ROBERT W. KRAUSS University of Maryland, College Park, Md.
Photosynthesis in the Algae Present understanding of photosynthetic efficiency, requirements, and products in the algae has stimulated research in two important fields-water purification and mass culture of algae for food use. The two are not necessarily incompatible, There is no question but that the algae may provide the most efficient sustained photosynthetic system known. The hope for a maximal production o f oxygen or a maximum production of protein by the algae rests equally on engineering ingenuity and basic research concerning the metabolism o f the organisms.
I N R E C E S T years the efforts of many laboratories have been directed toivard studies concerning the photosynthetic capacity of bacteria, algae, and higher plants. Such studies are the lineal descendents of the initial observations of Priestley (49)in 1771 that green plants in some way “freshen” the air in xshich they grow. Though experimental technique has become more sophisticated during succeeding decades, some aspects of Priestley’s puzzle are as perplexing now as they were then. Though the
complexity of the photosynthetic process has stimulated and now taxes some of the best biochemists of our time, Priestley’s inference that green plants can improve or “freshen” a n environment has received only general recognition as the obvious result of oxygen evolution. Comparatively little experimental effort has been allocated to a n evaluation of this effect. either as a direct or as a n indirect result of photosynthesis. T h e capacity of the algae to effect the purity of Lvaters as the result of photosynthesis
suggests an examination of those facets of the process Tvhich may be most significant to those interested in the biological purification of polluted Lsaters. The purpose of this paper is to review briefly the photosynthetic chardcteristics of the algae within the broader framexork of their requirements for growth and reproduction. S o attempt will be made to digest the manv volumes of literature that deal with the topic of photosynthesis even though much of it would be pertinent. A major portion VOL. 48, NO. 9
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of the information concerning the process has come from experimental work with the algae. Reviews of research in this field (20, 26, 36, 50, 57) have dealt in detail with attempts to dissect the mechanism into its component parts. Attention here is restricted to that information which may have a bearing on the roles that algae are playing or might eventually play in the biological purification of water.
Classification by Energy Source It is first necessary to recognize that not all algae are capable of photosynthesis. Of the seven major divisions, only the Phaeophjta or brown algae are all colored, photosynthetic forms. Within each of the other divisions there are species that are completely dependent on reduced carbon as a n energy source. There are also species which are dependent on other organic compounds, such as vitamins or amino acids, but which retain their photosynthetic capacity. Consequently a physiological classification of the algae has evolved, based on the energy source and the relative synthetic capacity of the organism in question (37). O u r primary concern is with the normally photosynthetic algae requiring only inorganic nutrients, carbon dioxide. and {vater, {vhich are technically classed as autotrophic-phorolithotrophs. \Vhen growing on rich organic substrates, heterotro2hic-photolithorrophs (requiring gro;vth factors to SUSrain photosynthesis) or even chenioorganotrophs (which obtain energy from reduced carbon like most bacteria) can be of importance. Often a n alga can pia)- a dual role. As a n example, the green alga Chlorella, Xvhich is of such current interest, metabolizes either as a phototroph in the light or as a facultative chemo-organotroph in the dark. Consequently not only may the time of day affect the type of metabolism but also the depth, the turbidiLy, and the organic content of the \cater will jointly determine the extent to \vhich the algae will be phototrophic or chemotrophic. The rate of oxygen evolution and carbon dioxide absorption in the light usually exceeds by a factor of 20 the reverse reaction which takes place in the dark in the absence of organic energy sources (39). If endogenous respiration is supplemenred by a n organic carbon source, the difference \vi11 be much less. Nevertheless some algae such as Chlamjdomonas eugametos are obligate autotrophs and cannot utilize exogenous organic carbon in the light or dark ( 3 5 ) . T h e complication of alternative pathways can be resolved by erecting experimental conditions which \vi11 satisfy only one. Ho\vever, data obtained in the laboratory must be subjected to interpretations
1 450
when applied to an ecologically involved situation.
tural unit CHzO will be approximately 112,000 cal. I t follows that f.h.A‘ = 112,000 X J (2)
Energy Relations
And in terms of wave length
The electromagnetic energy available for photosynthesis is of much greater magnitude than that which is used even under optimum conditions (63). T h e earth intercepts at its surface, approximately 5 X 10*0 kcal. per year (74) or a n average of 1.896 gram-ca1.i square cm./minute outside the atmosphere a t the mean solar distance at normal incidence. This will be reduced a t any given location on the earth’s surface depending on the atmospheric conditions, time of year, and latitude. Using the average hourly radiation given for Washington, D . C.., by Hand (ZZ), it is possible to estimate the energy striking the earth’s surface in similar temperate climates. If the total energy is calculated as if it kvere evenly distributed throughout the 24-hour day, the intensity ranges from 0.349 gram cal.,/square cm.,’minute for June 25 to 0.097 gram cal., square cm./minute for December 24. Less than 40% of this energy is distributed in the visible spectrum betv.een 3800 and 7600 .2. When such energy is intercepted b:r a n absorbing pigment, like chlorophyll Lvhich is photosensitive, several la\vs of physics bear on the resulting reactions. First, only the energy which is absorbed by a system can cause a chemical change (the Grotthus-Draper la\v) and secondly, only one photon of light is required- to activate a molecule of the substance to its reaction level (the Stark-Einstein law). T h e structure of the absorbing molecule will determine the quantum energy required to raise it to a n excited state. This higher energy level, resulting from the displacement of electrons from their normal orbits, persists only briefly-probably in the neighborhood of 10-9 to 10-’0 second for chlorophyll in the plant, If the quantum energy is too high-i.e., in the frequency of ultraviolet or x-rays, a n electron may be ejected and the ionized molecule disrupted chemically. If the quantum energy is too lo\\, 10 cause ionization but higher than the minimum required to cause excitation, the excess energy is lost as heat. A dkect approach to a n understanding of the energy relationship can be made from a calculation of the minimum quantum energy and equivalent tvave length required if only one quantum \vere necessary to reduce one molecule of carbon dioxide in the photosynthetic equation:
If the energy of 1 mole of glucose is 673.000 cal . then the energy of the stiuc-
INDUSTRIAL AND ENGINEERING CHEMISTRY
where f = frequency; h = Planck’s constant; S = Avogadro’s number ; J = mechanical equivalent of heat; and c = velocity. Such a calculation indicates that a wave length of 2535 4 . \\;odd be required for one photon to have sufficient energy to reduce 1 mole of carbon dioxide to CHzO (7). Radiation of such frequency is beyond the absorption band of chlorophyll and is also lethal to protoplasm. Action spectra indicate that quanta with energy contents as low as 41,000 cal., found a t 7000 .4. in the red, are sufficient to activate chlorophyll. .4bsorbed radiation a t higher frequencies results in loss of part of the energ!- of the photon. T h e chlorophyll molecule appears to act in photosynthesis from the excitation level obtained by red light. Photons of higher frequency are equal in unit effectiveness to, but no greater than those in the red. Therefore, more than 1 mole quanta or 1 Einstein must be involved in the reduction of 1 mole of CH20. Further calculations indicate that a minimum of 2.8 or 3 quanta are required assuming an effective quantum energy of 41,000 cal. T h e experimental difficulty in accurate measurement of the quantum yield and the latitude present for interpretation of the data have resulted in major disagreement concerning the actual requirement. At present there are three schools of thought on this matter. Warburg, Burk, and their colleagues maintain that the quantum yield is very high. Their earlier measurements indicated a requirement of 3 to 4 quanta per molecule of oxygen. HoLcever, more recent experiments have led them to the figure of 2.8 quanta per molecule of oxygen (70! 7 7 , 62). This requires a perfect conversion of light energy into chemical energy and the methods and interpretation of these experiments have been severely criticized as being incompatible Lvith the basic concepts of physical chemistry. Among the critics Franck (79) has calculated that the energy losses involved in the transition to the stable, triplet state of activation ivould
b Figure 1 . Representation of rnechanisrn proposed by Calvin (72) and Bassham (8) for photosynthetic assimilation of carbon. Cycle requires four reducing hydrogen atoms and energy from three ATP molecules derived from an initial photoreaction. Cycle yields one molecule of fructose diphosphate after introduction of six molecules of carbon dioxide
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be enough to raise the energy requirement for the system to 194 kcal., with no allotvance for entropy loss. This is more than the 164 kcal. carried by 4 photons of red light capable of efficient photosynthesis. Lumry, Spikes, and Eyring (36) re-estimated the probable energy loss and reached a n over-all energy requirement of 162 kcal. which would barely allow for a 4-quanta system but concluded that 8 was a safer estimate. Emerson: Arnold, and others (2, 77, 78) have measured the lower efficiency of 8 to 12 quanta. Recently Bassham, Shibata, and Calvin ( 9 ) have obtained data shoiving a 4-quantum conversion in algae which are able to obtain adenosine triphosphate from respiration, in contrast to a minimum requirement of 6 to 7 quanta where all the energy available comes from photosynthesis. In view of the Einstein law of photochemical equivalence, the mechanism by which several quanta can activate the photosynthetic reaction has also been perplexing. Burk (70) and M'arburg have proposed a cycle in which individual quanta are employed in the production of single molecules of oxygen. This suggestion, still open to experimental proof. has been criticized by Gaffron (20). Calvin (72) also has discussed the involvement of several quanta in different reactions. In the absence of general agreement concerning quantum yield? the maximum efficiency of photosynthesis must be estimated \vith caution. Under optimum conditions in red light. ivhere all the energy of the photon is utilized. the efficiency may be as high as 707, ( 4 quanta) or at least as high as 35Yc (8 quanta). Under solar radiation where many of the absorbed quanta are from the blue end of the spectrum, a maximum of 4S% might be hoped for under optimum conditions. but 20 to 25% (close to that found in the best long term experiments) appears to be the most conservative and useful estimate a t this time. Of considerable theoretical interest is the fact that algal pigments other than chlorophyll may be effective in absorbing energy Ivhich may be available in photosynthesis. The carotenoids, phycocyanins, and phycoerythrins-all pigments associated in chromatophoresabsorb and transfer energy to chlorophyll Lvhich in turn effects carbon reduction (3, 23. 21: 58). This absorption and transfer may occur in the presence of large amounts of chlorophyll which for some reason is itself made incapable of utilizing light directly ( 7 6 ) . I t is difficult to explain why a portion of the chlorophyll should be in a n inactive state under certain conditions. A more intimate understanding of the biochemistry of the process may illuminate this and certain other aspects of the problem of efficiency.
1452
-Chlorophyll
CFlL
PS CARBON-TPNH CYCLE
1
-
It\
,-Cytochrome
'
.(
Xh
*
Pod'+ ADP
2 or 3 ATP m o l e c u l e s per 2 electrons
f
I
,'
* \
ATP
-,
'.
KREBS.
c Y CLE
Figure 2. Diagram of suggested nature of photochemical apparatus and its relation to other functions O x i d a t i v e pathways
- - - ; reductive pathways-
The Biochemical Pathway Since the initial observation by Hill (2.7) that isolated chloroplasts \vi11 liberate oxygen from ivater in the presence of a suitable hydrogen acceptor, the photosynthetic process has been considered to consist of t\vo phases-the photolysis of Ivater and the reduction of carbon dioxide. Ruben, Randall, Kamen, and Hyde (52)were able to clearly demonstrate by the use of 0 ' 8 that all of the oxygen given off in normal photosynthesis comes from water. This has necessitated a modification of the photosynthetic formula to account for the loss of the oxygen of carbon dioxide. I t is obvious that four h)-drogen atoms ivould be required for the production of C H 2 0 . The major effort, however, has centered around the fate of the carbon initially assimilated. There are several possible points of entry of carbon dioxide into the metabolism of the cell. hlost suggestive has been the possibility of a reverse reaction effecting carboxylation in the Krebs cycle. i2lood and M'erkman (65) demonstrated such a reaction using a propionic acid bacterium which utilized carbon dioxide to form succinic acid. Malic enz)-me, in the presence of reduced rriphosphopyricline nucleotide (TPK). can catalyze a similar reaction in green plants (64). HoM.ever. experiments by Calvin and his group have indicated that a reverse cyclophorase cycle is not the major photosynthetic route (72). The photosynthetic cycle proposed by Calvin is diagrammed in Figure 1. The majority of the experiments leading to the construction of this cycle were performed Ivith either Chlorella or Scenedesmus. Essentially the technique has been to kill the cells immediately after exposure to labeled C l 4 0 ~which was bubbled through an illuminated culture. The pattern of distribution of organic compounds, obtained by chromatographs of all extracts, reflected the duration of
INDUSTRIAL AND ENGINEERING CHEMISTRY
(13)
the photosynthetic period. Time sequence studies, coupled Ivith organic analysis of thc radioactive components. prrmitted the development of the probable sequence of synthesis. Products of the c)-cle can be led directly into the oxidative metabolism of the alga. Phosphoglyceric acid (PG.4): in the absence of a thioctic acid block, may yield a high energy phosphate to adenosine diphosphate after enolization to give pyruvic acid which is decarboxylated to form acetyl-coenzyme A. In the light the normal course is to the forniation of glucose! sucrose, and polysaccharides \vhich> in turn: are made available as energy sources to the cell in the dark or during periods \Then photosynthesis is not possible. Study of the cyclic process for the fixation of carbon has also resulted in a suggestion concerning the link betxveen the h>-drogen produced in the chloroplast reaction and the photosynthetic cycle. Calvin observed that in the light very little PGA was permitted to enter the Krebs cycle but that; immediately after light was removed, PGA flowed rapidly into the oxidative system. The reaction which blocks the utilization of PGA may be a reduction of the coenzyme of coenzyme A, thioctic acid, to the disulfide form. T h e disulfide can be oxidized to the thiol in the dark and only in this form can i t link the acetyl group to coenzyme A. Thioctic acid, therefore, can act as a gate for PGA which is closed in the light and open in the dark. It appears that thioctic acid can also act as a hydrogen (or electron) acceptor in the Hill reaction and that some other compound acts as the oxygen acceptor. Calvin further suggests that the electrons removed in photolysis migrate to one side of a carotenoid-chlorophyllcarotenoid lamina and can be maintained at this interface until removed by thioctic acid. This presupposes a built-in field in the lamina, which would
WATER P U R I F I C A T I O N permit separation of positive and negative chdrges of the migrating atoms and which Tvould effect a larger retention of the excitation energy of the photoreaction than had been thought possible. Such a throry has support from earlier studies (771 and in recent electron microscope piciures of a laminar structure of the grana such as thosr of ChlamJdomonas and Eugifrici (.53. .iA'l. Calvin's representation Of this system is given in Figure 2. Direct erperiintntal testing of this type of system is difficult. and further examination of this stimulating hypothesis \vi11 require additional effort. Indirect support of the mechanism comes from the demonstration of a (,-quanta requirement for the process ( 9 ) . This matches thc minimum amounl of energy which has brrn calculated to be required for the cycle---i.e.. I quantum for each of the four reducing hydrogens and 2 quanta to yield 2 molecules of reduced triphosphopyridine nucleotide (TPNH) which. in turn. can be oxidized to yield the 3 nx~les of adrnosine triphosphate (.4TP) required to run the cycle. An apparent 4-quanta requirement could be obtained as long as there \vas a sufficient reservt energy source \rhich could suppl) sufficient .4TP through respiration o r other\vise. This observation could be rrconciled \vith those of LVarburp and co\iorkers if i t could be demonstrated that a sufficient energy reserve exists in algae Lvhich have been cultured a t optimum conditions prior to manometric experiments. Intermittent Illumination
Thus far our discussion has dealt \cith photosynthesis as it may operate normally in algae growing under optimum conditions. Hoivever, in nature or even in the laboratory. the alga will usually be in a n environment where one or more of the factors necessary for photosynthesis will be limiting. C n d e r such circumstances efficiency \+-ill be reduced and abnormalities in the photosynthetic mechanism can be anticipated. I t is of particular interest here to examine certain asprcts of the process \Then this is the case. Of first importance is light. Unicellular algae in natural waters are continually subjected to fluctuations in light intensity. quality. and duration. Depending on the degree of agitation or f l o ~ i .they may be subjected to hours of full sunlight. hours of complete darkness. or to any briefer intervals of light and darkness. The maximum light intensity. for the saturation of photosynthesis in Chlowlin. has been sho\v to be about 600 ft.-C. or 2.5 X lo4 ergs,.'sq. cm.sec. in contrast to a compensation point for groivrh a t about 24 ft.-C. or 10 X 10' rrgs 'sq. cm.-sec. (98). \Vith regard
to higher light intensities the response of the alga depends to a considerable estenr on the rate of photosynthesis achieved directly before being subjected to the high intensities. I n general some reduction in the rate of photosynthesis can be expected to set in somewhere bettteen 1000 and 4000 ft.-C.. usually after 15 to 30 minutes' exposure. h?yers and Burr 140) have shoizn that either Chlorella or ProtoLoccus. previously grown a t high light intensities with sufficienL carbon dioxide for optimum photosynthesis. did not sho\c a reduction in oxygen evolution under 7000 to 10,000 ft.-C. I t is clear that although injury may not set in above 600 to 1000 ft.-C. that any energy received by the cells in excess of this is not used in photosynthesis. Under conditions of light and carbon dioxide saturation the total photosynthetic reaction is limited by the socalled "dark reaction." If a masimum energy yield per unit of radiation is to be obtained from dense cultures exposed to normally high solar radiation, each cell should move in and out of the illuminated field a t a rapid enough rate to permit cells to receive the maximum quanta of light. complete the dark reaction in the dark. and be replaced in the illuminated field just soon enough to be able to utilize the next complement of quanta. This would make it possible for cells to occup!- the illuminated surface only long enough to receive the requisite energy and would allow the illuminated surface to be used more efficiently. Such a system if perfectly executed would effect a n ideal integration of intensity X time. Studies have been made to determine the period of the light flash which will permit integration of intensity X time (47: 48). With short light flashes of 0.001 second, Myers found almost perfect integration in Chloreila. Longer flashes are less perfectly integrated by the alga, but considerable gain can be achieved even a t 0.067 second. This is in agreement Lvith the earlier observations of Kok (30). From the data available it can be concluded that a n alternation of 0.001 second of light Liith 0.01 second of dark \\ill produce groitth or ox\,gen evolution q u a l t o about 9Owc of that produced bv continuous illumination. I n short. a greater photosynthetic rate can be achieved by agitation or turbulence in a natural or laboratorv culture Tvhich is dark enough and dense enough to produce a gradient of light from full intensitv to near darkness. Carbon Dioxide Supply
T h e necessity for adequate levels of carbon dioxide to saturate the photo-
synthetic reaction has been mentioned previously. Unfortunately, the determination of that level is dependent on a number of factors. It has been shown that 0.0i7Q of carbon dioxide, normally found in air, is sufficient to permit a n optimum rate of photosynthesis in algae (75: J.2). This is somewhat of a surprise considering the general use of 5.0% of carbon dioxide-in-air in laboratory studies. The explanation lies in the observation that only by driving a very strong stream of air through a small culture can a 0.03% mixture be sufficient. Lt'henever a large volume of a relatively dense culture is in contact \vith a small area of the gas phase a higher carbon dioxide concentration will be necessary. T h e level \There toxicity becomes allparent depends somewhat on p H but \vi11 generally be encountered some\ Hyde. J. L., .J. Am. Chetii. pij v i h Kernink Enzoon N . V. Soc. 63, 877 (1941). Domplein 2. Utrecht. Ger. 1952. (53) Sagar, R., Palade, G. E.; Exfit/. ( 1 - ) Emerson, R . B., .%mold, \V.%J . Gen. Cell. Research 7, 584 (1954). Physiol. 15, 391 (1941 ) , (54) Sorokin, C . : Myers, J., Science 117, (18 ) Emerson. R . E.; Lewis; C. lf.,Am. J. 330 (1953 1. Botany 28, 78': (1941). ( 5 5 ) Spoehr. H. A , , Milner, H. FV . i1 9 ) Franck, .J., Arch. Biochern. arid BisPlant Physiol. 1, 120 (1949). p h ~ s .45, 190 (1953). (56) Steffen, K., Walter. F., Planta 45, 3'13 ( 2 0 ) Gaffron. H.. in ".Autotrophic hficro(1955). organisms" (B. .A. Fry and J. L. (57) Tamiya, H., Hase, E., Shihara. K.. Minuya, '4..Iwamura, T.! Xihri. Peel, editors), pp. 152-85. CamT., Sasa, T.: in Publ. 600, pp. bridge Univ. Press. Cambridge. 204-32. Carnegie Institiition of Eng., 1954. Washington. 1953. (21 I Gotaas. H . B.: Oswald, LV. J.. LudTanada. T.? Am. J . Hotanj 38, 2'6 wig. H. F.. Scr'. .\fmth/! 79, 368 (1951 ). (1954). Tarzwell, C. hi., Palmer, C. lf.!.I. ( 2 2 1 Hand. I . F.. .\io. Tleather Rezm. 65, .4m. i t h t e r Tl'orks .4ssnc. 43, 568 415 (1937). (1951). (23) Hawo. F. T . Blinks, L. R.. J Gen. Thomas, W.H., Krauss, R. W.: Plarrt Phlrioi. 33, 389 (1950) Physiol. 30, 113 (1955). ( 2 4 ) Haxo. F. T.. O'hEocha. C.. Norris. Van der Honert, T. H., Rec. ttaz'. Phyllis. Arch. Riochm. and Biobotan. nperl. 27, 149 (1930). phys. 54, 162 (1955). Lt'arburg, O., Burk, D., Schocken, ( 2 5 ) Hill. R.. and Scarisbrick, R.. .\ht/c,-e, V., Riochem. P t Binphys. Acfa 4 , 335 146, 61 (1940). 11950). 9 6 ) Hill, R.. LVhittingham, C. P.. "Pho163) Lt'hssink. E. C.. Kok. B., Oorschot. tosynthesis." hfethuen. London, and J. L. P. van, in Publ. 600, pp. M'iley. New York. 1955, 55-62, Carnegie Institution of Washington, 1953. (2') Hutner, S. H., Provasoli, L.. Schatz. A , . Haskins, C . P.. P ~ o c . ~ m . (64) Whatley, F. R . , 'vert' Ph?.'foi. 50, 244 (1951 ). Philosophical Soc. 94, 152 (1951 ), ( 2 8 ) Ketchum, B, H., R ~ plarlt ~ , (65) M'ood, H. G., [Verkman, c:. H., Biochem. J . 30, 48 (1936). Physiol. 5 , 55 (1954). ( 6 6 ) Yoneda, Y., .2femoirs Coll. Agriculbiip, 129) Kok, B., Enzjmalogia 13, 1 (1948,. Kyoto 1;nii'. 62, 1 (1952). 130) Kok. B.. in Publ. 600, pp, 63-75, Carnegie Institution of IVashingRECEIVEDfor review January 1 2 , 3956 ton. 1953. .ACCEPTED July 2, 1956 8'
Conclusions
pf
Kratz, W. h.)Myers, J., rim. .I. Botany 42, 282 (1955). Krauss, R . N'.,in Publ. 600, pp. 85 102. Carnegie Institution of Wadiington. 1953. Krauss. R. LV., Sei. ,lIonfhly 80, 11 (1955). Krogh, .A,. Lange, E., Smith, \t'.. Biochem. J . 24. 1666 11930,. Lewin. Joyce d., Science 112, 652 (1950). Lumry, R., Spikes, J. D.. Ilyrinq. H., Ann. Re&$.Plant Physrol. 5, 2'1 ( 1 954). (37) Lwoff, A., "Biochemistry and Physiology of Protozoa." .icadcmic Press, Piew York, 1951. ' lfoon, P., J . Franklin Inst. 230, 583 (1940). htyers, J., ,471~ Rer,. .\ficrobiol. 5 , 13(1951). Gii. Myers, J., Burr, G. 0.: Physiol. 24, 45 (1940). hlyers, J. Phillips, J. N., Graham. .1. R., Plant Physzol. 26, 539 (1951 I . Sielsen, E. S.. Physiol. Plnniarzcirr 8 , 31- (19%). Osterlind. S.. Ibid.,1, 1'0 ( 1 9 4 8 !. Zbid.. 3, 353 (1950). Oswald, LV. J.. Gotaas, H . B. Ludw i g H . F.. Lynch, Victor age and I d . Il'astes 25, 26 (1 Oswald, W.J . . Gotaaa, H. B.. Sanitary Engr. Div.. Am, Soc. Civil Enyrs.. 1954. (Available from Univrrsitv of California, Berkeley, Calif.) Phillips, J. 17.: Myers. J.. Planf Phi > i d . 29, 148 (19541. Ibid.. D. 152. Priestley, J.! Phil. Trans. KO!. S u i . London 62, 147 (1772). Rabinowitch, E. I., "Photosynthcsis and Related Processes," vol. 1, Interscience, New Y o r k , 1945. , I
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DISCUSSION
.. .
Photosynthesis in the Algae 4
R e s e a r c h on photosynthesis has made evident in recent years the great complexity of the process. Instead of carbon dioxide and water combining directly to form a carbohydrate with oxygen as a by-product, there are now considered to be as many as twenty successive steps and numerous variations in these according to the particular environment or plant involved. The algae have been found to be well adapted for metabolic studies because of their simplicity of body structure, their rapid growth, their aquatic habitat, and the opportunity to work with them as pure cultures, free from all other organisms. Krauss has given an excellent description of the present knowledge of the photosynthetic mechanism in algae. Comments referring to his paper are grouped under the following headings: carbohydrates, carbon dioxide, oxygen, chlorophyll, light, and photosynthesis.
Carbohydrates Algae are commonly recognized as the best organisms for the production of carbohydrates and other organic compounds from polluted water. Because of the potential economic value of these compounds, it seems probable that most sewage treatment processes. in the future, will be judged for their effectiveness and efficiency in changing wastes into useful products rather than into inorganic materials which invariably stimulate the excessive grolvth of nuisance and interference organisms. There are indications that many uses might be found for the products or algal photosynthesis. One of the biggest dra\vbacks a t present is the lack of a practical method for harvesting the algae-i.e., for separating the algae from the water.
Carbon Dioxide The author has pointed out that carbon dioxide utilization in phorosynthesis is not limited merely to rhe initial step. Instead, there are several points for its entry into the process. It’hile the 0.03y0 of carbon dioxide normally found in air is sufficient to permit an optimum rate of photosynthesis in algae. concentrations as high as 5% of carbondioxide may be needed for dense laboratory cultures. This naturally raises the question as to whether the concentration of carbon dioxide in streams and lakes might be a factor influencing the development of algal blooms.
1 456
Krauss mentions that in natural water steep gradients of carbon dioxide occur which may range from toxic levels in some areas to deficiency in others. Algal photosynthesis in such water can change the carbon dioxide content which affects the amounts of half-bound and bound carbonates as well as the pH of the water. It is obvious that this relationship must be taken into consideration when figures are given concerning any one of these factors, since they can vary according to the amount of photosynthesis taking place a t that particular time.
Oxygen A s stated by Krauss algae may release twenty times as much oxygen in photosynthesis as they utilize in metabolism. The significance of this in the biological purification of polluted waters is obvious since rapid decomposition of the wastes depends primarily on aerobic bacteria. Oxygen becomes a limiting factor in the speed of decomposirion when the concentration drops much belo\v 1 p.p.m., and higher levels may be required for some materials. It is necessary to point out that the older photosynthetically inactive algal cells can be a detriment rather than an aid in the release of oxygen. Figures on the B O.D. of polluted streams are indicaLive of the need for available oxygen \
