Jul 2, 2007 - A thin glass fibre, rigidly held at one end, ... glass wool and glued to a glass rod, which was clamped in a Singer High Power ... l and H caused uncertainties of about ± 10~ in the use of (1). There is ... adhesion, but the diatom's size and shape also matter. ... with more pointed ends could do this more easily.
British Phycological Bulletin
ISSN: 0374-6534 (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/tejp18
Measurements of diatom adhesion and their relationship with movement M.A. Harper & J.F. Harper To cite this article: M.A. Harper & J.F. Harper (1967) Measurements of diatom adhesion and their relationship with movement, British Phycological Bulletin, 3:2, 195-207, DOI: 10.1080/00071616700650051 To link to this article: http://dx.doi.org/10.1080/00071616700650051
Published online: 02 Jul 2007.
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Date: 31 January 2017, At: 12:33
Br. phycoL Bull. (1967) 3 (2) 195-207 October 31, 1967
M E A S U R E M E N T S OF D I A T O M A D H E S I O N A N D THEIR RELATIONSHIP WITH MOVEMENT By M. A. HARPER Department of Botany and J. F. HARPER Department of Mathematics University of Bristol Adhesive and tractive forces of diatoms have been measured by the bending of a glass fibre. The results show that motility depends on attachment to the substrate. Movement on girdle bands occurs when the raphe acts against lumps of trail substance. Resistance to shear stress in laminar flow through a capillary tube is found to he particularly high in large epipsammic spp., e.g. Amphora ovalis, and is related to strong adhesion. The mechanism of movement is discussed in relation to the physical properties of the raphe and trail. Mucilage threads of pennate diatoms, first described by Lauterborn (1894), have been shown to form continuous trails behind all moving diatoms investigated (see D r u m & Hopkins, 1966; Hopkins & Drum, 1966). The latter authors suggest that the trail makes the diatom stick to its substrate and that the forces exerted in its secretion against the point(s) of adhesion push the diatom along. Two methods were used for measuring these forces. A thin glass fibre, rigidly held at one end, can be pushed against a diatom until it is forced away from its substrate to measure the adhesive force, or held in place to measure the tractive force of a diatom pushing it. The forces are calculated from the observed bending of the fibre. The second method uses properties of laminar flow of water through a tube containing diatoms to give minimum values of the adhesion over periods of time. There appear to be no previous direct measurements of the forces exerted by any micro-organisms. Epipsammic diatoms (Round, 1965) are distinguished from others by their strong adhesion to grains of sand. Mucilage pads attach the non-motile forms, but motile species seem to adhere by their trails. Unless epipsammic diatoms stick on unusually well, they could hardly withstand the washing used to isolate the sand grains from finer sedimentary material (Round, 1965); measurements show that they do. They also explain why Amphora spp. in two springs have been found to remain after increases in the flow which removed other diatoms (Dr J. W. Eaton, personal communication). MATERIALS Sediment was collected from Langford Rising (O.S. ST 466594), Rickford Rising (ST 488592), Abbot's Pool (ST 536733) and Shear Water (ST 850421). The sediment was left overnight irL 195
M. A. H A R P E R A N D J. F. H A R P E R
196
Petri dishes with coverslips or squares of lens-cleaning tissue on its surface. These were removed, and the motile diatoms washed off them into watch glasses for the experiment with fibres. For the capillary tube experiment, diatoms were sucked into the tube from the watch glasses.
M E A S U R E M E N T OF S H O R T - T E R M FORCES METHOD
A cylindrical fibre 1.2 cm long and about 20/~ in diameter was taken from glass wool and glued to a glass rod, which was clamped in a Singer High Power Micromanipulator (Barer & Saunders-Singer, 1945). To measure the adhesion of a diatom (either moving or at rest), it was pushed with the free end of the fibre until it was removed from the substrate and the fibre sprang back (Fig. 1).
F R
I E FIG. 1. Arrangement for measuring forces by fibre deflection. E, equilibrium position; F, thin fibre; H, deflection; R, rigid support
Deflections were measured with an eyepiece micrometer. The measured forces acted on the diatom only if the fibre was wholly immersed in water, the force exerted was at right angles to its length, and it was not rubbing on the substrate. This prevented measurements on a rough or particulate substrate. Tractive forces in the direction of motion can be measured as well as adhesive forces. Sometimes the fibre became stuck to the trail substance on an upper raphe and was pulled along by the diatom. THEORY
To calculate the force, the fibre is assumed to be a uniform circular cylinder o f diameter d, length ! and Young's modulus E, which obeys Hooke's law. Unless the bending is excessive, the force F exerted is given in terms of the deflection H by the equation 37rEd4H F -64l~, (1) (see Lamb, 1949). A convenient unit for diatom forces, used throughout this paper, is the millidyne (mdyne), which is approximately 1.02 micrograms weight. Measurements of deflections of two fibres with wire weights gave Young's moduli of 4.61 and 4.38 x 1011 dyne/cm z for one and 5.84 and 5.31 x 10n for the other, which are at the lower end of the usual range for glass (see Kaye &
Diatom adhesion
197
Laby, 1956). They also showed that the experimental errors in measuring d, l and H caused uncertainties o f a b o u t ± 1 0 ~ in the use o f (1). There is also a constant e r r o r o f a b o u t 0.02 millidyne because very small deflections could not be m e a s u r e d accurately. A Y o u n g ' s m o d u l u s o f 5 x 10 it d y n e / c m 2 was used in all the calculations. RESULTS All the d i a t o m s showed a wide range o f values for b o t h a d h e s i o n and traction, so that even with m a n y m e a s u r e m e n t s the results are tentative. The m a x i m u m , m i n i m u m a n d (if sufficient readings were m a d e ) m e a n a n d s t a n d a r d deviation o f the forces were calculated for a t t a c h e d diatoms. F o r u n a t t a c h e d ones the force----0. T a b l e I shows t h a t the e p i p s a m m i c species, Amphora ovalis and Diploneis ovalis, a d h e r e d m u c h m o r e strongly t h a n the others. Tractive forces are generally s o m e w h a t lower t h a n adhesive, b u t only for A. ovalis does the ratio reach 1 : 10. This d i a t o m is the only one with all f o u r raphes in contact with a flat substrate. W h e n p u t in a cell containing translucent B.D.H. sand it m o v e d a m o n g the grains, p u s h i n g t h e m a p a r t : their average weight in water was a p p r o x i m a t e l y 10 millidynes. TABLEI. Ranges of adhesive and tractive forces for diatoms on a horizontal surface. Mean and standard deviation (S.D.) not given if number of readings (N) less than 7.
Species
ADHESION in millidynes state N Max. Mean Min. S.D.
Diploneis ovalis stationary Amphora ovalis mowng stationary Surirella ovalis moving stationary Nitzschia moving stationary linearis !Navicula oblonga mowng stationary mowng Cymatopleura solea stationary Nitzschia moving stationary sigmoidea
2 2900 3 480 11 630
100 230 410 140
170
7 16 21 25 21 2 9 24 23
4.6 1'9 4.3 0"83 2.0
4.0 2.0 3'4 1-1 1'7
14 6.6 12 4.2 6-8 4.8 4"5 9.3 14
1.0 0.36 0.1 0.02 0.24 0.23 0.91 0.02 1"3 0.02 1-7 0-05
TRACTION in millidynes N Max. Mean Min. S.D. 48
48
4 7.3
2.1
2
1-4 2"0 3.1
8* 21"
1.1
0.05 0.94
24 0.57 0.15 0.01 0.14 2 2.2
0-25
20* 1-9" 0-08 0-01 0.08
In these cases, one measurement was far higher than all others and is excluded from the mean and S.D. because the fibre became stuck to the diatom. There is clearly a close relation between l o c o m o t i o n a n d adhesion, as rem a r k e d on b y H o p k i n s & D r u m (1966). I n a total o f over 500 observations, whenever a d i a t o m m o v e d it was a d h e r i n g to the substrate. Tables I a n d II indicate t h a t s t a t i o n a r y d i a t o m s often have higher values o f a d h e s i o n than m o v i n g ones. The latter p r o b a b l y only have a limited a m o u n t o f trail in contact with the substrate at any one time. However, strong a d h e s i o n does n o t prevent m o v e m e n t : .4. ovalis cells were able to m o v e n o r m a l l y while exerting adhesions o f over 400 millidynes.
Agar Substrates D i a t o m s were studied on a g a r because they are slowed down. W a g n e r (1934)
M. A. HARPER AND J. F. HARPER
198
and Peteler (1940) found the m a x i m u m velocities of Nitzschia putrida and N. closterium on agar reduced to ½ and ~n respectively o f the speeds on glass. With Navicula oblonga we found ratios o f 3 on 2 ~ agar and ½ on 1 ~ agar. Adhesion to the latter was much less than to glass, and to 2 ~ agar somewhat less (Table II). Decreased m o v e m e n t is thus found on a substrate unsuitable for adhesion, but the diatom's size and shape also matter. W a g n e r (1934) found that Nitzschiapalea (c. 10 ~2 cross-section) penetrated 2 ~ agar, but our Navicula oblonga (c. 600/z 2) did not, although they would move into 0.5 ~ agar, digging tunnels for themselves and making force measurements impossible. Diatoms with more pointed ends could do this more easily. TABLEII. Adhesion of Navicula oblonga to glass and agar.
Substrate Glass 2 % Agar 1% Agar
Diatom
N
Moving Stationary Unattached Moving Stationary Unattached Moving Stationary Unattached
25 21 9 20 25 6 22 23 25
FORCE in millidynes Max. Mean Min. 4.2 6.8 0 2-2 9 0 0.8 4-2 0
S.D.
0'83 2.0
0"02 0.24
1"06 1"7
0.45 1.6
0-02 0.02
0.54 1.9
0.070
