When we look casually at the sea around us, we may be hardly aware that living concealed beneath the surface, especially in harbour and coastal waters, are millions of tiny organisms— both plant and animal— which together make up the plankton. Each plankton organism is known as a plankter. The plant plankton, or phytoplankton, is composed of unicellular algae, the chief constituents of which are diatoms (encased by a siliceous wall); dinoflagellates (tiny organisms with one transverse and one longitudinal flagellum which ‘screw’ themselves through the water); and even tinier micro-flagellates which are now considered to be of basic importance in providing the bulk of primary marine food production. As Professor C. C. Davis (1955) puts it, the phytoplankters are the producers and the zooplankters the consumers.

Like land plants, the plankton diatoms must have light for their existence. Unlike most land and seashore plants, however, they are forced to float in order to survive, for their existence is possible only in those layers of the ocean through which the sun's rays penetrate. We find that these minute algae are peculiarly adapted to life as single cells. Each cell is capable of existing as an independent unit, even in the many species where the cells are joined together to form long chains. Size is a disadvantage for phytoplankters. A small object will float more readily if it possesses a larger surface area in proportion to its total volume. This will increase resistance through friction and also present the largest possible surface to the energy-giving light from the sun. Several distinctive types of cell construction to assist with flotation have been classified. These are the bladder type (e.g. Coscinodiscus), the ribbon type (e.g. Streptotheca), the hair type (e.g. Rhizoselenia) and the branching type (e.g. Chaetoceros - Plate II, Figs. 1, 5-7, 8-10, 11).

Those species which flourish away from any land mass are said to be oceanic while those inhabiting coastal waters are known as neritic. In general, oceanic diatoms cannot reproduce in coastal waters and hence their occurrence in the neritic zone is limited to one short-lived generation. Diatoms growing attached to a substrate in the littoral or sublittoral regions may break off, float upwards and be collected along with other pelagic (free-floating) forms. Such plants are called tychopelagic.

Quite apart from their importance in the food chain. diatoms play a fundamental role in the world's economy through their contribution to ocean floor deposits which, under great pressure, have become reservoirs of ‘mineral’ oil. Within limits, these organisms can provide physical oceanographers with valuable information about movement and mixing of page 108 different water masses by acting as indicator species. They are also the main constituents of diatomaceous earth, revealed after the sea has receded. The uses of diatomaceous earths are many and varied, including the preparation of dynamite and polishing powders, and the filtration of liquids such as acids, beer, gelatine, milk, oils and penicillin. They help to form asphalt, ceramics and concrete, as well as acting like thermal insulators to save fuel in boilers, kilns, ovens, etc. Such a wide range of application in industry is due largely to the lightness, purity, inertness, heat-resistance, uniformity and extraordinarily high porosity of the fossil diatom frustules (q.v. Okuno 1954).

Professor A. C. Hardy (1956) has very aptly described a microscopic view of living diatoms. They look to him like ‘crystal caskets filled with jewels as the strands of sparkling protoplasm and groups of amber chloroplasts catch the light’.

Distribution

Diatoms are among the most cosmopolitan of plant groups, owing to the much more continuous nature of the environment of the sea in comparison with terrestrial habitats. All the same, local differences in species composition are quite to be expected, and until more is known about year-round seasonal variation in all latitudes, it is difficult to generalise on this topic. It has been stated that production of marine phytoplankton in the arctic and antarctic greatly exceeds that in tropical waters (e.g. Hart 1934). Davis (1955) points out, however, that most of the sampling in high-latitude waters has taken place during the summer by ‘hit and run’ expeditions. At such a time the following conditions encourage tremendous productivity: nearly twenty-four hours of light for photosynthesis, weak thermal stratification, plus a super-abundance of nutrient salts from deep upwelling water and from melted ice. It is obvious, however, that total annual production of plant growth cannot be adequately assessed from summer analyses alone.

In the open Pacific, Graham (1941) found that in regions where upwelling of nutrient-rich water occurred, production was greater in tropical than in average temperate waters. Further, Allen (1939) found as many as 500,000 diatoms in a litre of sea-water off the Pacific coast of Panama— a high figure for any latitude. It would seem, therefore, that each locality must be studied in conjunction with the variable physical properties of the local marine environment before broad generalisations can be made on geographic distribution. page break

PLATE I
Fig. 1: Diagrammatic sketch of a centric diatom frustule (Coscinodiscus) in girdle view. Fig. 2: The same in valve view. Fig. 3: Diagrammatic sketch of a pennate diatom frustule (Navicula) in broad girdle view. Fig. 4: The same in valve view. (Figs. 1-4 after Cupp, 1943, Figs. A and B in part, pp. 4-5.)

Seasonal Occurrence

In temperate waters, one of the most absorbing problems presented by the phytoplankton is that of sudden appearances and disappearances of one or several species in tremendous quantities. This phenomenon is known as a ‘bloom’. Although the precise factors which trigger off a bloom are not yet fully understood, the general pattern of seasonal fluctuation is influenced by the physical factors of light, temperature and availability of nutrient salts (mainly nitrates and phosphates). The chief biological factors involved are those produced by grazing zooplankton, including relationships between individuals of the same species (intraspecific) and between different species (interspecific).

The picture is further complicated by other factors such as turbulence, salinity, pH and dissolved oxygen. Turbulence, produced by instability of the vertical water column or by horizontally induced wind and tide movements, may act as a hindrance to phytoplankton production in transporting the organisms out of the photic zone to the dimly lit layers below, or as a help in supplying fresh nutrients for new growth. Salinity is probably not a limiting factor except in its effect on the density structure and hence vertical stability of the water. Factors like dissolved oxygen and pH are chemical manifestations of biological interactions such as the balance between photosynthesis and respiration.

In winter, the water is unstable owing to surface cooling, denser layers sinking below warmer and less dense ones. Aided by storms, the layers of water are turned over many times until the sea temperature is almost uniformly low. Weak, low-angled rays from the sun are reflected from the surface or penetrate obliquely and not far enough down to form an effective photosynthetic layer. In such an environment, where nutrients are abundant but heat and light are inadequate, no blooming occurs.

With the onset of spring, nutrients are still present after winter turbulence, and the sun increases daily in altitude, raising the surface heat of the water and illuminating deeper layers. Here, then, are the conditions under which blooming may occur, through rapid multiplication by cell division— up to six times a day or more in warm temperate latitudes (Wood 1958) —of the species which is best able to take advantage of the prevailing environmental conditions. There follows a succession of organisms, each rising to dominance, and giving way just as rapidly to others, and each possibly conditioning the presence of its successor by the liberation of certain organic substances such as hormones, antibiotics and vitamins. These substances, together with excretory and respiratory products, have been grouped together under the heading of external metabolites (Lucas 1947).

As summer advances, nutrients become scarce, since they have been used up by the spring bloom. With heating of surface layers, the water mass is more stabilised, therefore turbulence due to instability is greatly reduced. However, turbulence caused by winds will tend to produce an isothermal, page 111 warmer layer, the lower boundary of which is the thermocline (or discontinuity layer), a zone of abrupt transition between cooler, denser water below and warmer, less dense water above. The actual location of the thermocline varies from place to place, but may be found between about 10 and 100 metres. In such an environment where heat and light are strong but nutrients are scarce, no blooming may be expected. The original nutrients are made unavailable since they are in the bodies of the herbivorous zooplankters, which will also increase in warmer water.

During autumn, unstable conditions again prevail with surface cooling and equinoctial gales. Once again, turbulence brings fresh phosphates and nitrates from decayed organic remains to the upper layers, in lesser quantities than in winter; but with the still ample light and heat, conditions are favourable for another bloom though on a lesser scale than that in spring.

Towards higher latitudes the season of maximum production is progressively later, until in polar seas there is only one summer peak. In the tropics, however, the seasonal effects are not pronounced, a more regular and sparser growth of many different species taking place through a much deeper photic zone at all times of the year.

Classification

Most authors recognise two main groups: —

Structure

So main terms are used in describing a diatom cell or frustule that precise definitions are required to gain a clear picture of its structure and organisation. (Plate I, Figs. 1-4.)

• Valves— parts of frustule corresponding with the top and bottom of a box
• Valve Mantle— part of valve bent over at the sides
• Epivalve or Epitheca— older, larger, upper valve
• Hypovalve or Hypotheca— younger, smaller, lower valve
• Girdle or connecting bands— two-ended, overlapping side faces of frustule joined to edges of valves— corresponding with the sides of a box
• Intercalary bands— incomplete ring-like or scale-like bands between valve and girdle, as in Rhizoselenia
• Pervalvar (cell) axis— imaginary line joining the two valve centres
• Apical axis— longer axis of valve
• Transapical axis— shorter axis of valve
• Setae— long delicate spines at corners of Chaetoceros and other cells
• Septa— ingrowths from intercalary bands, as in Grammatophora
• Nodules— small internal rounded or conical thickenings in pennate diatoms
• Raphe— a slit through the valve joining two nodules
• Pseudoraphe— a clear longitudinal space separating two vertical rows of transverse markings.
• Keel— a wing-like expansion of valve, containing raphe— in Nitzschia and Surirella

PLATE II - CENTRIC DIATOMS
Fig. 1: Coscinodiscus excentricus Ehrenberg; Hauraki Gulf. Fig. 2: Stephanopyxis orbicularis Wood (in press); Cape Campbell. Fig. 3: Thalassiosira hyalina (Grunow) Gran, with auxospore; Hauraki Gulf. Fig. 4: Planktoniella sol (Wallich) Schutt; Napier. Fig. 5: Chaetoceros affine Lauder; Wellington Harbour. Fig. 6: Chaetoceros concavicorne Mangin; Cape Campbell. Fig. 7: Chaetoceros decipiens Cleve; Hauraki Gulf. Fig. 8: Rhizoselenia alata Brightwell; Hauraki Gulf. Fig. 9: Rhizoselenia setigera Brightwell; Hauraki Gulf. Fig. 10: Rhizoselenia stolterfothii H. Peragallo; Hauraki Gulf. Fig. 11: Streptotheca thamensis Shrubsole; Castle Point. Fig. 12: Lauderia annulata Cleve; Hauraki Gulf. Fig. 13: Ditylum brightwellii (West) Grunow; Cape Campbell. Fig. 14: Biddulphia chinensis Greville: Hauraki Gulf. Fig. 15: Biddulphia mobiliensis Bailey; Hauraki Gulf.

• Pores— perforations in the walls
• Spinulae— small spines formed from marginal pores, as in Thalassiosira
• Areolae— cavities closed by a thin siliceous membrane and separated by partitions
• Costate— strong ribs, closed outside and open inside (coarse striae)
• Punctae— linear series of tiny dots resembling striations or striae
• Pectin— complex, gel-forming polysaccharide, comprising the inner wall layers of the frustule
• Silica— hard, insoluble solid with a high melting point, making up most of the outer wall layers of the frustule
• Pyrenoid— a protein centre, surrounded by a starch sheath, present in chromatophores of algae
• Auxospore— thin-walled spore, sometimes sexual in nature, larger than parent cell or cells
• Microspore— small flagellated spore
• Resting spore— medium, thick-walled spore

Basically, the structure of each diatom cell is that of a box. The lid and bottom of the box are the valves, and the sides are formed by the girdle or connecting bands which overlap slightly, one half of the cell being larger than the other. The overlapping connecting bands are known as the girdle (Plate I, Fig. 1). But this box-like cell differs from all other algal cells in being encased by a skeleton of silica which is often modified to form intricate shapes and is transversed by numerous canals and pores which connect the inner protoplasmic contents with the external aqueous environment. Recent studies under the electron microscope made at very much higher magnifications than are possible under an ordinary light microscope have shown that the siliceous wall is an amazingly complex structure. A thin cytoplasmic layer lines the pectic membrane of the wall. The nucleus may be suspended in the threads crossing the central vacuole, or may lie adpressed to the wall. Chromatophores, constant in shape and size for each species, vary greatly from small discs to large, simple or complex anastomosing plates. Some chromatophores of pennate diatoms contain one or several pyrenoids. The olive green pigment is made up of chlorophyll a and c, B-carotene, fucoxanthin, neofucoxanthin a and b, diatomin and diatoxanthin (Strain 1951). A fatty oil, the principal food reserve product of diatoms, is thought to be the main cause of ‘slicks’— glassy patches or page break

PLATE III - PENNATE DIATOMS
Fig. 1: Pleurosigma naviculaceum Brebisson; Hauraki Gulf. Fig. 2: Asterionella japonica Cleve; Lyttelton. Fig. 3: Pleurosigma formosum W. Smith; Hauraki Gulf. Fig. 4: Grammatophora marina (Lyngbye) Kutzing; Port Chalmers. Fig. 5: Achnanthes longipes Agardh; Cape Campbell. Fig. 6; Amphora chinensis A. Schmidt: Hauraki Gulf. Fig. 7: Nitzschia seriata Cleve; Castle Point. Fig. 8: Nitzschia closterium (Ehrenberg) W. Smith; Hauraki Gulf. Fig. 9: Thalassiothrix nitzschoides Grunow; Hauraki Gulf. Fig. 10; Surirella gemma Ehrenberg; Hauraki Gulf.

page 116 streaks on the surface of oceanic water— especially near the Antarctic where blooming of diatoms occurs in the summer months on a gigantic scale (Dietz and La Fond 1950).

Reproduction

Average diatom size actually becomes less as a population increases in numbers. When a diatom divides by mitosis, nuclear division is usually followed by the separation of the two halves of the parent cell and cleavage of the cytoplasm into two parts. Each new protoplast is thus partly naked and partly encased by part of the parent cell wall, which becomes the new epitheca. A new wall, fitting inside the parent one, is rapidly secreted by the protoplast. This smaller wall is the new hypotheca.

Owing to the box-like organisation of the frustule, with successive divisions the descendents of one half of the box become progressively smaller until further division by ordinary vegetative means becomes impossible. Consequently a curious process of spore formation has evolved whereby on rare occasions the protoplast escapes from the firm surrounding wall and expands to about the original size of the cell. Among pennate diatoms a sexual fusion occurs (two parent cells conjugate in Navicula and Nitzschia). In centric forms like Thalassiosira, the two halves of the cell wall become separated by the enlarging protoplast, which surrounds itself with a thin, mainly pectic, membrane. Valves and girdle bands are soon formed inside this, completing the formation of the new and enlarged individual. It is now known that centric diatoms also reproduce sexually, i.e. they too are diploid, and auxospore formation is a sexual process (see summary of research on this topic in Papenfuss 1955). Thus the auxospore is a true zygote.

Other spores, called microspores, are produced much more frequently as a result of nuclear division without cell division. Up to 32 flagellated microspores are known in Biddulphia mobiliensis. Resting spores, one per cell, with thick walls and concentrated contents, may tide many diatoms over harsh periods. In Chaetoceros these are covered with spines (Plate II, Fig. 5). Under certain conditions in Ditylum resting spores may germinate to form auxospores (Gross 1940).

Collecting, Preserving and Mounting

All that is required to collect these little plants is a fine-meshed conical tow-net of bolting silk or nylon of about 200 meshes to the inch, with either a plankton bucket or simply an open jar placed tightly in the narrow, cylindrical canvas sleeve at the base of the net. The wide end of the net needs to be attached to a metal hoop and fixed by three strong cords to a towing rope. A tow at slow speed (not more than two knots), just below the surface, for fifteen minutes should be ample for a good catch. 3% to 5% neutral formalin is adequate to preserve the plant material, but if there is an abundance of small animal larvae or copepods, up to 10% is needed.

For many of the delicate plant forms, e.g. Rhizoselenia and Lauderia, further treatment is unnecessary, even harmful, wet mounts on a clean slide covered by an 0.0 or 1.0 coverslip being quite satisfactory. For the more heavily silicified types like Coscinodiscus and most pennate forms much severer treatment must be used to eliminate the organic contents and enable the intricate cell wall structurs on which identification largely depends to be seen clearly.

Several methods for clearing diatoms have been described. Perhaps the most usual one is that of boiling the material in hydrochloric, nitric or sulphuric acid, and adding potassium chloride or sodium nitrate. However, the process is violent and must be performed in a fume cupboard. An easier method is that involving the reaction between hydrogen peroxide and potassium dichromate or potassium permanganate (Van der Werff, 1955).

The material is centrifuged and washed in distilled water several times, then placed in a porcelain dish, flooded with 30% hydrogen peroxide, and covered with a piece of glass. After standing for about five minutes, 1 mg, of powdered potassium dichromate or potassium permanganate is added carefully. After a few minutes bubbles are formed and then a more violent reaction occurs, during which the temperature rises to about 80 deg. Centigrade. The gases emitted are not dangerous, and no heating is necessary. If there is any precipitation (of hydrated manganese dioxide) after the manganese reaction, it can be dissolved by adding some 10% acetic acid. One or two rinses in distilled water should be adequate, after which the material to be examined is allowed to settle, placed in drops on a coverslip (care should be taken to avoid massing of organisms by blowing gently on the drop) and left to dry completely. A drop of mounting medium is placed on the slide, preferably one with a high refractive index, e.g. hyrax or pleurax, though Canada balsam will do. Diatoms can be mounted straight from distilled water into hyrax, but must be transferred to pleurax from 95% alcohol. The coverslip is dropped diatom-face downwards, on to the gently warmed slide, and left for several days to harden.

Acknowledgments

The writer would like to express her thanks to Mr. C. T. T. Webb and Mr. R. Brazier for assistance with the illustrations, to Dr. E. E. Cupp for permission to reproduce Figures A and B in part from her (1943) paper, and to her husband and Dr. R. E. Norris for helpful criticism of the text.