Internal nitrogen pools in thalli of Gracilaria tikvahiae McLachlan were examined in three experiments as a function of total nitrogen content of the thallus, nitrogen deprivation, and nitrogen resupply. Amino acids and proteins appeared to form the major nitrogen storage pools in G. tikvahiae, while DNA appeared to be relatively unimportant in this regard. Inorganic nitrogen in the forms of NH4+ and NO3− was found in the thalli; however, its contribution to the total nitrogen, pools was small. Within the protein pool, the phycoerythrin pigments appear important as a source of nitrogen when thalli are initially becoming nitrogen limited. In general, there was an inverse relationship between the levels of nitrogen and the carbohydrate content of the algal thalli.

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Laboratory experiments were conducted to determine the effectiveness of three floating and six emergent aquatic macrophytes in improving domestic wastewater quality, based on their capacities for O2 transport into the effluent. Oxygen transport into the rooting zone of the plants created an oxidized microenvironment, thereby stimulating C and N transformations critical to wastewater treatment. Plants were cultured in flasks containing deoxygenated primary and secondary sewage effluent for an 8-d period. Oxygen transport by the plants was measured in terms of both O2 consumed by the effluent (biological 02 demand reduction--BODs) and increased effluent dissolved 02. Two floating plants, pennywort (Hydwcotyle nmbellata L.) and waterhyacinth [Eichkornia crasslpes (Mart.) Solms], and the emergent plants pickerelweed (Pontederla cordata L.) and common arrowhead (Sagittaria latifolia L.), were superior in improving primary sewage effluent quality, by reducing BOD5 up to 88%, NH4-N up to 77%, and increasing dissolved O2 up to 6.1 mg L -I. Nitrification rates in pennywort-and water hyacinth-based water treatment systems were calculated to be in the range of 12 to 47 kg NH 4-N ha -1 d -!. Oxygen transport through plants accounted for up to 90% of the total O2 transported into the effluent. In separate batch experiments, the effectiveness of diffuse mechanical aeration (5 and 50 mL air min -t) and of biological aeration (02 transport by selected plants including pennywort, waterhyacinth, pickerelweed, and common arrowhead) on the rate of contaminant removal from deoxygenated primary sewage effluent were compared for a 26-d period. Biological and mechanical aeration effected similar BODs removal. First-order reaction rate constants for BOD~ removal were from 0.0066 to 0.0079 h -~ and from 0.0041 to 0.0051 h -1 for biological and mechanical aeration, respectively. Rate constants for NH4-N removal were from 0.0024 to 0.0107 h -~ for the plant treatments. Virtually complete BOD~ removal occurred in biological and mechanical aeration treatments within 20 d. Complete nitrification of NH4-N had occurred within 12 d after mechanical aeration was initiated, but subsequent N-loss by denitrification was inhibited. In the biological aeration treatments, negligible effluent (NO 3 + NO2)-N levels were measured, but 65 to 100% NH4-N loss occurred both by plant assimilation and by sequential nitrification-denitrification reactions.A QUATIC PLANTS rooted in anaerobic sediments and anoxic waters transport 02 through stems and leaves into their rooting zones. The mechanism ofO2 transport through aquatic plants into the rooting zone has been demonstrated by several researchers (Armstrong

Research, pilot-scale and operational studies conducted within the past 15 years have shown that aquatic macrophyte-based treatment systems offer a promising, low-cost method for removing contaminants from wastewaters and polluted natural waters. The vascular plants cultured in such treatment systems perform several functions, including assimilating and storing contaminants, transporting O2 to the root zone, and providing a substrate for microbial activity. Among the various types of aquatic treatment systems, pond systems containing floating macrophytes such as the water hyacinth are most commonly utilized for wastewater treatment in tropical and sub-tropical regions, whereas in temperate regions, emergent plants cultured in artificial wetlands (e.g., root zone method, nutrient film techniques) appear to be more appropriate. However, due to concerns about system management and reliability, aquatic plant treatment systems are currently used only on a limited basis throughout the world. This review discusses the general performance, contaminant removal processes and criteria for plant selection in aquatic macrophyte wastewater treatment systems. Case studies on the use of floating plants for domestic wastewater treatment and the renovation of eutrophic lake water are presented, and future research needs for aquatic macrophyte-based wastewater treatment systems are discussed.

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