Anaerobic digestates are useful nutrient sources for microalgae cultivation: functional coupling of energy and biomass production

We investigated the extent to which nitrogenous and phosphorus nutrients from liquid anaerobic digestates could be recycled for photosynthetic growth of a microalga, Scenedesmus sp. AMDD. Digestates recovered from the anaerobic digestion of cow manure and swine manure and a co-digestion of swine manure and algal biomass were diluted in distilled water and used for algal growth with and without supplemental CO₂ addition. Nutrient assimilation and final biomass yield were retarded in all but the swine manure/algae co-digestate cultures supplemented with high CO₂. Swine manure digestate cultures supplemented with the typical complement of micronutrients normally added with a commonly used growth medium or with Fe/EDTA failed to grow any better than unamended controls. When the culture medium was prepared by blending swine manure digestate with 25 or 50 % algal biomass digestate, diluting it with lake water or by supplementing with magnesium, nutrient assimilation and final algal biomass yields were maximized, indicating that magnesium was critically limiting for algal growth in swine manure digestates. Magnesium amendment thus appears to be essential if nutrients from swine manure digestates are recycled for algal growth. No such requirement is necessary for recycling nutrients from digestates generated wholly or in part from algal biomass.     

Regulation of blue-green algal buoyancy and bloom formation by light,inorganic nitrogen,C02, and trophic level interactions

In highly eutrophic ponds, buoyancy of the gas-vacuolate blue-green alga Anabaenopsis Elenkinii (Miller) was regulated by complex interactions between chemical and physical parameters, as well as by biological interactions between various trophic levels. Algal buoyancy and surface bloom formation were enhanced markedly by decreased light intensity, and to a lesser extent by decreased CO₂ availability and increased availability of inorganic nitrogen. In the absence of dense populations of large-bodied Cladocera, early season blooms of diatoms and green algae reduced light availability in the ponds thus creating conditions favorable for increased buoyancy and bloom formation by A. Elenkinii. The appearance of blue-green algal blooms could be prevented by a reduced density of planktivorous fish, which allowed development of dense cladoceran populations. The cladocerans limited the growth of precursory blooms of diatoms and green algae, and given the resulting clear-water conditions, buoyancy of A. Elenkinii was reduced, and blue-green algal blooms never appeared.     

Rising CO2 Levels Will Intensify Phytoplankton Blooms in Eutrophic and Hypertrophic Lakes

Harmful algal blooms threaten the water quality of many eutrophic and hypertrophic lakes and cause severe ecological and economic damage worldwide. Dense blooms often deplete the dissolved CO₂ concentration and raise pH. Yet, quantitative prediction of the feedbacks between phytoplankton growth, CO₂ drawdown and the inorganic carbon chemistry of aquatic ecosystems has received surprisingly little attention. Here, we develop a mathematical model to predict dynamic changes in dissolved inorganic carbon (DIC), pH and alkalinity during phytoplankton bloom development. We tested the model in chemostat experiments with the freshwater cyanobacterium Microcystis aeruginosa at different CO₂ levels. The experiments showed that dense blooms sequestered large amounts of atmospheric CO₂, not only by their own biomass production but also by inducing a high pH and alkalinity that enhanced the capacity for DIC storage in the system. We used the model to explore how phytoplankton blooms of eutrophic waters will respond to rising CO₂ levels. The model predicts that (1) dense phytoplankton blooms in low- and moderately alkaline waters can deplete the dissolved CO₂ concentration to limiting levels and raise the pH over a relatively wide range of atmospheric CO₂ conditions, (2) rising atmospheric CO₂ levels will enhance phytoplankton blooms in low- and moderately alkaline waters with high nutrient loads, and (3) above some threshold, rising atmospheric CO₂ will alleviate phytoplankton blooms from carbon limitation, resulting in less intense CO₂ depletion and a lesser increase in pH. Sensitivity analysis indicated that the model predictions were qualitatively robust. Quantitatively, the predictions were sensitive to variation in lake depth, DIC input and CO₂ gas transfer across the air-water interface, but relatively robust to variation in the carbon uptake mechanisms of phytoplankton. In total, these findings warn that rising CO₂ levels may result in a marked intensification of phytoplankton blooms in eutrophic and hypertrophic waters.     

Effect of pH on Inorganic Carbon Uptake in Algal Cultures

Biomass production by the green algae Scenedesmus obliquus and Chlorella vulgaris in intensive laboratory continuous cultures was considerably affected by the pH at which the cultures were maintained. Carbon photoassimilation experiments revealed that pH values in the range of 8 to 9 were important for determining the free CO₂ concentrations in the medium. With higher pH values, additional pH effects were observed involving a decrease in the relative high affinity of low CO₂-adapted algae to free CO₂. The carbon uptake rate by high CO₂-adapted algae after transfer to low free CO₂ medium was characterized by a lag period of about 30 min, after which the affinity of the algae to CO₂ increased considerably. Both continuous growth and carbon uptake experiments indicated that artificially maintained high free CO₂ concentrations are recommended for maximal production in intensive outdoor algal cultures.     

An inexpensive inorganic medium for the mass cultivation of freshwater microalgae

A new and inexpensive inorganic medium (‘DS medium’) for the mass cultivation of freshwater blue-green algae (Cyanobacteria) and green algae has been developed. It consists basically of distilled or demineralized water (90%) and seawater (10%) and requires only little addition of pure purchasable chemicals (phosphate, trace elements, if necessary nitrate). No addition of macronutrients (NaCl, MgCl₂ or MgSO₄, KCl or K₂SO₄, CaCl₂) and of boron is required because they are sufficiently provided by the seawater. Decalcified water may also be suitable instead of demineralized water. For the cultivation of green algae, a higher trace element concentration is recommended than for blue-green algae. Because of its low total salt concentration the DS medium is freshwater-like. It is easy to prepare and effects rapid algal growth. It may be of special value for algal mass culture in regions which are close to the sea.     

WATER-BLOOMS

1. Peculiarities in the ecology of planktonic blue-green algae are reviewed in relation to recent advances in understanding their physiological characteristics. 2. Dense water-blooms are always the result of buoyant migration of existing populations to the lake surface under calm weather conditions. The size of the population is the direct result of photoautotrophic growth, and is dependent upon light and the availability of inorganic nutrients; it is apparently enhanced by moderately high water temperatures, high pH, low oxygen tensions and possibly, the presence of organic solutes. The relative effectiveness of these factors is untested. 3. Buoyancy is imparted by gas vacuoles whose principal function is to regulate the position of the alga in the water column. Control is effected by two mechanisms: (i) ‘dilution’ of newly produced vacuoles during active cell division; (ii) changes in cell turgor-pressure acting on the gas-vacuole structure. Gas-vacuole production is greatest at low light intensities and the alga becomes more buoyant; at higher light intensities, increased turgor-pressure collapses the weaker vacuoles causing the alga to lose buoyancy. 4. Potentially, algae are able to poise themselves at an optimum point in the light gradient, usually towards the bottom of the euphotic zone, where the algae are likely to encounter the conditions most favouring their growth. 5. Different species of blue-green algae differ in the typical sizes of their colonies and, hence, in their rates of controlled movement. These differences are interpreted as hydrodynamic adaptations to the variations in turbulent water movements to which the algae are subject. 6. Populations of single-filamentous Oscillatoria agardhii and O. rubescens come to occupy the stable metalimnia of stratified lakes, provided that they are located within the euphotic zone. 7. The large stream-lined colonial forms occur mainly in polymictic lakes and in the unstable epilimnia of stratified lakes where light penetration is restricted to the superficial layers. These algae are adapted to sink or float rapidly to the optimum depth when turbulence subsides. Because of their potentially high rates of movement, it is the large colonial forms that commonly form blooms. 8. Bloom formation can occur when most of the algae possess excess buoyancy. Excess buoyancy is acquired when the photosynthetic rate is insufficient to develop the necessary turgor-pressure to cause collapse of the vacuoles. Photosynthesis may be sufficiently impaired under four circumstances: (i) during turbulent circulation of the population over a depth that significantly exceeds the euphotic depth; (ii) in the absence of light (e.g. at night): (iii) at limiting concentrations of carbon dioxide: and (iv) when the algal population is senescent. 9. Because bloom-formation depends upon the coincidence of persistent algal overbuoyancy with calm weather, its occurrence is incidental, and serves no vital function in the biology of blue-green algae. 10. Some possible causes for the occurrence of blue-green algal blooms in a relatively restricted range of water bodies are discussed. Large bloom-forming populations are probably restricted to moderately rich, mildly alkaline, thermally unstable lakes in all regions, except those which are permanently cold. Extremes of poverty or richness of nutrients, short water-retention times and low pH seem to be factors which select against planktonic blue-green algae.     

Preference of carbon absorption determines the competitive ability of algae along atmospheric CO2 concentration

Although many studies have focused on the effects of elevated atmospheric CO₂ on algal growth, few of them have demonstrated how CO₂ interacts with carbon absorption capacity to determine the algal competition at the population level. We conducted a pairwise competition experiment of Phormidium sp., Scenedesmus quadricauda, Chlorella vulgaris and Synedra ulna. The results showed that when the CO₂ concentration increased from 400 to 760 ppm, the competitiveness of S. quadricauda increased, the competitiveness of Phormidium sp. and C. vulgaris decreased, and the competitiveness of S. ulna was always the lowest. We constructed a model to explore whether interspecific differences in affinity and flux rate for CO₂ and HCO₃ ⁻ could explain changes in competitiveness between algae species along the gradient of atmospheric CO₂ concentration. Affinity and flux rates are the capture capacity and transport capacity of substrate respectively, and are inversely proportional to each other. The simulation results showed that, when the atmospheric CO₂ concentration was low, species with high affinity for both CO₂ and HCO₃ ⁻ (HCHH) had the highest competitiveness, followed by the species with high affinity for CO₂ and low affinity for HCO₃ ⁻ (HCLH), the species with low affinity for CO₂ and high affinity for HCO₃ ⁻ (LCHH) and the species with low affinity for both CO₂ and HCO₃ ⁻ (LCLH); when the CO₂ concentration was high, the species were ranked according to the competitive ability: LCHH > LCLH > HCHH > HCLH. Thus, low resource concentration is beneficial to the growth and reproduction of algae with high affinity. With the increase in atmospheric CO₂ concentration, the competitive advantage changed from HCHH species to LCHH species. These results indicate the important species types contributing to water bloom under the background of increasing global atmospheric CO₂, highlighting the importance of carbon absorption characteristics in understanding, predicting and regulating population dynamics and community composition of algae.     

The possible roles of algae in restricting the increase in atmospheric CO2 and global temperature

ABSTRACT

Anthropogenic inputs are increasing the CO₂ content of the atmosphere, and the CO₂ and total inorganic C in the surface ocean and, to a lesser degree, the deep ocean. The greenhouse effect of the increased CO₂ (and, to a lesser extent, other greenhouse gases) is very probably the major cause of present global warming. The warming increases temperature of the atmosphere and the surface ocean to a greater extent than the deep ocean, with shoaling of the thermocline, decreasing nutrient flux to the surface ocean where there is greater mean photosynthetic photon flux density. These global changes influence algae in nature. However, it is clear that algae are important, via the biological pump, in decreasing the steady state atmospheric and ocean surface CO₂, and thus decreasing radiative forcing, a reduction enhanced by algal increases in albedo. As well as these natural processes there are possibilities that algae can, with human intervention, partly offset the increase in atmospheric CO₂. One possibility is to grow algae as sources of fuel for transport, in principle providing an energy source that is close to CO₂-neutral. The other possibility is to increase the role of algae in sequestering CO₂ as organic C over periods of hundreds or more years in the deep ocean and marine sediments and/or increasing albedo and decreasing radiative forcing of temperature. There are problems, currently unresolved, in the economically viable production of algal biofuels without carbon trading subsidies. Enhanced algal CO₂ sequestration also has costs, both in resource input (phosphorus (P) from high P content rocks, a limited resource with a competing use as an agricultural fertilizer) and adverse environmental effects. For example, ocean anoxic zones producing N₂O and increased algal production of short-lived halocarbons by algae that both, through breakdown, destroy O₃ and increase UV flux to the Earth’s surface.     

Photosynthesis of submersed macrophytes in acidified lakes II. Carbon limitation and utilization of benthic CO2 sources

Photosynthetic characteristics of carbon-dioxide limitations were analyzed for leaf tissue in a Cartesian-diver system, in which irradiance could be stringently controlled, and with whole plants in electrode macrosystems for submerged macrophytes (Juncus bulbosus L., Sphagnum auriculatum Schimp. var. inundatum (Russow) M. O. Hill) and other benthic moss and algae (Drepanocladus, Batrachospermum, and an algal mat) from acidified lakes. Light compensation points were extremely low for Juncus (1.5–6 μE m^?2 s^?1) and Sphagnum (3–10), and higher for shallow-inhabiting Batrachospermum (22–33). Leaf tissue, whole plants, and algal populations were rapidly limited by CO₂ availability under closed submersed, acidified conditions (pH 4–6).Controlled and in situ experiments were performed, in which the rooting tissue of Juncus bulbosus was partitioned from the leaves and the rates of photosynthetic carbon fixation of the foliage, utilizing dissolved inorganic ¹⁴C-carbon from the water, were analyzed under different conditions of CO₂ enhancement in the rhizosphere of the sediments. Results demonstrated that: (a) from 25 to 40% of the carbon fixed in the leaves can originate from the rhizosphere, diffuse to the leaves via internal gas lacunae, and be fixed photosynthetically; (b) photosynthetic utilization of CO₂ from the water surrounding the leaves is reduced markedly when the CO₂ concentration of the rhizosphere was increased by direct additions of CO₂, bacteria, or organic compounds (glucose, acetate) that stimulate bacterial growth. Shifts to predominance of submersed benthic primary producers with low light compensation points and adapted to acidified lakes are related in part to circumvention of carbon limitation in the water by utilization of enhanced CO₂ availability in the rhizosphere and at the sediment—water interface from bacterial degradation of organic matter, and in part to physiological mechanisms that conserve and recycle CO₂ of respiration and photorespiration.     

INTERACTIONS BETWEEN LIGHT,NH4+, AND CO2 IN BUOYANCY REGULATION OF ANABAENA FLOS-AQUAE (CYANOPHYCEAE)1

Buoyancy of the gas-vacuolate alga Anabaena flosaquae Brébisson was measured under various levels of light, NH₄⁺, and CO₂. At high irradiance (50 μE · m^?2·^?1) the alga was non-buoyant regardless of the availability of CO₂ and NH₄⁺. At low irradiance (≤10 μE · m ^?2· s^?1) buoyancy was controlled by the availability of NH₄⁺ and CO₂. When NH₄⁺ was abundant, algal buoyancy was high over a wide range of CO₂ concentrations. In the absence of NH₄⁺, algal buoyancy was reduced at high CO₂ concentrations, however as the CO₂ concentration declined below about 5 μmol · L^?1, algal buoyancy increased. These results help explain why gas vacuolate, nitrogen-fixing blue-green algae often form surface blooms in eutrophic lakes.     

Inverse relationship of epilithic algae and pelagic phosphorus in unproductive lakes: Roles of N2 fixers and light

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Process-based simulation library SALMO-OO for lake ecosystems. Part 1: Object-oriented implementation and validation

The process-based simulation library SALMO-OO represents an object-oriented implementation of mass balance equations for pelagic food webs consisting of diatoms, green algae, blue-green algae and cladocerans, as well as nutrient cycles based on PO₄–P, NO₃–N, DO and detritus in lakes. It gains its structural flexibility from alternative process representations for algal growth and grazing, and zooplankton growth and mortality.Generic model structures of SALMO-OO were determined for three lake categories classified by circulation types reflecting the local climate and morphometry of lakes, and by trophic states reflecting community structures and habitat properties of lakes.Data of six lakes belonging to either of the three lake categories were used to validate the lake category specific structures of SALMO-OO.The study has demonstrated that: (1) JAVA suits well for object-oriented implementations of ecological simulation libraries, (2) object-oriented simulation libraries facilitate the identification of generic model structures for ecosystem categories.     

Phosphorus availability and nitrogenase activity in aquatic blue-green algae

Metabolically active phosphorus-starved cultures of blue-green algae assimilate ³²P rapidly in the light and in the dark. The uptake of phosphorus results in a rapid (within 15 min) stimulation in acetylene reduction by Anabaena cylindrica, A. flosaquae, Anabacnopsis circuiaris and Chlorogloea fritschii, with a response being obtained to less than 5 μg/1 of phosphorus. Uptake of phosphorus also causes a rapid increase in respiration in the dark but not in photo respiration, and the size of the cellular ATP pool and the ¹⁴CO₂ fixation rate both increase more slowly. The metabolism of phosphorus-sufficient cells, which assimilate phosphorus more slowly, shows little response when phosphorus is provided. Excess phosphorus is stored in the vegetative cells of blue-green algae as polyphosphate bodies which may form within 60 min of adding phosphorus to phosphorusstarved cells and which serve as a source of phosphorus for the algae when exogenous phosphorus is limiting. Preliminary results from Scottish waters suggest that urban effluents are important sources of available-phosphorus for algal growth and that the levels entering fresh waters from agricultural land are, per unit volume, lower. In both types of water the levels of available-phosphorus are rather similar to the levels of orthophosphate-phosphorus present. Most detergents tested serve as a source of phosphorus for nitrogen-fixing blue-green algae and cause a rapid stimulation in reduction when added to phosphorus-starved cultures. Of the detergents assayed, the biological types were richest in available phosphorus. The addition of detergents may result in a rapid increase in number of polyphosphate bodies present in the algae. Detergents in general also contain an inhibitor of algal metabolism. Whether a stimu-lation or an inhibition occurs depends on the quantities of detergent added and on whether or not the alga is phosphorus-deficient.     

Inorganic carbon promotes photosynthesis,growth, and maximum biomass of phytoplankton in eutrophic water bodies

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Root growth and functioning under atmospheric CO2 enrichment

This paper examines the extent to which atmospheric CO₂ enrichment may influence growth of plant roots and function in terms of uptake of water and nutrients, and carbon allocation towards symbionts. It is concluded that changes in dry matter allocation greatly depend on the experimental conditions during the experiment, the growth phase of the plant, and its morphological characteristics. Under non-limiting conditions of water and nutrients for growth, dry matter partitioning to the root is not changed by CO₂ enrichment. The increase in root/shoot ratio, frequently observed under limiting conditions of water and/or nutrients, enables the plant to explore a greater soil volume, and hence acquire more water and nutrients. However, more data on changes in dry matter allocation within the root due to atmospheric CO₂ are needed. It is concluded that nitrogen fixation is favored by CO₂ enrichment since nodule mass is increased, concomitant with an increase in root length. The papers available so far on the influence of CO₂ enrichment on mycorrhizal functioning suggest that carbon allocation to the roots might be increased, but also here more experiments are needed.Abbreviations LAR leaf area ratio - LWR leaf weight ratio - SWR stem weight ratio - RGR relative growth rate - R/S root/shoot - RWR root weight ratio     

Observations of Toxic Blue-Green Algae (Cyanobacteria) in Some Scandinavian Lakes

Blooms of blue-green algae from 51 eutrophic Scandinavian lakes were investigated during the period 1978–1984, to ascertain the occurrence of toxinogenic species. Toxicity assays were performed by intraperitoneal injection of suspensions of freeze-dried algal material in mice. Toxin-producing blue-green algae were found in 30 lakes. They belonged to 11 different species of the six genera Anabaena, Aphanizomenon, Gomphosphaeria, Microcystis, Nodularia and Oscillatoria. The presence of toxinogenic strains of blue-green algae seemed quite constant in several of the localities studied. In some lakes, more than one toxic species were found to develop simultaneously. The level of toxicity showed large variation (MLD100, 6 to > 2500 mg/kg), but clinical and pathological changes were quite uniform. The results indicate that water-blooms of toxin-producing blue-green algae, in the geographical area in question, are regionally widespread. In some localities, blooms of blue-green algae are apparently always toxic. Several aspects of the toxic blue-green algae problem are discussed.     

Stoichiometric impacts of increased carbon dioxide on a planktonic herbivore

The partial pressure of carbon dioxide (pCO₂) in lake ecosystems varies over four orders of magnitude and is affected by local and global environmental perturbations associated with both natural and anthropogenic processes. Little is known, however, about how changes in pCO₂ extend into the function and structure of food webs in freshwater ecosystems. To fill this gap, we performed laboratory experiments using the ecologically important planktonic herbivore Daphnia and its algal prey under a natural range of pCO₂ with low light and phosphorus supplies. The experiment showed that increased pCO₂ stimulated algal growth but reduced algal P : C ratio. When feeding on algae grown under high pCO₂, herbivore growth decreased regardless of algal abundance. Thus, high CO₂‐raised algae were poor food for Daphnia. Short‐term experimental supplementation of PO₄ raised the P content of the high CO₂‐raised algae and improved Daphnia growth, indicating that low Daphnia growth rates under high pCO₂ conditions were due to lowered P content in the algal food. These results suggest that, in freshwater ecosystems with low nutrient supplies, natural processes as well as anthropogenic perturbations resulting in increased pCO₂ enhance algal production but reduce energy and mass transfer efficiency to herbivores by decreasing algal nutritional quality.     

Assessment of <Emphasis Type="Italic">Nannochloropsis gaditana</Emphasis> growth and lipid accumulation with increased inorganic carbon delivery

Algal biomass refineries for sustainable transportation fuels, in particular biodiesel, will benefit from algal strain enhancements to improve biomass and lipid productivity. Specifically, the supply of inorganic carbon to microalgal cultures represents an area of great interest due to the potential for improved growth of microalgae and the possibility for incorporation with CO₂ mitigation processes. Combinations of bicarbonate (HCO₃^?) salt addition and application of CO₂ to control pH have shown compelling increases in growth rate and lipid productivity of fresh water algae. Here, focus was placed on the marine organism, Nannochloropsis gaditana, to investigate growth and lipid accumulation under various strategies of enhanced inorganic carbon supply. Three gas application strategies were investigated: continuous sparging of atmospheric air, continuous sparging of 5% CO₂ during light hours until nitrogen depletion, and continuous sparging of atmospheric air supplemented with 5% CO₂ for pH control between 8.0 and 8.3. These gas sparging schemes were combined with addition of low concentrations (5 mM) of sodium bicarbonate at inoculation and high concentration (50 mM) of sodium bicarbonate amendments just prior to nitrogen depletion. The optimum scenario observed for growth of N. gaditana under these inorganic carbon conditions was controlling pH with 5% CO₂ on demand, which increased both growth rate and lipid accumulation. Fatty acid methyl esters were primarily comprised of C16:0 (palmitic) and C16:1 (palmitoleic) aliphatic chains. Additionally, the use of high concentration (50 mM) of bicarbonate amendments further improved lipid content (up to 48.6%) under nitrogen deplete conditions when paired with pH-controlled strategies.     

Specific bleaching of phycobiliproteins from cyanobacteria and red algae at high temperature in vivo

Exposure of blue-green or red algal cells to temperatures exceeding 60–65°C for several minutes resulted in bleaching of all phycobilin absorption in the visible range, with virtually no alteration in chlorophyll or carotenoid absorption. Difference spectra of non-bleached vs bleached cells appeared identical to absorption spectra of purified phycobilisomes isolated from the same cell culture in high phosphate medium. All phycobilin chromophores were bleached at approximately the same rate during heating. There were no changes in apparent molecular weights or relative amounts of the phycobilisome apoproteins during chromophore bleaching. Phycobilisomes in cell extracts from Anacystis nidulans resisted bleaching when suspended in medium of high phosphate concentration, but were bleached at 60–65°C within a few minutes when placed in diluted medium. The results indicate that phycobilisomes in vivo are stabilized by a mechanism other than high osmotic and ionic strength. This represents a rapid and quantitative method to characterize the phycobiliprotein content of cyanobacteria and red algae in vivo.Abbreviations Chl chlorophyll - APC allophycocyanin - PC phycocyanin - PE phycoerythrin - SPM medium, 0.2 M sucrose, 15 mM MgCl₂, 0.75 M Na/KPO₄, pH 7.8     

Effect of growth and subsequent decomposition of blue-green algae on the transformation of iron and manganese in submerged soils

N₂-fixing blue-green algae (Cyanobacteria), besides enriching soils with N and organic carbon, may modify a number of chemical and electro-chemical properties of the soils resulting in a change in availability of some micronutrient elements. Keeping this in view, an experiment was conducted to study the effects of growth and subsequent decomposition of blue-green algae on changes in the different forms of Fe and Mn in four soils under submerged condition. A mixed algal culture containing Anabaena, Nostoc, Cylindrospermum, and Tolypothrix was used as inoculum. It was allowed to grow for 2 months, after which the soils were sequentially extracted with (i) M NH₄OAc (pH 7.0), (ii) M K₄P₂O₇, (iii) 0.1 M NH₂OH.HCl (pH 2.0), (iv) 0.2 M (NH₄)₂C₂O₄ (pH 3.0) and (v) 0.1 M ascorbic acid to obtain water-soluble plus exchangeable, organically bound, easily reducible, amorphous oxides-and crystalline oxides-bound forms of Fe and Mn, respectively, both during the growth as well as the subsequent in-situ decomposition of the algal biomass in soils. Iron and Mn in the extracts were estimated by atomic absorption spectrophotometry.The results showed that growth of blue-green algae in submerged rice soils caused a decrease in the NH₄OAc-extractable forms of Fe and Mn with concomitant increases in all the other four determined forms of the elements. Such decreases and/or increases in different forms of Fe and Mn in soils were explained as being due to release of O₂, addition of organic matter and liberation of extracellular organic compounds by the blue-green algae during their growth. The decomposition of algal biomass resulted in an increase in the NH₄OAc-, K₄P₂O₇- and (NH₄)₂C₂O₄-extractable forms of Fe and Mn with a simultaneous decrease in the NH₂OH · HCl- and ascorbic acid-extractable forms. Development of strong reducing conditions and formation of organic acids with chelating properties were suggested as being the cause of the above changes. The implication of these changes in the forms of Fe and Mn for the Fe and Mn nutrition of rice plants were discussed.