versão On-line ISSN 0717-6643
Gayana Bot. v.67 n.1 Concepción 2010
Gayana Bot 67(1): 141-144, 2010 ISSN 0016-5301
Asterosiphon dichotomus (Kützing) Rieth (Vaucheriales, Ochrophyta, Xanthophyceae) in sub-arid agricultural soils of Catalonia (Spain)
Asterosiphon dichotomus (Kützing) Rieth ( Vaucheriales, Ochrophyta, Xanthophyceae) en suelos sub-áridos de Cataluña (España)
Departamento de Biología Vegetal, Facultad de Biología, Universidad de Barcelona, Av. Diagonal 645, 08028 Barcelona, España. email@example.com
En una campaña de muestreo en la provincia Lleida (Cataluña, España), se han recolectado diversas algas edáfcas en campos agrícolas. En una de las localidades estudiadas se encontró Asterosiphon dichotomus (Kützing) Rieth (Vaucheriales, Ochrophyta, Xanthophyceae), y es la primera vez que se observa este taxón para la fora algológica ibérica. Las algas crecían sobre suelos húmedos, en plantaciones de almendros. Se describe su morfología y su crecimiento en cultivos, y se analiza su distribución geográfca en Europa.
A botanical survey was performed in the Lleida sub-arid agricultural soils (Catalonia, Spain) during 1995-96. In this study the edaphic alga Asterosiphon dichotomus (Kützing) Rieth (Vaucheriales, Ochrophyta, Xanthophyceae) appeared abundantly between Botrydium sp. vesicles. The genus Asterosiphon is very close to Vaucheria. Macroscopically, it forms green rosettes of 5-20 mm in diameter and it is organised in plurinucleated flaments (siphonal-siphonocladal organization), with a conspicuous dichotomous branching system, the branches of which usually are radially arranged. The reproduction is asexual, by the formation of akinetes in rows at the end of the filaments.
Asterosiphon dichotomus was firstly described by Kützing (1841) as Protonema dichotomum Kützing and later as Gongrosira dichotoma Kützing. However, Dangeard (1940) suggested that this taxon did not belong to the genus Vaucheria, Protonema or Gongrosira, based on the growth morphology and the reproduction type. Dangeard (1940) thus created a new genus: Asterosiphon. Later, Dangeard (1942) gave more data about the biology of Asterosiphon, using the combination A. terrestre Dangeard. Finally, Rieth (1962) cultured Asterosiphon, giving more information about the akinete ontogeny and germination. Moreover, Rieth (1962) proposed the new combination Asterosiphon dichotomous (Kützing) Rieth. After Rieth (1962, 1980), the asexual life cycle of this alga is well known. It forms a vegetative haploid thallus which is attached to the soil by a long colourless rhizoid. The green flaments can form rows of akinetes in dichotomous branched flaments and this shape was called Gongrosira-stadium.
The algae were collected by hand between October 1995 and February of 1996. In both cases after rain. They were growing in felds of almond trees in Torregrossa (Catalonia, Spain), which is located in a sub-arid mediterranean region. In Torregrossa, the climate is mediterranean, with an average precipitation of 351.7 mm/year. The average temperature is 14.8ºC. There is little precipitation except in spring and autumn. The material is preserved in the BCN-Phyc.
The microscopic study was performed with a Nikon Labophot-2 light microscope. The Asterosiphon samples were washed in distilled-sterilised water and transferred to a WoodsHole (WC) medium (Guillard 1975) agarized culture medium in Petri dishes. The culture conditions were 2000 lux, 12h:12h photoperiod and a temperature of about 18ºC. The culture was checked daily. To improve the growth rate, 2-3 ml of liquid WoodsHole media was added to the cultures after the frst week. This procedure stimulated the growth of the Asterosiphon thallus.
The thalli were round, about 5-20 mm in diameter, with conspicuous radially arranged flaments (Fig. 1A, B). The main branches were 42-48 µm in diameter, forming an irregular dichotomous branching system (Fig. 2 A, B), which was radially arranged from the centre of the thallus to the periphery. Nearly all branches formed ellipsoidal and uniseriate akinetes at the ends, which were 45-50 µm in diameter and 84-92 µm in length (Fig. 2 C, D).
figure 1. A, B. Asterosiphon dichotomus, detail of macroscopic morphology of thalli from living fresh material.
figure 2. A-D. Asterosiphon dichotomous, branching pattern and uniseriate akinetes in the three weeks cultured and living fresh material specimens.
figura 2. A-D. Asterosiphon dichotomous, detalle de las ramifcaciones y acinetas uniseriadas a partir de material fresco y en cultivo después de tres semanas.
The samples collected in February 1996 were cultured for three weeks. In this period, the vegetative growth was inconspicuous. The addition of liquid media to the culture stimulated flament growth. The new flaments showed rapid longitudinal growth and a few presented irregular and scarce dichotomous branching. These long branches were tapered towards the apex. The flament ends were hyaline, with few chloroplasts and they usually grew into the agar layer. These endings may be the origin of the new rhizoidal system.
After three weeks, the flaments were still scarcely dichotomously branched and usually ended with a rhizoid. The akinete formation was not limited to the end segments, the whole vegetative flaments started to produce uniseriate akinetes. This character differs slightly from the dichotomous rosette typical of this taxon. This difference possibly could be produced by the depletion of nutrients in the agar plates.
The distribution of this taxon is still not known. It has been recorded in Argentina (Lacoste de Díaz 1981), north Africa (Gauthier-Lièvre 1954), Asia (Hu & Wei 2006), France (De Puymaly 1924; Dangeard 1940, 1942), Germany (Rieth 1962), Greece (Christensen 1986), Switzerland, Denmark and Norway (Langangen 1994). It seems that this taxon has a cosmopolitan distribution (Ettl & Gärtner 1995), appearing mostly in wet naked soils in autumn or winter. In our opinion the scarce information of its distribution could be explained by its ephemeral life cycle.
Asterosiphon dichotomus can establish populations in Scandinavian or boreo-alpine soils, even under the snow (Langangen 1994), as well as in mediterranean semi-arid edaphic systems (Gauthier-Lièvre 1954). It seems that this taxon could live in strongly different habitats. However, it remains to be determined whether these populations belong to the same taxon, as well as check other morphological features like akinete germination, which was not observed in our cultures.
The author is indebted to Prof. X. Llimona (Barcelona Univ.) who collected the material. Thanks are also due to C. Balaguer, R. Gual and J. Puig, who helped in cultured the samples.
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