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Utilization of various carbon sources for the growth of waterborne conidial fungi

     Department of Botany, Kumaun University, Nainital–263002, India

	ABSTRACT

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Four isolates of waterborne conidial fungi (Tetracheatum elegans, Tetracladium marchalianum, Pestalotiopsis submersus and Flagellospora penicillioides) were investigated for their carbon requirement, using eight different carbon sources (viz. glucose, fructose, sucrose, xylose, starch, cellulose, dextrin and lactose). All fungi tested grew sparsely on the basal medium lacking in carbon, which was the control. However these fungi were found to vary in their ability to use the supplied sources of carbon. Glucose and sucrose were found to be suitable sources of carbon for all four fungal isolates, whereas fructose proved good for T. marchalianum and P. submersus. Starch and xylose also supported growth of T. marchalianum, P. submersus and F. penicillioides. Cellulose, a polysaccharide, was a poor source of carbon for the growth of these isolates. Four g/L of glucose was recorded as the most useful concentration that gives the maximum dry weight of selected fungi (262 mg and 400 mg for T. elegans and P. submersus respectively after 15 d).

Key words: aquatic hyphomycetes, carbon utilization, conidial fungi, fungal growth

	INTRODUCTION

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Waterborne conidial fungi, previously known as aquatic hyphomycetes (Ingold 1942), represent the major microbial elements of decaying leaves in running freshwater. These fungi are characterized by their magnificent conidial types (Ingold 1975). Submerged plant litter serves as substrates for the nutrition of these fungi. They use a wide range of simple and polymer carbohydrates from abscised leaves or litter.

In studies on the nutrition of aquatic hyphomycetes (Ranzoni 1951) reported that thiamine and a carbohydrate source are required for their growth. Aquatic hyphomycetes can satisfactorily use a wide range of carbohydrates, which includes starch, cellulose, cellobiose, sucrose, mannose, xylose, maltose, glucose and galactose. Thornton (1963) analyzed leaf extract carbohydrates, and these leaf extract carbohydrates were tested for utilization by aquatic hyphomycetes in vitro. Jones and Stewart (1972) reported that some aquatic hyphomycetes are able to use cellobiose and starch and can degrade cellulose effectively.

The occurrence and distribution of these fungi have attracted attention worldwide (Webster and Descals 1981, Marvanova 1997), but there is a paucity of knowledge on their physiology (Sridhar et al 1992, Sati and Pant 2000). Limited studies on carbon nutrition of aquatic hyphomycetes are available (Jones and Stewart 1972; Gulis and Suberkropp 2003, 2004). Carbon nutrition of aquatic fungi, especially on waterborne conidial fungi, is still an area that requires investigation.

Thus investigation of the nutritional requirements of some commonly occurring waterborne conidial fungi, isolated from the streams of the temperate zone in Kumaun Himalaya, India, (1600–2000 m asl, 29°5'–31°25'N, 77°48'–81°6'E), was carried out to determine the use of various carbohydrates for the growth of four selected waterborne conidial fungi.

	MATERIALS AND METHODS

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Four fungal species of waterborne conidial fungi (viz. Flagellospora penicillioides Ingold, Pestalotiopsis submersus Sati and Tiwari, Tetracheatum elegans Ingold and Tetracladium marchalianum de Wildeman) were selected for the present investigation. Monohyphal cultures of these isolates were obtained by single spore isolation. The cultures were maintained at 20 ± 2 C on 2% malt extract agar slants with initial pH 6.5 and in Petri dishes containing malt extract agar.

The basal medium consisted of KH₂PO₄, 1g; MgSO₄ · 7H₂O, 0.2 g; FeCl₃ · 6H₂O, 0.02 g; Difco yeast extract, 1.0 g/L distilled water. To find the optimal concentration of carbon sources for the best growth of waterborne conidial fungi, preliminary experiments were conducted by growing T. elegans and P. submersus in different concentrations of glucose, which is a good source of carbon. The range of concentration was taken arbitrarily, and 4 g/L of glucose in the basal medium was found to be the desirable concentration that gives maximum dry weight of waterborne conidial fungi (TABLE I). Eight carbon-containing compounds (glucose, fructose, sucrose, xylose, starch, cellulose, dextrin and lactose) were taken separately in sufficient quantity to create a concentration equivalent to 4.0 g of carbon/L of medium. Three replicates were used in each experiment for each fungal isolate. Uniform circular agar blocks (5 mm diam) containing mycelial mat were cut from a 15 d old culture plate and transferred into sterilized conical flasks (100 mL cap.) containing 25 mL of basal medium. The inoculated flasks were incubated at 20 ± 2 C in the dark and casually shaken for aeration. After 15 d of incubation the net hyphal growth of the fungus in terms of mycelial dry weight in the basal medium was determined. Adhered agar medium from the mycelial mat was removed by straining through a filter paper (Whatman No. 1). The mycelial mat was rinsed with distilled water 3–4 times to remove traces of basal medium and it was placed in a drying oven at 80 C for 4 h. The fungal biomass was weighed with a digital electronic balance.

	RESULTS

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View larger version (34K): [in this window] [in a new window]   FIG. 1. Dry weight yields of the four fungal isolates in different carbon sources after 15 d incubation at 20 C.

	DISCUSSION

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In the present study out of eight carbon sources glucose supported maximum growth of P. submersus and F. penicillioides. The remaining fungal isolates also showed comparatively higher growth. These observations conform with studies of (Ranzoni 1951) on glucose as a good source of carbon for the growth of Anguillospora longissima and A. gigantea. Thornton (1963) similarly found glucose as a good source of carbon for the growth of Articulospora tetracladia, Flagellospora penicillioides, Tricladium angulatum, T. splendens, Tetracladium setigerum, Var-icosporium elodeae and V. aurantiaca.

Starch, fructose and dextrin are good sources of carbon for the growth of P. submersus and T. marchalianum (FIG. 1). Sucrose supports maximum growth of T. elegans as well as of T. marchaliunum and moderate growth of P. submersus and F. penicillioides. These results support the findings of (Ranzoni 1951). Thornton (1963) also recorded that fructose, xylose and starch are good sources of carbon for some species of aquatic hyphomycetes. He concluded that starch could be an alternative source of carbon for these fungi. Mer (1982), while studying the carbon requirements of zoosporic fungi (watermolds), found that dextrin is a good source for the growth of Leptolegnia caudata and Saprolegnia subterranea. Lactose was found to promote moderate growth of P. submersus and F. penicillioides; however it resulted in poor growth for the remaining two fungal species (FIG. 1).

It is interesting to note that cellulose is a poor source of carbon for the growth of these hyphomycetous fungi (FIG. 1). This supports (Ranzoni 1951) who also recorded the absence of growth of A. gigantea and A. longissima in cellulose as a source of carbon in medium. These fungi probably lack the enzyme necessary to degrade cellulose. The fast colonization of these fungi on leaf litter might be explained on the basis of the availability of nutrients and polysaccharides preferred by them.

ANOVA calculated for the observed data showed significant variations in the dry weight production of different fungal species grown with different carbon sources (p < 0.01). This indicates that different fungal species have different priorities for their carbon source and one carbon source that is less preferred by one fungal species could be preferred by another fungal species. This might be due to the fact that simple carbon compounds are assimilated directly while complex ones (i.e. polysaccharides) must be converted into simpler forms before their use. Glucose (a monosaccharide) and sucrose (a disaccharide) are well used by the waterborne fungi.

	ACKNOWLEDGMENTS

 
The authors thank Prof. S.P. Singh, head, Department of Botany, Kumaun University, Nainital, for providing laboratory facilities and Dr M. Belwal for his help in conducting the experiments. We also thank Prof. S. Chandra (Botany) and Dr Madhu Joshi (English) for their careful reading of the manuscript and improving it.

	FOOTNOTES

 
Accepted for publication June 25, 2006.

¹ Corresponding author. E-mail: saraswatibisht{at}yahoo.com

	LITERATURE CITED

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Gulis V, Suberkropp K. 2003. Interactions between stream fungi and bacteria associated with decomposing leaf litter at different levels of nutrient availability. Aqua Microb Ecol 30:149–157.[CrossRef]

———, ———. 2004. Effects of whole-stream nutrient enrichment on the concentration and abundance of aquatic hyphomycete conidia in transport. Mycologia 96:57–65.[Abstract/Free Full Text]

Ingold CT. 1942. Aquatic hyphomycetes on decaying alder leaves. Trans Brit Mycol Soc 25:339–417.

———. 1975. An illustrated guide to aquatic and water-borne hyphomycetes (Fungi imperfecti) with notes on their biology. London: Freshwater Biology Association, Publication No. 30. 96 p.

Maravanova L. 1997. Freshwater Hyphomycetes: a survey with remarks on tropical Taxa. In: Janardhanan KK, Rajendran C, Natrajan K, Hawksworth DL, eds. Tropical Mycology. USA: Science Publishers Inc. p 169–226.

Mer GS. 1982. Taxonomic and physiological studies of watermolds of Sat tal (Nainital) [Doctoral thesis]. Naintal, India: Kumaun University.

Ranzoni FV. 1951. Nutrient requirements for two species of aquatic hyphomycetes. Mycologia 43:130–141.[CrossRef]

Sati SC, Tiwari N. 1997. Glimpses of conidial aquatic fungi in Kumaun Himalaya. In: Sati SC, Saxena J, Dubey RC, eds. Recent Researches in ecology, environment and pollution X. New Delhi: Today & Tomorrow Printers & Publishers. p 17–37.

———, Pant N. 2000. Microbial decomposition of leaf litter in a fast flowing freshwater stream of Kumaun Himalaya. In: Maheshwari DK, Dubey RC, Prasad G, Navneet, eds. Microbes, agriculture, industry and environment. Dehra Dun, India: Bishan Singh Mahendra Pal Singh. p 197–207.

———, ———, Belwal M. 2002. Conidial aquatic fungi of Kumaun Himalaya, India. Mycotaxon 80:445–455.

Sridhar KR, Chandrashekhar KR, Kaveriappa KM. 1992. Research on the Indian sub-continent. In: Barlocher F., ed. The ecology of aquatic hyphomycetes. Heidelberg, Germany: Springer-Verlag. p 182–211.

Thornton DR. 1963. The physiology and nutrition of some aquatic hyphomycetes. J Gen. Microbiol 33:23–31.[Abstract/Free Full Text]

Webster J, Descals E. 1981. Morphology, distribution and ecology of conidial fungi in freshwater habitats. In: Cole GT, Kendrick B, eds. The biology of conidial fungi. New York City: Academic Press. p 295–355.

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