Peter F. M. Coesel

Biogeography of desmids

Compared with other groups of unicellular freshwater algae, desmids lend themselves well to biogeographical studies since, at species level, identification is often relatively easy, whereas high ecological demands use to curtain their geographical distribution. Considering some ten desmid floral regions as distinguished in the beginning of this century, Indo-Malaysia/Northern Australia, tropical America, and equatorial Africa come to the fore as most pronounced. Also well typified are Eastern Asia, New Zealand/Southern Australia, and North America. Less endemic species are met with in Southern Africa and extratropical South America, whereas temperate Eurasia, with respect to the other continents, is mainly negatively characterized. The so-called arctic-alpine desmid flora may be encountered on all continents, provided that adequate minimum temperatures occur. Its distribution seems to be determined microclimatologically rather than macroclimatologically. Arguments for a tropical origin of the desmids as an algal group are adduced.

Desmids of the Lowlands

The present flora describes all desmid taxa known from the Netherlands and adjacent lowland areas: over 500 species and more than 150 additional varieties. Because of the predominantly cosmopolitan nature of most species the flora may also be of use outside this geographic region

5. Biogeography of desmids

Compared with other groups of unicellular freshwater algae, desmids lend themselves well to biogeographical studies since, at species level, identification is often relatively easy, whereas high ecological demands use to curtain their geographical distribution. Considering some ten desmid floral regions as distinguished in the beginning of this century, Indo-Malaysia/Northern Australia, tropical America, and equatorial Africa come to the fore as most pronounced. Also well typified are Eastern Asia, New Zealand/Southern Australia, and North America. Less endemic species are met with in Southern Africa and extratropical South America, whereas temperate Eurasia, with respect to the other continents, is mainly negatively characterized. The so-called arctic-alpine desmid flora may be encountered on all continents, provided that adequate minimum temperatures occur. Its distribution seems to be determined microclimatologically rather than macroclimatologically. Arguments for a tropical origin of the desmids as an algal group are adduced.

STRUCTURAL CHARACTERISTICS AND ADAPTATIONS OF DESMID COMMUNITIES

SUMMARY (1) Nearly 400 samples containing desmids, previously referred to three groups by the application of a classification program, were ordinated on the basis of their desmid species composition. (2) In alkaline, open water habitats, as eutrophication increased, there was a decrease in desmid diversity, and an increase in the proportion of planktonic forms. (3) Submerged fen-hollows were dominated, in relatively eutrophic sites, by small desmids with short generation times, whereas in relatively oligotrophic sites, larger desmids with longer generation times prevailed. The greatest desmid diversity occurred in the middle part of the trophic range; this is explained by a greater niche differentiation resulting from frequent fluctuations in nutrient concentrations in the habitat. (4) In emergent fen habitats, the desmid diversity decreased with decreasing moisture content and rising acidity. The relative contribution of omniradiate forms with a small surface: volume ratio increased. (5) The ecological significance of these characteristics, and the relationship between genera: species ratio and habitat are discussed.

Diversity and geographic distribution of desmids and other coccoid green algae

Taxonomic diversity of desmids and other coccoid green algae is discussed in relation to different species concepts. For want of unambiguous criteria about species delimitation, no reliable estimations of global species richness can be given. Application of the biological species concept is seriously hampered by lack of sexual reproduction in many species. Molecular analyses demonstrated cases of close affiliation between morphologically highly different taxa and, contrary, examples of little relationship between morphologically similar taxa. Despite the fact that desmids and chlorococcal algae, because of their microbial nature, can be readily distributed, cosmopolitan species are relatively scarce. The geographic distribution of some well-recognizable morphospecies is discussed in detail. Of some species a recent extension of their area could be established, e.g., in the desmids Micrasterias americana and Euastrum germanicum, and in the chlorococcaleans Desmodesmus perforatus and Pediastrum simplex.

COMPETITION FOR PHOSPHORUS AMONG PLANKTONIC DESMID SPECIES IN CONTINUOUS-FLOW CULTURE1

When grown under stringent P limitation, affinity for P uptake and growth in Staurastrum pingue Teil. and Staurastrum chaetoceras (Schr.) G. M. Smith (both originating from eutrophic lakes) were of the same magnitude, whereas these parameters for Cosmarium abbreviatum Rac. var. planctonicum W. & G. S. West (isolated from a meso‐oligotrophic lake) were significantly higher in value. On the other hand, at all growth rates tested, maximum P uptake rates were lower in C. abbreviatum than in the two Staurastrum species. The outcome of competition between either Staurastrum species and C. abbreviatum in mixed chemostats under different levels of continuous P limitation was in agreement with what could be predicted from the species‐specific affinity parameters: Staurastrum was outcompeted at dilution rates lower than 0.012 h−1, calculated to correspond with external inorganic P concentrations lower than 0.02 μM P, but won out at higher dilution rates. When P was added in two pulses of 2.5 μmol · L−1 a week instead of continuously, S. chaetoceras outcompeted C. abbreviatum at a slow rate. When P was supplied as a daily pulse of 0.7 μmol · L−1, a stable coexistence of S. chaetoceras and C. abbreviatum was established, Staurastrum predominating the culture in cell numbers at ca. 85%. The results show that P uptake and growth characteristics of the three species can predict the outcome of competition under various P‐limited conditions. Specific growth kinetic parameters as found in this study may also explain distribution patterns of the species observed in the field.

The relevance of desmids in the biological typology and evaluation of fresh waters

SummaryA biological evaluation of an aquatic environment based upon the rate of replaceability of the biocoenosis can be achieved by the separate estimation of the parameters trophism, saprobity, and diversity (SCHROEVERS, 1973). In those cases where the trophic and saprobic degrees are determined by an analysis of the composition of the phytoplankton, mostly the indicative value of the Desmidiaceae is used. It is emphasized that the often undiscriminating application of this parameter in practice can easily lead to unwarranted conclusions. An alternative approach in which the ecological differentiation within this group of algae is better accounted for, is proposed in the form of a (simple) typology of communities of desmids to be used as a direct basis of a biological rating.

The significance of desmids as indicators of the trophic status of freshwaters

Reports concerning the ecological demands of desmids are compared with pertaining results obtained from various habitat types in the Netherlands. The significance and usefulness of desmids as indicators of the trophic level is discussed.

TAXONOMIC REVISION OF THE CONJUGATOPHYCEAN FAMILY PENIACEAE ON THE BASIS OF CELL WALL ULTRASTRUCTURE1

An ultrastructural investigation of the cell wall of Penium silvae‐nigrae Raban. and P. spinospermum Josh. showed that these species possess true pores with a pore apparatus and overlapping semi‐cell walls. It follows that these two taxa belong not to the Peniaceae, but to the Desmidiaceae sensu stricto; they are referred to the genus Actinotaenium Teil. on account of the shape of their cells and chloroplasts. Two other species previously included in Penium Bréb. are referred to Actinotaenium. Although their cell wall structure could not be studied, they are distinguished from “typical” representatives of Penium by the following photomicroscopically observable complex of features: (pseudo‐) girdle bands none, cell wall pores in longitudinal rows, zygospores not globose but of irregular shape. The following new combinations ensued: Actinotaenium borgeanum (Skuja), A. phymatosporum (Nordst.), A. silvae‐nigrae (Raban.), A. silvaenigrae var. parallelum (Krieger) and A. spinospermum (Josh.). In addition the diagnosis of the genus Penium was emended and P. margaritaceum (Ehr.) ex Bréb. was selected as the lectotype species. The family Gonatozy‐gaceae is merged into the Peniaceae on the basis of cell wall structure.

Different response mechanisms of two planktonic desmid species (Chlorophyceae) to a single saturating addition of phosphate

Two planktonic algal species, Staurastrum chaetoceras (Schr.) G. M. Smith and Cosmarium abbreviatum Rac. var. planctonicum W. et G. S. West, from trophically different alkaline lakes, were compared in their response to a single saturating addition of phosphate (P) in a P‐limited growth situation. Storage abilities were determined using the luxury coefficient R = Qmax/Q0. Maximum cellular P quotas differed, depending on whether cells were harvested during exponential growth at μmax (Qmax, R being 26.7 and 9.1 for C. abbreviatum and S. chaetoceras, respectively) or harvested after a saturating pulse at P‐limited growth conditions (Q′max, R being 53.5 and 20.2 for C. abbreviatum and S. chaetoceras, respectively). At stringent P‐limited conditions, maximum initial uptake rates were higher in S. chaetoceras than in C. abbreviatum (0.094 and 0.073 pmol P·cell−1·h−1, respectively), but long‐term (net) uptake rates (over ∼20 min) were higher in C. abbreviatum than in S. chaetoceras (0.048 and 0.019 pmol P·cell−1·h−1, respectively). Before growth resumed after the onset of a large P addition (150 μmol·L−1), a lag phase was observed for both species. This period lasted 2–3 days for S. chaetoceras and 3–4 days for C. abbreviatum, corresponding with the time to reach Q′max. Subsequent growth rates (over ∼10 days) were 0.010 h−1 and 0.006 h−1 for S. chaetoceras and C. abbreviatum, respectively, being only 20%–30% of maximum growth rates. In conclusion, S. chaetoceras, with a relatively high initial P‐uptake rate, short lag phase, and high initial growth rate, is well adapted to a P pulse of short duration. Conversely, C. abbreviatum, with a high long‐term uptake rate and high storage capacity, appears competitively superior when exposed to an infrequent but lasting pulse. These characteristics provide information about possible strategies of algal species to profit from temporarily high P concentrations.