Atrazine catabolism by a combined bacterial association (KRA30) under carbon- and nitrogen-limitations in a retentostat.

AIMS: Nutrient-limited atrazine catabolism study in continuous cultures with biomass retention to mimic in situ environmental conditions and thus gain insight of the efficacy of biosupplementation/biostimulation to eliminate reduced herbicide bioavailability. METHODS AND RESULTS: Carbon- and nitrogen-limited retentostat (1 and 5 l) cultivation of a combined atrazine (100 mg l-1)-catabolizing association KRA30 was made. As a nitrogen source, through citrate supplementation, increased herbicide catabolism resulted and was complete in the absence of NH4-N. Co-metabolism of the molecule in the presence of succinate was identified. Population characterization by polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) indicated component species numerical dominance shifts in response to changes in nutrient limitation, mineral salts composition and biofilm formation, although the total species complement and catabolic potential were retained. CONCLUSIONS: Biomass and catabolic capacity maintenance, through cost-effective biosupplementation/biostimulation, should promote atrazine bioavailability and so ensure successful amelioration. SIGNIFICANCE AND IMPACT OF THE STUDY: All planning, implementation and monitoring of bioremediation programmes should be underpinned by a combination of molecular and (continuous) culture-based methods.

Relationships between microbial community structure and hydrochemistry in a landfill leachate-polluted aquifer.

Knowledge about the relationship between microbial community structure and hydrogeochemistry (e.g., pollution, redox and degradation processes) in landfill leachate-polluted aquifers is required to develop tools for predicting and monitoring natural attenuation. In this study analyses of pollutant and redox chemistry were conducted in parallel with culture-independent profiling of microbial communities present in a well-defined aquifer (Banisveld, The Netherlands). Degradation of organic contaminants occurred under iron-reducing conditions in the plume of pollution, while upstream of the landfill and above the plume denitrification was the dominant redox process. Beneath the plume iron reduction occurred. Numerical comparison of 16S ribosomal DNA (rDNA)-based denaturing gradient gel electrophoresis (DGGE) profiles of Bacteria and Archaea in 29 groundwater samples revealed a clear difference between the microbial community structures inside and outside the contaminant plume. A similar relationship was not evident in sediment samples. DGGE data were supported by sequencing cloned 16S rDNA. Upstream of the landfill members of the beta subclass of the class Proteobacteria (beta-proteobacteria) dominated. This group was not encountered beneath the landfill, where gram-positive bacteria dominated. Further downstream the contribution of gram-positive bacteria to the clone library decreased, while the contribution of delta-proteobacteria strongly increased and beta-proteobacteria reappeared. The beta-proteobacteria (Acidovorax, Rhodoferax) differed considerably from those found upstream (Gallionella, Azoarcus). Direct comparisons of cloned 16S rDNA with bands in DGGE profiles revealed that the data from each analysis were comparable. A relationship was observed between the dominant redox processes and the bacteria identified. In the iron-reducing plume members of the family Geobacteraceae made a strong contribution to the microbial communities. Because the only known aromatic hydrocarbon-degrading, iron-reducing bacteria are Geobacter spp., their occurrence in landfill leachate-contaminated aquifers deserves more detailed consideration.

Microorganisms with a taste for vanilla: microbial ecology of traditional Indonesian vanilla curing.

The microbial ecology of traditional postharvesting processing of vanilla beans (curing) was examined using a polyphasic approach consisting of conventional cultivation, substrate utilization-based and molecular identification of isolates, and cultivation-independent community profiling by 16S ribosomal DNA based PCR-denaturing gradient gel electrophoresis. At two different locations, a batch of curing beans was monitored. In both batches a major shift in microbial communities occurred after short-term scalding of the beans in hot water. Fungi and yeast disappeared, although regrowth of fungi occurred in one batch during a period in which process conditions were temporarily not optimal. Conventional plating showed that microbial communities consisting of thermophilic and thermotolerant bacilli (mainly closely related to Bacillus subtilis, B. licheniformis, and B. smithii) developed under the high temperatures (up to 65 degrees C) that were maintained for over a week after scalding. Only small changes in the communities of culturable bacteria occurred after this period. Molecular analysis revealed that a proportion of the microbial communities could not be cultured on conventional agar medium, especially during the high-temperature period. Large differences between both batches were observed in the numbers of microorganisms, in species composition, and in the enzymatic abilities of isolated bacteria. These large differences indicate that the effects of microbial activities on the development of vanilla flavor could be different for each batch of cured vanilla beans.

Control of glycolytic dynamics by hexose transport in Saccharomyces cerevisiae.

It is becoming accepted that steady-state fluxes are not necessarily controlled by single rate-limiting steps. This leaves open the issue whether cellular dynamics are controlled by single pacemaker enzymes, as has often been proposed. This paper shows that yeast sugar transport has substantial but not complete control of the frequency of glycolytic oscillations. Addition of maltose, a competitive inhibitor of glucose transport, reduced both average glucose consumption flux and frequency of glycolytic oscillations. Assuming a single kinetic component and a symmetrical carrier, a frequency control coefficient of between 0.4 and 0.6 and an average-flux control coefficient of between 0.6 and 0.9 were calculated for hexose transport activity. In a second approach, mannose was used as the carbon and free-energy source, and the dependencies on the extracellular mannose concentration of the transport activity, of the frequency of oscillations, and of the average flux were compared. In this case the frequency control coefficient and the average-flux control coefficient of hexose transport activity amounted to 0.7 and 0.9, respectively. From these results, we conclude that 1) transport is highly important for the dynamics of glycolysis, 2) most but not all control resides in glucose transport, and 3) there should at least be one step other than transport with substantial control.

Spatiotemporal Stability of an Ammonia-Oxidizing Community in a Nitrogen-Saturated Forest Soil.

Elevated levels of nitrogen input into various terrestrial environments in recent decades have led to increases in soil nitrate production and leaching. However, nitrifying potential and nitrifying activity tend to be highly variable over space and time, making broad-scale estimates of nitrate production difficult. This study investigates whether the high spatiotemporal variation in nitrate production might be explained by differences in the structure of ammonia-oxidizing bacterial communities in nitrogen-saturated coniferous forest soils. The diversity of ammonia-oxidizing bacteria of the b-subgroup Proteobacteria was therefore investigated using two different PCR-based approaches. The first targeted the 16S rRNA gene and involved temporal temperature gradient electrophoresis (TTGE) of specifically amplified PCR products, with subsequent band excision and nucleotide sequence determination. The second approach involved the cloning and sequencing of PCR-amplified amoA gene fragments. All recovered 16S rDNA sequences were closely related to the culture strain Nitrosospira sp. AHB1, which was isolated from an acid soil and is affiliated with Nitrosospira cluster 2, a sequence group previously shown to be associated with acid environments. All amoA-like sequences also showed a close affinity with this acid-tolerant Nitrosospira strain, although greater sequence variation could be detected in the amoA analysis. The ammonia-oxidizing bacterial community in the nitrogen-saturated coniferous forest soil was determined to be very stable, showing little variation between different organic layers and throughout the year, despite large differences in the total Bacterial community structure as determined by 16S rDNA DGGE community fingerprinting. These results suggest that environmental heterogeneity affecting ammonia oxidizer numbers and activity, and not ammonia oxidizer community structure, is chiefly responsible for spatial and temporal variation in nitrate production in these acid forest soils.

16S rDNA-based characterization of BTX-catabolizing microbial associations isolated from a South African sandy soil.

In the presence of different selection pressures, particularly pH and electron donor concentration, indigenous microbial associations which catabolize selected petroleum hydrocarbon components (benzene, toluene and o-, m- and p-xylene (BTX)) were enriched and isolated from a petroleum hydrocarbon-contaminated KwaZulu-Natal sandy soil. Electron microscopy revealed that, numerically, rods constituted the majority of the populations responsible for BTX catabolism. Molecular techniques (polymerase chain reaction (PCR) and 16S rDNA fingerprinting by denaturing-gradient gel electrophoresis (DGGE)) were employed to explore the diversities and analyze the structures of the isolated microbial associations. Pearson product-moment correlation indicated that the different, but chemically similar, petroleum hydrocarbon molecules, effected the isolation of different associations. However, some similar numerically-dominant bands characterized the associations. A 30% similarity was evident between the m- and o-xylene-catabolizing associations regardless of the molecule concentration and the enrichment pH. PCR-DGGE was also used to complement conventional culture-based microbiological procedures for environmental parameter optimization. Band pattern differences indicated profile variations of the isolated associations which possibly accounted for the growth rate changes recorded in response to pH and temperature perturbations.

Maintenance energy demand and starvation recovery dynamics of Nitrosomonas europaea and Nitrobacter winogradskyi cultivated in a retentostat with complete biomass retention.

Nitrosomonas europaea and Nitrobacter winogradskyi (strain "Engel") were grown in ammonia-limited and nitrite-limited conditions, respectively, in a retentostat with complete biomass retention at 25 degrees C and pH 8. Fitting the retentostat biomass and oxygen consumption data of N. europaea and N. winogradskyi to the linear equation for substrate utilization resulted in up to eight-times-lower maintenance requirements compared to the maintenance energy demand (m) calculated from chemostat experiments. Independent of the growth rate at different stages of such a retention culture, the maximum specific oxygen consumption rate measured by mass spectrometric analysis of inlet and outlet gas oxygen content always amounted to approximately 45 micromol of O2 mg-1 of biomass-C x h-1 for both N. europaea and N. winogradskyi. When bacteria were starved for different time periods (up to 3 months), the spontaneous respiratory activity after an ammonia or nitrite pulse decreased with increasing duration of the previous starvation time period, but the observed decrease was many times faster for N. winogradskyi than for N. europaea. Likewise, the velocity of resuscitation decreased with extended time periods of starvation. The increase in oxygen consumption rates during resuscitation referred to the reviving population only, since in parallel no significant increase in the cell concentrations was detectable. N. europaea more readily recovers from starvation than N. winogradskyi, explaining the occasionally observed nitrite accumulation in the environment after ammonia becomes available. From chloramphenicol (100 microg x ml-1) inhibition experiments with N. winogradskyi, it has been concluded that energy-starved cells must have a lower protein turnover rate than nonstarved cells. As pointed out by Stein and Arp (L. Y. Stein and D. J. Arp, Appl. Environ. Microbiol. 64:1514-1521, 1998), nitrifying bacteria in soil have to cope with extremely low nutrient concentrations. Therefore, a chemostat is probably not a suitable tool for studying their physiological properties during a long-lasting nutrient shortage. In comparison with chemostats, retentostats offer a more realistic approach with respect to substrate provision and availability.

Growth, maintenance and fermentation pattern of the salt-tolerant lactic acid bacterium Tetragenococcus halophila in anaerobic glucose limited retention cultures.

The homofermentative lactic acid bacterium Tetragenococcus halophila showed mixed acid fermentation at low growth-rates under glucose limiting conditions and in the presence of 10% NaCl. Maximum growth yields in fermentors with cell retention were not affected by pH, but maintenance requirement was at pH 5.2 four times higher than at pH 7.0. Despite the high salt-concentration of the medium, maintenance requirements were low compared to other lactic acid bacteria. The possible causes of the observed differences in maintenance requirements are discussed.

Sulfur production by obligately chemolithoautotrophic thiobacillus species.

Transient-state experiments with the obligately autotrophic Thiobacillus sp. strain W5 revealed that sulfide oxidation proceeds in two physiological phases, (i) the sulfate-producing phase and (ii) the sulfur- and sulfate-producing phase, after which sulfide toxicity occurs. Specific sulfur-producing characteristics were independent of the growth rate. Sulfur formation was shown to occur when the maximum oxidative capacity of the culture was approached. In order to be able to oxidize increasing amounts of sulfide, the organism has to convert part of the sulfide to sulfur (HS(sup-)(symbl)S(sup0) + H(sup+) + 2e(sup-)) instead of sulfate (HS(sup-) + 4H(inf2)O(symbl)SO(inf4)(sup2-) + 9 H(sup+) + 8e(sup-)), thereby keeping the electron flux constant. Measurements of the in vivo degree of reduction of the cytochrome pool as a function of increasing sulfide supply suggested a redox-related down-regulation of the sulfur oxidation rate. Comparison of the sulfur-producing properties of Thiobacillus sp. strain W5 and Thiobacillus neapolitanus showed that the former has twice the maximum specific sulfide-oxidizing capacity of the latter (3.6 versus 1.9 (mu)mol/mg of protein/min). Their maximum specific oxygen uptake rates were very similar. Significant mechanistic differences in sulfur production between the high-sulfur-producing Thiobacillus sp. strain W5 and the moderate-sulfur-producing species T. neapolitanus were not observed. The limited sulfide-oxidizing capacity of T. neapolitanus appears to be the reason that it can convert only 50% of the incoming sulfide to elemental sulfur.

Influence of environmental factors on endo-beta-1,4-glucanase production by Bacillus HR68, isolated from a Zimbabwean hot spring.

The production of endo-beta-1,4-glucanase by a Bacillus strain isolated from a hot spring in Zimbabwe was studied in batch culture, chemostat culture, and carbon dioxide-regulated auxostat (CO2-auxostat). The bacteria produced the enzyme in the presence of excess glucose or sucrose, but not under carbon-limited conditions in a chemostat using mineral medium. There was a specific growth rate dependent linear increase in enzyme production in glucose excess, nitrogen-limited chemostat cultures. A high specific growth rate of 2.2 h-1 and a high rate of enzyme production of 362 nkat (mg dry mass.h)-1 were attained under nutrient rich conditions in the CO2-auxostat. The bacteria had the highest specific growth rate and endo-beta-1,4-glucanase enzyme production at 50 degrees C. The maximum specific growth rate and the rate of enzyme production increased when yeast extract and tryptone were added in increasing amounts to the mineral medium used for cultivation in separate experiments. Increasing the glucose concentration in the CO2-auxostat cultures increased the rate of enzyme production but did not affect the specific growth rate.

Characterization of Tetragenococcus halophila Populations in Indonesian Soy Mash (Kecap) Fermentation.

Chinese- and Japanese-type fermented soy sauces are made of different plant materials. The lactic acid bacterium Tetragenococcus halophila is present and grows in both types. On the basis of the difference in sugar composition and content in the plant materials, differences in the populations of T. halophila bacteria were expected. However, obvious differences were found only regarding the utilization of l-arabinose. In the Chinese type, almost all isolates utilized l-arabinose, while in the Japanese type only 40% of the isolates did. Also, the population in the Japanese type was more heterogeneous regarding substrate utilization. Random amplified polymorphic DNA analysis revealed that the heterogeneous population at the Japanese-type industrial manufacturer was derived from only three strains at maximum. Genetic relatedness among isolates from different soy sauce manufacturers was low, but protein fingerprinting indicated that the isolates still belonged to one species.

Organic acid production by Aspergillus niger in recycling culture analyzed by capillary electrophoresis.

Wild-type Aspergillus niger N402 and glucoamylase++ overproducing transformant A. niger N402[pAB6-10]B1 have grown in maltodextrin- and xylose-limited recycling culture at pH 4.5 on mineral medium. The only products formed were organic acids and proteins, among which glucoamylase. The production of organic acids by the fungus has been analyzed qualitatively and quantitatively using capillary electrophoresis. The only organic acids produced in these cultures were substantial amounts of citric acid. This is the first demonstration of abundant oxalic acid production and a very low citric acid production by submerged cultures of A. niger. In the maltodextrin-limited culture the oxalic acid production rate increased during the first 80 h of cultivation and decreased after that time. In xylose-limited recycling culture the oxalic acid production rate always increased in time and highest values were found in the last samples taken from the culture after about 140 h of cultivation. Oxalic acid production rates were highest by the wild-type strain grown on xylose as carbon source, i.e., when the lowest glucoamylase production rates were observed. A clear negative correlation was found between the oxalic acid production rate and the respiration quotient (RQ). An increase in the oxygen consumption rate, due to the production of strongly oxidized oxalic acid, caused the RQ to be lowest at those stages of recycling cultivation when highest oxalic acid production rates were observed.

Growth behaviour and glucoamylase production by Aspergillus niger N402 and a glucoamylase overproducing transformant in recycling culture without a nitrogen source.

When wild-type Aspergillus niger N402 and a glucoamylase-overproducing transformant were grown in recycling culture without a nitrogen source, hyphal tip extension and glucoamylase production still occurred, but overproduction of glucoamylase by the transformant strain stopped. The mycelium retained a low metabolic activity. Light micrographs of mycelial samples showed that some hyphae were broken at their tip and partially empty, while after continuing recycling fermentation for more than 500 h many small and empty pieces of broken mycelium could be found. A model has been developed to calculate the mycelial growth and death rates. The mycelial death rate just exceeded the mycelial growth rate and as a consequence the amount of biomass in the fermentor vessel slightly decreased. It is concluded that the cytoplasmic contents of broken mycelial threads were released into the medium and acted as a nitrogen source for the growing parts of the mycelium.

Genetic and phenetic analyses of Bradyrhizobium strains nodulating peanut (Arachis hypogaea L.) roots.

Seventeen Bradyrhizobium sp. strains and one Azorhizobium strain were compared on the basis of five genetic and phenetic features: (i) partial sequence analyses of the 16S rRNA gene (rDNA), (ii) randomly amplified DNA polymorphisms (RAPD) using three oligonucleotide primers, (iii) total cellular protein profiles, (iv) utilization of 21 aliphatic and 22 aromatic substrates, and (v) intrinsic resistances to seven antibiotics. Partial 16S rDNA analysis revealed the presence of only two rDNA homology (i.e., identity) groups among the 17 Bradyrhizobium strains. The partial 16S rDNA sequences of Bradyrhizobium sp. strains form a tight similarity (> 95%) cluster with Rhodopseudomonas palustris, Nitrobacter species, Afipia species, and Blastobacter denitrificans but were less similar to other members of the alpha-Proteobacteria, including other members of the Rhizobiaceae family. Clustering the Bradyrhizobium sp. strains for their RAPD profiles, protein profiles, and substrate utilization data revealed more diversity than rDNA analysis. Intrinsic antibiotic resistance yielded strain-specific patterns that could not be clustered. High rDNA similarity appeared to be a prerequisite, but it did not necessarily lead to high similarity values between RAPD profiles, protein profiles, and substrate utilization. The various relationship structures, coming forth from each of the studied features, had low compatibilities, casting doubt on the usefulness of a polyphasic approach in rhizobial taxonomy.

Nicotinate catabolism is dispensable and nicotinate anabolism is crucial in Azorhizobium caulinodans growing in batch culture and chemostat culture on N2 as The N source.

When Azorhizobium caulinodans was grown in chemostat cultures with N2 as the N source at a constant dilution rate of 0.1 h-1 in media with a constant concentration (50 mM) of succinate and variable concentrations (1.5 to 585 microM) of nicotinate, neither the growth yield on succinate, the specific rate of O2 consumption, nor the specific rate of CO2 production showed linear regression with the concentration of nicotinate. Moreover, for transient continuous cultures in which the nicotinate concentration was gradually lowered, growth parameters remained unchanged until an apparently critical level of 0.7 microM nicotinate was reached. Below this nicotinate level, an immediate washout of the chemostat population began. A. caulinodans nicotinate hydroxylase-negative mutant 61007, unable to catabolize nicotinate, and the wild type behaved similarly. Thus, for continuous cultures supplied with N2 as the N source, submicromolar concentrations of nicotinate both sustained pyridine nucleotide biosynthesis at sufficient levels and precluded the use of nicotinate as a catabolic substrate. Furthermore, when more nicotinate was provided, dual succinate-nicotinate limitation in continuous cultures did not occur. Finally, when nicotinate is present in suboptimal concentrations, the specific growth rate is directly proportional to the amount of nicotinate present per unit of biomass. By contrast, in batch cultures with different nicotinate concentrations and with either succinate or lactate as the carbon and energy source, anomalous growth curves were obtained. With a low concentration (1.5 microM) of nicotinate, growth on N2 occurred, albeit at low rates. With a high concentration (195 microM) of nicotinate, growth on N2 was temporarily stimulated, but nicotinate was quickly exhausted and growth was thereafter nicotinate limited. Continuous supplementation of batch cultures with nicotinate allowed only transient exponential growth followed by linear growth. Thus, also for batch cultures, nicotinate catabolism is dispensable, although a high concentration of nicotinate temporarily stimulates growth on N2. Ut us concluded that A. caulinodans is a true diazotroph.

Simultaneous NH3 oxidation and N2 production at reduced O2 tensions by sewage sludge subcultured with chemolithotrophic medium.

The ammonia oxidation rate by sewage sludge was determined as a function of the dissolved oxygen tension. Samples of sludge were taken from a domestic waste water treatment pilot plant in which sludge was completely retained by membrane filtration. The samples were subcultured chemolithotrophically in recycling reactors. The gas supplied was a mixture of pure argon and oxygen. The KO2 for ammonia oxidation was estimated to be 0.97 (+/- 0.16) kPa dissolved oxygen. Together with ammonia oxidation and oxygen consumption, dinitrogen gas was produced. So, aerobic denitrification occurred. At dissolved oxygen tensions of 1.25 kPa and higher, the dinitrogen production rate (per N-mole) equalled 20% of the ammonia oxidation rate. This proportion was even 58% at 0.3 kPa dissolved oxygen. At 0.15 kPa dissolved oxygen, however, nitrification hardly proceeded, while dinitrogen production soon stopped. Most likely, a nitrifier concomitantly oxidized ammonia and reduced nitrite to dinitrogen.

Nitrogen Fixation and Hydrogen Metabolism in Relation to the Dissolved Oxygen Tension in Chemostat Cultures of the Wild Type and a Hydrogenase-Negative Mutant of Azorhizobium caulinodans.

Both the wild type and an isogenic hydrogenase-negative mutant of Azorhizobium caulinodans growing ex planta on N(2) as the N source were studied in succinate-limited steady-state chemostat cultures under 0.2 to 3.0% dissolved O(2) tension. Production or consumption of O(2), H(2), and CO(2) was measured with an on-line-connected mass spectrometer. In the range of 0.2 to 3.0%, growth of both the wild type and the mutant was equally dependent on the dissolved O(2) tension: the growth yield decreased, and the specific O(2) consumption and CO(2) production increased. A similar dependency on the dissolved O(2) tension was found for the mutant with 2.5% H(2) in the influent gas. The H(2)/N(2) ratio (moles of H(2) evolved per mole of N(2) consumed via nitrogenase) of the mutant, growing with or without 2.5% H(2), increased with increasing dissolved O(2) tensions. This increase in the H(2)/N(2) ratio was small but significant. The dependencies of the ATP/N(2) ratio (moles of ATP consumed per mole of N(2) fixed) and the ATP/2e ratio [moles of ATP consumed per mole of electron pairs transferred from NAD(P)H to nitrogenase] on the dissolved O(2) tension were estimated. These dependencies were interpreted in terms of the physiological concepts of respiratory protection and autoprotection.

Transcription regulation of ribosomal protein genes at different growth rates in continuous cultures of Kluyveromyces yeasts.

We have investigated the relationship between the growth rate of two Kluyveromyces strains that differ in their maximum growth rate, namely K. lactis (mumax = 0.5 h-1) and K. marxianus (mumax = 1.1 h-1), and the transcription rate of ribosomal protein (rp) genes in these strains. The growth rate of either strain was varied by culturing the cells in a chemostat under conditions of glucose limitation at different dilution rates. Although the steady-state levels of transcription of the rp-genes of both Kluyveromyces strains were tightly coupled to the cellular growth rate, no clear relationship between the level of rp-gene transcription and the amount of in vitro binding of the RAP1- and ABF1-like proteins to the promoters of these rp-genes was observed. Upon a sudden increase in the growth rate of a steady-state culture, the transcription of rp-genes of K. lactis showed a different response from that in K. marxianus. Whereas a substantial overexpression of the K. lactis rp-genes was found during at least 4-5 h, the level of expression of the K. marxianus rp-genes was almost immediately adjusted to the new growth rate.

Growth and product formation in chemostat and recycling cultures by Aspergillus niger N402 and a glucoamylase overproducing transformant, provided with multiple copies of the glaA gene.

Continuous and recycling cultures were carried out with Aspergillus niger N402 wild-type and a glucoamylase overproducing transformant to investigate growth and product formation characteristics. In shake flask cultures, the amount of glucoamylase produced by the transformant was about five times more than by the wild-type strain. In contrast with these results, a twofold overproduction was found in glucose-limited continuous cultures, while no overproduction was found under maltodextrin-limitation. Two regions of specific growth rates could be distinguished, one at specific growth rates lower (domain I) and one at specific growth rates higher than 0.12 h-1 (domain II). In domain I changes in mycelium morphology and conidia formation were observed. It has been concluded that maintenance requirements are dependent on the specific growth rate over the whole range of measured growth rates. The deviation in linearity in the linear equation of substrate utilization, caused by this phenomenon, should be considered when continuous cultures with filamentous fungi are performed. In recycling cultures, xylose as limiting carbon source repressed glucoamylase production very strongly. Under maltodextrin-limitation a fivefold overproduction was found. After about 150 h , the total amount of glucoamylase produced was still increasing, while total amount of product, measured as carbon, remained constant. After this time no increase in the amount of biomass formed was observed. These results suggest autolysis and cryptic growth taking place in a recycling fermenter and cell death rate equalling growth rate.