G. C. C. DAMANT
y\
High-water equinoctial springs 44 ft.
tI1
Zone
Ugh water 37-6 ft.
Uppe limit of nodonm,325ft.
High water oeapt
(Hydr •tatio pnwaiirt
^20 ft.
'3L
1 UydiMUtie'pnMtin
nodostm* / / V/t ? -
|26*fuh«do.Ulb.Cr
Half4idelerel21-2ft.
t
Lower limit of nodomm 17-6 ft.
Low-water neap* 1 0 7 ft.
Mean low water 5 3 f t .
Port Datum Zero
Text-fig. 4. Tidal levels and limits of growth of Ascophyllum nodosum
at Battery Point, Portishead.
Table III. Comparison of the thickness of the walls of the bladders of specimens of
Ascophyllum nodosum taken from the upper and lower margins respectively of
the zone of growth; each measurement being the mean of seven normal samples
Volume
of bladders
c.c.
1
1 to 2
2 to 3
3 to 4
4 to 5
5 to 6
Thickness of walls
In plants from
upper margin
mm.
In plants from
lower margin
mm.
06
07
i-o
09
i-o
i-o
'•4
1-9
2-2
2-S
2-7
3-2
Oxygen in the Bladders of the Seaweed
207
The thickening or " armouring'' shown in Table III enables the bladders of plants
in the lower part of the zone to protect their contained gas from the hydrostatic pressure which would otherwise cause it to diffuse away as described on p. 204. The
armoured bladders are naturally less buoyant than the thin-walled bladders of the
upper part of the zone and consequently afford less support to the frond which,
however, compensates by forming them at closer intervals, as may be seen on comparing the two types of plants shown in PI. I, fig. 1. The necessity for this armouring
can be demonstrated by transplanting a bunch of the weed from the upper to the
lower margin of the zone, a distance of 15 ft. vertically; the plants may be cut from
PortUbetd
lidaj raogc
32 ft.
Cowes with
scale
A J - T I D E LEVEL; • : ". 1
H«lf4ide level
Text-fig. 5. Comparison of the tidal range and position of zone of growth of Ascophyllum nodosum
(cross-hatched) at Cowes and at Portishead. On the right (in dotted outline) the Cowes diagram is
enlarged in scale so as to fit the Portishead range and show that the weed does not grow so far down
towards low water mark at Portishead as at Cowes.
their attachment and secured in their new position with string, for their roots are
merely organs of attachment. In such experiments loss of gas from the unadapted
plants can be observed after one tide, and by the third day about half of their
bladders will be deeply dimpled. I have not been in a position to watch the process
continuously, but have found after 3 weeks every bladder had collapsed into a
concave form like the bowl of a spoon (PI. I, fig. 2), was practically empty of gas and
devoid of buoyancy. Even among adapted plants the margin of safety seems to be
very small, for, searching along the lowest level of growth, one finds numerous
stunted or dying individuals whose bladders have been crushed flat, presumably
by some unusually high tide. It seems likely indeed that at Portishead hydrostatic
208
G. C. C. DAMANT
pressure is the limiting factor which determines the downward spread of the plant,
for, if one compares the zone of growth as it exists there with the zone at Cowes
(Text-fig. 5) it is apparent that, relative to the tidal range, the upper margin
occupies the same position at both places but that the lower margin is shifted
upwards at Portishead as though to avoid pressure.
The simplicity and directness of this case of adaptation are noteworthy. In a
belt of seaweed a few yards wide one finds the adverse environmental factor of hydrostatic pressure increasing from a trifle at the upper margin to a limiting value at the
lower, and the plants protecting themselves by thickening their bladder walls in
corresponding degree according to the level at which they grow. Those plants
which need protection have it, the others do not. At present there is little evidence
upon which to decide whether the thickening is a direct response which every plant
is capable of making if exposed to severe hydrostatic pressure at the beginning of its
life or whether, as seems more likely, it is a manifestation of different constitutions
in the young plants which natural selection sifts into the upper or lower part of the
zone according to the thickness of the bladders they are capable of developing. In
this connexion it may be mentioned that the bladders of plants from the middle of
the zone while, on the average, intermediate in thickness vary among themselves in
this character more than do their neighbours of the two margins, not only from
plant to plant but even from side to side of the same bladder.
SUMMARY
1. Ascophyllum nodosum secretes oxygen into its bladders so freely that the
total gas pressure within them is usually above atmospheric pressure. The plant
draws on this store of oxygen during the night and when shaded; the advantage of
storing the gas under pressure is that withdrawals do not diminish the volume or
buoyancy of the bladder whose efficiency as a float is of importance to the plant
when competing for sunlight.
2. As'the bladder walls are somewhat permeable, the contained gas tends to
diffuse out into the sea. Disastrous loss of gas may take place if, during an abnormally high tide, hydrostatic pressure overcoming the resistance of the bladder causes
it to collapse with formation of a "dimple". In this event the gas inside takes up
the pressure of the water outside and diffuses away more rapidly than before the
collapse took place. The bladders lose buoyancy and can no longer support the
plant properly; as a result it is starved of sunlight, fails to replace the lost gas and
becomes permanently crippled.
3. Plants growing in situations where they are exposed to severe hydrostatic
pressure show an adaptive thickening of the bladder walls which enables them to
resist deformation and the sequence of events detailed in (2).
4. This adaptation is so closely fitted to the environment that in a zone of
A. nodosumfifteenfeet wide the plants of the upper level are unable to withstand
the hydrostatic pressure obtaining at the lower and, if transplanted there, lose all
their gas and perish.
|URNAL OF EXPERIMENTAL BIOLOGY, XIV, 2
PLATE I
- H
DAMANT—STORAGE OF OXYGEN IN THE BLADDERS OF THE SEAWEED ASCOPH'YLLUM
NODOSUM AND THEIR ADAPTATION TO HYDROSTATIC PRESSURE (pp. 198—209).
Oxygen in the Bladders of the Seaweed
209
I desire to thank Surgeon-Commander F. R. Mann, R.N. for the drawing from
which PI. I, fig. 2 was made.
REFERENCES
LANGDON (1916). Publ. Puget Sd Mar. (biol.) Sta. 1, No. 23.
LUCAS, A. H. S. (1911). Proc, Linn. Soc. N.S.W. 36, 626-31.
WILLIE, N. (1889). Biol. Form. Fork., Stockh., 1, 63-5.
ZELLKR & NEIKEHK (1915). Publ. Puget Sd Mar. (biol.) Sta. 1, No. 5.
EXPLANATION OF PLATE I
Fig. 1. Typical specimens of Ascopkyllum nodotum from Portishead. A, From the upper margin of
the zone of growth. B, From the lower margin of the zone of growth having bladders with thickened
walls and set closer together than A.
Fig. 2. Right, normal bladders of AtcophyUum nodosum. Left, the same "dimpled" after losing gas
by diffusion while deeply submerged.