Photosynthesis of Phytoplankton

The intensity of light decreases gradually with depth; yet for the purpose of biological investigations it is convenient to consider the sea as divided vertically into three zones with respect to the amount of light that is present. The depths given are approximations and vary with latitude and season.

1. The euphotic zone, which is abundantly supplied with light sufficient for the photosynthetic processes of plants. This zone extends from the surface to 80 or more meters. All of the eulittoral zone lies within this layer.

2. The disphotic zone, which is only dimly lighted and extends from about 80 to 200 or more meters. No effective plant production can take place in this zone, and the plants found here have mostly sunk from the layer above.

3. The aphotic zone, the lightless region below the disphotic zone. In the deep sea it is a very thick layer in which no plants are produced and the animal life consists only of carnivores and detritus feeders.

Until the relationship between rate of photosynthesis and light intensity at different wave lengths is more thoroughly understood than at present, it will be necessary to restrict discussion to the relationship between rate of photosynthesis and total light intensity in the sea as expressed in energy units or in units of illumination.

When using energy units the light intensity is expressed in ergs per unit surface and unit time, for example, ergs/cm²/sec; or in gram calories or joules per unit surface and unit time (1 g cal = 4.18 × 10⁷ ergs, 1 joule = 10⁷ ergs). The penetration of radiant energy in the sea was discussed in chapter IV (p. 104). It was stated there that no direct measurements are available of the energy which under different conditions reaches different depths below the surface, but that these amounts could be computed from observations of the extinction coefficients for radiation of different wave lengths and from the intensity of the radiation of stated wave lengths reaching the sea surface (table 27, figs. 21, 22). In all experiments in which the intensity of light has been examined in conjunction with studies of photosynthesis, the energy at different levels has

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been computed on such a basis. The light which penetrates to any considerable depth in the sea falls within a narrow band of the spectrum between approximate wave lengths of 0.45 μ and 0.60 μ, corresponding to light in the blue-to-yellow range. In marine plants, photosynthesis takes place within this part of the spectrum, for which reason the photosynthesis can be expected to stand in a fairly definite relation to the total energy.

In a number of published reports on photosynthesis in the sea, light intensity is expressed in units of illumination, that is, in units based on the power of light to produce visual sensation. Illumination units such as 1 meter-candle = 1 lux = 0.093 foot-candle have been widely used, and it is therefore necessary to explain their relation to the energy units. Because the unit of illumination is directly based on the reaction of the human eye, a mechanical equivalent of the illumination unit can be established only at a specified wave length. At moderate illumination the maximum sensitivity of the human eye lies at a wave length of 0.56 μ, and at this wave length the “luminous equivalent” has been established (Smithsonian Tables, 1933) at

There exists no simple relation, however, between the total energy of radiation of different wave lengths and the corresponding illumination in luxes, the reason being that the human eye is not at all sensitive to infrared radiation and is only slightly sensitive to radiation in the violet part of the spectrum. More than half of the radiation from sun and sky which reaches the sea surface falls in the infrared, and the eye is not very sensitive to a great portion of the remaining half, so that the luminous equivalent as defined above has to be multiplied by a factor of about 0.15 in order to give the illumination in luxes corresponding to a given intensity of the radiation in energy units. In the sea the infrared part of the radiation is absorbed in the upper few centimeters and, owing to the selective absorption in the visible part of the spectrum, the energy which penetrates to greater depths becomes more and more concentrated at wave lengths to which the human eye has its greatest sensitivity. Consequently, the factor by which the luminous equivalent must be multiplied in order to give the illumination in luxes increases with depth, reaching a value of approximately 0.75 at depths where the incident energy is reduced to 1 g cal/cm²/hour or less.

It is not possible to give any table by means of which the energy reaching a certain depth can be converted into units of illumination, because the conversion factors will depend upon the quality of the light. Thus, in turbid coastal water and in the clearest oceanic water the same amount of energy is found at the depths of 10 and 100 m, respectively (table 27), but at 10 m in the coastal water the maximum intensity lies

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at a wave length of 0.55 μ, whereas at 100 m in the oceanic water the maximum intensity is at a wave length of 0.48 μ. At these intensities the maximum sensitivity of the human eye will be at about 0.55 μ, and the illumination at 10 m in the coastal water would therefore be greater than at 100 m in the clear oceanic water, in spite of the energies being equal.

Table 92 has been prepared in order to show approximately corresponding values of energy and illumination at different depths and in different types of water. For the sake of simplicity the intensity of the radiation penetrating the surface has been selected at 100 g cal/cm²/hour, corresponding approximately to the radiation from sun and sky with the sun at zenith. The illumination at the surface is then about 120,000 luxes. The illumination decreases more slowly with depth than does the energy, but it is realized that the values in the table do not correspond exactly to each other. They give only the approximate relationship and can serve in obtaining a rough estimate of the energy if the illumination is stated, or vice versa.

The effect of light (somewhat modified by the effect of temperature) on photosynthesis can be illustrated by an experiment in which a series of suitable clear glass bottles are filled with sea water containing a substantial phytoplankton population and are submerged in the day time at various depths in the euphotic zone. It will be found that at a few meters below the surface the oxygen content of the water will increase as a result of the photosynthetic activities of the plants within the bottles.

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With increasing depth and accompanying diminution of light intensity, the amount of oxygen produced becomes gradually less until a depth is reached where the respiration of the plants exactly balances the oxygen produced through photosynthesis, so that no free oxygen is given off to the water. The amount of oxygen used by the plants in respiration during the experimental period is determined by a similar accompanying series of plankton samples in bottles covered by dark cloth to exclude any light for photosynthesis. The amount of oxygen consumed from the water in these bottles gives a measure of the respiration at the respective depths. Adding this figure to the amount of oxygen produced in the exposed bottles gives the total amount of oxygen produced in photosynthesis.

When the temperature decreases with increasing depth, the rate of respiration decreases. As long as the light intensity is so high that it does not limit the rate of photosynthesis, this rate also falls somewhat with falling temperature, but at low light intensities the effect of temperature

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on photosynthesis is negligible. A typical assimilation experiment is given in table 93. In this experiment samples of mixed plankton from the surface layer were lowered to different depths in covered or exposed bottles. The oxygen consumption in the covered bottles was therefore due to the total respiration of the plants and the animals and bacteria included in the sample. No oxygen was produced in the exposed bottles at depths greater than about 27 m, and the total production of oxygen disappeared at a depth of about 44 m. If the bottles had contained pure cultures of plants, production of oxygen in the exposed bottles would have taken place to a depth somewhere between these two limits (p. 933).

The photosynthesis depends on the light intensity, and the light intensity at which oxygen production and oxygen utilization are equal is therefore called the compensation point, or the compensation light intensity. The illumination at this point is “the minimum intensity at which the plant could survive in nature and [is] still too low for any crop increase” (Jenkin, 1937). The more pronounced effect of lowered temperature on the rate of respiration as compared with the effect on the rate of photosynthesis leads to a slight lowering of the compensation point with decreasing temperature, at least in certain higher algae (Spoehr, 1926).

It is obvious that the compensation point is determined by physiological characteristics of the plants and may, therefore, be somewhat different for different species, just as the optimum light intensity is not the same for all species. The compensation point is independent of the time during which photosynthesis and respiration have been measured if the oxygen production per unit time remains proportional to the light intensity and the oxygen consumption per unit time remains constant. On these assumptions, the oxygen production dP in the short time interval dt equals aIdt, where a is a constant and I is the light intensity, and the oxygen consumption dR equals bdt where b is another constant. The compensation point, I_c, is defined by

The values of oxygen production P and consumption R in the time T are The average light intensity in the time T is Therefore, if P = R, it follows that

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that is, the compensation point equals the average light intensity during a period of time in which oxygen production and consumption balance. The conclusion is correct on the assumptions which were made, but measurements in the field will not lead to exact results unless the light intensity remains low during the experiment, because only then is the production of oxygen proportional to the light intensity.

The two factors a and b which have been introduced here are in general not determined, but measurements of the difference between production and consumption are made at different light intensities and the compensation point is found by interpolation. Since the compensation point depends upon the ratio b/a, it follows that it depends on temperature if photosynthesis and respiration are influenced differently by the temperature.

The depth at which the compensation point is found is called the compensation depth. This depth evidently varies with the season of the year, the time of day, the cloudiness, and the character of the water. In the experiment quoted in table 93, the compensation depth lies between 27 m and 44 m. For a given locality one can introduce such terms as daily compensation depth with a clear sky, daily compensation depth with an overcast sky, average annual compensation depth, and so on. With a clear sky the compensation depth will in a given locality reach its maximum value at noon and will rise to the surface shortly before sunset. The average daily compensation depth is less than the compensation depth at noon, except at the North Pole.

The diatoms and many other algae thrive best in somewhat subdued light, but the optimum light intensity is not the same for all species. For example, Schreiber (1927) found the optimum for the diatom Biddulphia mobiliensis to be 1600 luxes, and for the green flagellate Carteria sp. over 3200 luxes. Some diatoms are able to grow in much reduced light. This has been shown in nature by Vanhoeffen for arctic species found growing under the ice in March. From Schreiber's results with Biddulphia we learn that high illumination does not necessarily increase the rate of photosynthesis, since the rate of multiplication was proportional to light intensity only at the intermediate illuminations 200 to 400 luxes. That this is quite generally true is indicated by the habit exhibited by many diatoms of grouping the otherwise scattered chromatophores into knots when exposed to strong light. In this way they afford mutual protection during such periods of unusual exposure as might occur when being swept by ascending currents to the very surface on bright days. This condition whereby the chromatophores are concentrated or collected into a group is known as a systrophe, and was described by Schimper (Karsten, 1905). For the diatom Coscinodiscus excentricus this has been found to occur at a light intensity of about 9.6 g cal/cm²/hour, p. 781. That diatom photosynthesis falls off

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in bright light at the surface is shown by field-assimilation experiments of Marshall and Orr (1928) with “persistent diatom cultures.” A “persistent” culture is one consisting of only a single species of diatom, but may contain also bacteria and a few other small organisms such as flagellates (Allen and Nelson, 1910). These investigators concluded that the compensation depth in inshore waters in the latitude of Loch Streven, Scotland, in summer is at 20 to 30 m and in winter at some level close to the surface. Gran (1927) also found that the optimum light conditions for diatoms in northern waters are not at the surface but at a depth of 5 m. Gaarder and Gran (1927) determined that the balance point between photosynthesis and respiration in “raw plankton” samples (mixed culture of composite plankton collection which may contain some animals) occur at a depth of 10 m in Oslo Fjord during March. These authors show that auto-oxidation and bacterial oxidation are important factors to be considered in studies of phytoplankton respiration. Other field observations with raw plankton indicate that a balance of photosynthesis and respiration occurs at a depth of about 17 m in Passamaquoddy Bay during midsummer (Gran and Braarud, 1935), and in certain Puget Sound bays during the summer the compensation depth for raw plankton lies between 10 and 18 m (Gran and Thompson, 1930). The shallowness of the compensation depth in Puget Sound perhaps resulted partly from absorption by the rich diatom plankton which prevailed. The most rapid photosynthesis, as indicated by oxygen production, in this experiment was at a depth of 1 m. The great number of diatoms produced in the upper few meters thus formed a screen preventing normal penetration of light to the population at greater depths.

Assimilation experiments with raw plankton samples can be carried on only if the diatom population is sufficiently dense that the metabolic activities provide differences in oxygen content and pH values above the limits of experimental error of the method. Experiments with persistent cultures have the advantage that they make possible field-assimilation experiments even at seasons when in nature the phytoplankton is at a minimum. The absence of small planktonic animals and the possible control of bacterial populations in such cultures eliminates a source of error in computing oxygen utilization. On the other hand, the diatoms in persistent cultures may represent selective species with slightly different light optima and may therefore not be typical of the mixed population of raw cultures.

The considerable variation found for the depth of compensation of oxygen production and utilization by phytoplankton results from variations in latitude, season, weather, and turbidity, all of which influence the depth of submarine illumination. Theoretically, it should be possible to determine the depth of optimum diatom production for a given area during the growing season by a study of the vertical distribution of

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oxygen, since the maximum oxygen produced in situ represents the depth of optimum light conditions for photosynthesis, provided, other factors essential to growth are uniform in the water mass. During a diatom maximum of five-day duration, the oxygen content may increase by as much as 2.2 ml/L. However, vertical circulation of the water, solution of oxygen from the air, and consumption of oxygen by animal and bacterial respiration makes this a rather uncertain index. The occurrence of the maximum diatom population below the level of maximum oxygen or below the experimentally determined compensation depth indicates that the plants were not produced at that level but have sunk there. For instance, it has been pointed out that the finding of diatom maxima at 40 to 80 m by Schimper (Karsten, 1905) in the Antarctic on the Valdivia could be explained only as being due to sinking, since no effective photosynthesis can occur in these latitudes below 50 m. Actually, such a population must represent a disintegrating one originally produced near the surface. Diatoms do, of course, carry on some photosynthesis at considerably greater depths than the depth of compensation, but more energy is used than is stored and the plants must finally reach a state where they succumb when the stored-up energy is exhausted.

Not only is the quantity of light a factor to be considered, but so also is the quality. It is well known that as sunlight enters the sea the intensity is reduced, but there is moreover a differential absorption, certain light rays being absorbed before others. The red component of light is removed in the first few meters of water, while the blue light penetrates to much greater depths (p. 106). Therefore, diatoms in nature have their greatest growth at depths where the red component is much reduced; rich growths are found at depths (15 to 20 m) where only blue and green light prevail. A realization of this has led to experiments on the effect of light intensities selectively reduced.

In experimental cultures, using Nitzschia closterium, a diatom much used in culture experiments, Stanbury (1931) concluded that the precise wave length is not so important as the amount of energy transmitted.

Assuming that the total energy in all wave lengths within the visible spectrum provides the best measure of available energy for photosynthesis of diatoms, Jenkin (1937), working in the English Channel, computed the available energy in joules or gram calories at various depths and correlated these values with the amount of oxygen produced by Coscinodiscus excentricus in submerged experimental bottles at the various depths tested. The results indicate that with an energy flux less than 7.5 joules, or 1.8 g cal/cm²/hour, the oxygen production is directly proportional to the energy and the utilization of the available energy is about 7 per cent. With an energy flux greater than the above, the photosynthesis is gradually inhibited. Systrophe takes place at an

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energy flux of about 40 joules, or 9.6 g cal/cm²/hour. At the compensation point the figures are 0.55 joule, or 0.13 g cal/cm²/hour. In Gullmar Fjord, Sweden, the compensation light intensity for mixed plankton with some animals included was found to be about 400 luxes (about 0.07 g cal/cm²/hour) (Pettersson et al, 1934). In the clear waters of the English Channel the compensation depth was found to be about 45 m during summer.

In certain sessile shallow-water diatoms adapted to a depth of only a few centimeters the maximum oxygen production occurs at higher energy values, that is, 60 g cal/cm²/hour or 80,000 luxes (Curtis and Juday, 1937).

Diatoms tend to show chromatic adaptation when grown under different components of light, assuming colors that are complementary to those under which they are grown. They thus tend to become yellow-green when grown in red and yellow light, and dark brown in green and blue light. For benthic green algae the red component of light is essential to healthy growth, while brown algae and, especially, red algae characteristic of deep water are adapted to carry on photosynthesis with the red component absent or reduced. We have seen also (p. 295) that, in keeping with this, the order of vertical distribution of the benthic marine algae is greens, browns, and reds, from shallow to deeper water. The quantity of chlorophyll present in these classes of algae is also diminished in the same order (Lubimenko and Tikhouskaia, 1928). The depth at which these benthic algae can grow is also directly correlated with the depth of submarine illumination. In northern latitudes the maximum depth is about 40 to 50 m, whereas in the clear waters of the Mediterranean it is said to be over 100 m. It has been suggested (Klugh, 1930) that certain pigments present in plants growing some distance below the surface may act as photosensitizers to the chlorophyll, enabling it to function with less light intensity.