Treatments of inorganic nutrition tend to become listings of poorly related facts and observations. As more information becomes available, this should give way to a drawing together of observations in an attempt to find new insights into the metabolic roles of nutrients and the mechanisms of their assimilation. In the present review, an attempt is made to do this, borrowing information from the physiology of bacteria, fungi, and higher plants where they augment observations on algae. In doing so, some erroneous suggestions are probable, but hopefully areas of needed research will be pointed out.

Inorganic nutrient uptake and deficiency in algae.

The uptake and utilization of organic carbon by algae: an essay in comparative biochemistry*

Nitrogen removal from wastewater by algae provides the potential benefit of producing lipids for biodiesel and biomass for anaerobic digestion. Further, ammonium is the renewable form of nitrogen produced during anaerobic digestion and one of the main nitrogen sources associated with wastewater. The wastewater isolates Scenedesmus sp. 131 and Monoraphidium sp. 92 were grown with ammonium, nitrate, or urea in the presence of 5 % CO2, and ammonium and nitrate in the presence of air to optimize the growth and biofuel production of these chlorophytes. Results showed that growth on ammonium, in both 5 % CO2 and air, caused a significant decrease in pH during the exponential phase causing growth inhibition due to the low buffering capacity of the medium. Therefore, biological buffers and pH controllers were utilized to prevent a decrease in pH. Growth on ammonium with pH control (synthetic buffers or KOH dosing) demonstrated that growth (rate and yield), biodiesel production, and ammonium utilization, similar to nitrate- and urea-amended treatments, can be achieved if sufficient CO2 is available. Since the use of buffers is economically limited to laboratory-scale experiments, chemical pH control could bridge the gap encountered in the scale-up to industrial processes.

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Growth, nitrogen utilization and biodiesel potential for two chlorophytes grown on ammonium, nitrate or urea

Specific growth rates (SGR) of freshwater algae species (Chlorella vulgaris, Auxenochlorella protothecoides, and Chlorella sorokiniana) and the marine species Nannochloropsis oculata on various nitrogen sources (ammonium carbonate, ammonium chloride, sodium nitrate, and urea) could be determined by in vivo chlorophyll-a autofluorescence. These preferences could be determined before large pH changes occurred in the media, with no significant difference (P > 0.05) between buffered and non-buffered media. In all algal species, acclimatization was observed with no significant difference (P > 0.05) between SGRs of the second and third cultivations. ANOVA of SGRs in the acclimatized second and third cultivations revealed preferences for nitrogen sources among most of the algae; C. vulgaris preferred sodium nitrate over other nitrogen sources, A. protothecoides adapted to urea after no growth in the first cultivation, and the SGRs of N. oculata showed an aversion for sodium nitrate over other nitrogen sources (P < 0.05).

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Effect of nitrogen source and acclimatization on specific growth rates of microalgae determined by a high-throughput in vivo microplate autofluorescence method

An enzyme has been purified from the green alga Chlorella which transfers sulfate from APS onto glutathion and dithiotreitol. The enzyme does not utilize PAPS unless a crude fraction containing a 3′-phosphonucleotidse is used. From the data it is assumed that in photosynthetic sulfte reduction of plants APS rather than PAPS transfers its sulfte group onto a carrier-bound SH-group—replaceable in vitro by glutathion—to form an S-sulfocompound which then might be reduced to the level of sulfide.

On the mechanism of photosynthetic sulfate reduction

Urea metabolism was studied with nitrogen-starved cells of Chlorella vulgaris Beijerinck var. viridis (Chodat), a green alga which apparently lacks urease. Incorporation of radioactivity from urea-14C into the alcohol-soluble fraction was virtually eliminated in cell suspensions flushed with 10% CO2 in air. This same result was obtained when expected acceptors of urea carbon were replenished by adding ornithine and glucose with the urea. Several carbamyl compounds, which might be early products of urea metabolism and a source of the 14CO2, were not appreciably labeled. If cells were treated with cyanide at a concentration which inhibited ammonia uptake completely and urea uptake only slightly, more than half of the urea nitrogen taken up was found in the medium as ammonia. Cells under nitrogen gas in the dark were unable to take up urea or ammonia, but the normal rate of uptake was resumed in light. Since 3-(3,4-dichlorophenyl)-1,1-dimethylurea did not selectively inhibit this uptake, an active respiration supported by light-dependent oxygen evolution in these cells was ruled out. A tentative scheme for urea metabolism is proposed to consist of an initial energy-dependent splitting of urea into carbon dioxide and ammonia. This reaction in Chlorella is thought to differ from a typical urease-catalyzed reaction by the apparent requirement of a high energy compound, possibly adenosine triphosphate.

Robert C. Hodson

Papers

Metabolism of Urea by Chlorella vulgaris