Morphological and biochemical variability within eastern North American populations of Asterionella Hassall, possible taxonomic implications

Abstract

Morphological plasticity is an inherent property of most diatom taxa which complicates taxonomic identification and the use of diatoms as ecological indicators. Consequently, physiological, biochemical, cytological and ecological traits, in combination with phenetic characterization, are now all employed to delineate species. The objective of this study was to explore taxonomic relationships and morphological variability within the Asterionella formosa I ralfsii complex. Populations of Asterionella were examined from clear and coloured lakes in eastern North America, taxonomic characters were measured using light and scanning electron microscopy, and isolates were grown in culture for pigment extraction and alaysis.

Cell length data and shifts in population distributions were followed within and between lakes over several seasons. Length class distributions were seen to follow one of three patterns: 1) an increase in length from the spring 1990 sample to the spring 1991 sample, 2) an increase in length from the spring of 1991 to the fall of 1991, and 3) an increase in length from the fall of 1992 to the spring of 1993. Mean population cell length increases occurred when cells declined to one-third to one-half the maximum, suggesting that auxospore formation had taken place. A few of the lakes followed no obvious cycle but always had a wide distribution of length classes. Average cell length of populations were 1) less than 60 um, 2) approximately 70 um, or 3) greater than 80 um. In addition, the number of valves per colony were counted to determine if there were differences in colony size between clear and coloured lakes. Although differences were seen in populations from a few lakes, most colonies had between five and eight cells, typical for this genus. Within-lake variability was similar to between-lake variability, with clear and coloured lakes overlapping in most cases.

Five morphological character states which proved to be stable and conservative and therefore useful for taxonomic identification, were measured on cleaned Asterionella valves. Populations were compared seasonally as well as within and between lakes. In all cases, differences were seen in one or more characters as well as for values reported in the literature for A. formosa and A. ralfsii and its varieties. Using principal components analysis, variability in morphological characters could not be connected with any measured environmental variable.

Pigment analysis revealed distinct differences in ratios of xanthophylls and chlorophylls between A. formosa and A. ralfsii var. americana cells grown in culture. The dominant pigment in the A. formosa clones was fucoxanthin and the combined xanthophyll component was twice that which was seen for chlorophyll a. Asterionella ralfsii var. americana clones had higher ratios of chlorophyll a to any other single or combined pigments.

In conclusion, valve and colony morphology did not differentiate between any of the Asterionella populations sampled from the eastern North American sites investigated in this study. Intermediate morphological forms were seen that possessed traits common to A. formosa and A. ralfsii and its varieties. However, pigment complement as well as pH preference were different in clones clearly identifiable in the classical sense, as A. formosa and A. ralfsii var americana. Thus the Asterionella populations in the study form a morphological continuum with A. formosa at one extreme and A. ralfsii at the other extreme. Clearly, the Asterionella formosa I ralfsii complex is an example of adaptive radiation in diatoms. Therefore, the use of these widespread and abundant taxa as bioindicators and as environmental proxies in paleoecological reconstructions should be with great caution. If this taxon is evolving and changing morphologically as the data from this study indicate, then present methods used for taxonomic identification are inadequate and misleading. Greater emphasis needs to be placed on variability across and within populations in all aspects of the biology of the taxon under investigation. This work strongly suggests that diatomists should be more inclusive and less exclusive when erecting taxa. Local or geographic populations of a taxon should be considered ecotypes, with variability a function of differing ecological conditions, possible founder effects or genetic mutations.