Andreas Nicklisch

ALLELOPATHIC GROWTH INHIBITION OF SELECTED PHYTOPLANKTON SPECIES BY SUBMERGED MACROPHYTES1

Allelopathic effects of submerged macrophytes on the growth and photosynthesis of different unialgal cultures of planktonic cyanobacteria, a diatom, and a green alga were tested in coexistence experiments using dialysis cultures. The method applied allowed measurements under conditions similar to that in lakes but without nutrient and light limitation. Growth and photosynthesis were measured with a pulse amplitude modulated fluorometer as an increase of chl a fluorescence and activity of PSII, respectively. Eurasian water milfoil Myriophyllum spicatum L. and rigid hornwort Ceratophyllum demersum L. proved to inhibit the PSII activity and then growth of the investigated phytoplankton species, whereas sago pondweed Potamogeton pectinatus L. showed no effect. Growth inhibition was dependent on biomass of M. spicatum. Considerable differences between phytoplankton groups and among species of cyanobacteria were found regarding their response to M. spicatum. Members of the Oscillatoriales and Microcystis aeruginosa Kütz. emend. Elenkin were more sensitive than the cyanobacterium Aphanizomenon flos‐aquae Ralfs ex Born. et Flah., the diatom Stephanodiscus minutulus (Kütz) Cleve et Möller, and the green alga Scenedesmus armatus Chodat. A possible contribution of this result to changes in the phytoplankton succession of lakes after loss of macrophytes is discussed.

Acclimation of the diatom Stephanodiscus neoastraea and the cyanobacterium Planktothrix agardhii to simulated natural light fluctuations

Functional and structural characteristics of the photosynthetic apparatus were studied in the diatom Stephanodiscus neoastraea and the cyanobacterium Planktothrix agardhii which were grown semi-continuously under constant irradiance or under simulated natural light fluctuations. The light fluctuations consisted of 24 oscillations of exponentially increasing and decreasing irradiance over a 12-h light period. Maximum irradiance was 1100 μmol photons m−2 s−1 with the ratio of maximum to minimum intensities being 100, simulating Langmuir circulations with a ratio of euphotic to mixing depth of 1. S. neoastraea acclimated to the light fluctuations by doubling the number and halving the size of photosynthetic units (PS II) while the amount of chlorophylls and carotenoids remained unchanged. The chlorophyll-specific maximum photosynthetic rate was enhanced while the slope of the photosynthesis versus irradiance curves was not influenced by the light fluctuations. Acclimation of P. agardhii was mainly characterized by an increase in chlorophyll content. Both photosystems showed only little changes in number and size. Maximum photosynthetic rate, saturating irradiance and initial slope of the photosynthesis versus irradiance curves did not vary. Although both high and low light were contained in the fluctuating light, an analogy to low or high light acclimation was not found for the diatom nor for the cyanobacterium acclimated to light fluctuations. We suggest that the acclimation to fluctuating light is a response type outside the known scheme of low and high light acclimation.

FIRST RECORD OF NANNOCHLOROPSIS LIMNETICA (EUSTIGMATOPHYCEAE) IN THE AUTOTROPHIC PICOPLANKTON FROM LAKE BAIKAL1

Three new strains of eukaryotic picoplankton, isolated from Lake Baikal, were identified as Nannochloropsis limnetica Krienitz, Hepperle, Stich & Weiler. To date, N. limnetica had only been detected in small German and North American inland water bodies. On determination of the 18S rDNA sequence, the three new strains were found to be identical to each other and to the type strain KR 1998/3 (GenBank accession no. AF251496). Rapid analysis of polymorphic DNA–PCR revealed that the genotypes were different, although the Baikalian eustigmatophycean strains were more similar to each other compared with the type strain KR 1998/3 from Germany. Ecophysiological differences were also evident between the new strains from Lake Baikal and the type strain from growth rate determinations. The morphological characteristics were similar to that of a previous description of the species. However, although the cells of Eustigmatophyceae usually propagate by autosporulation, in these newly detected species germination of single daughter cells from thick‐walled cells was observed for the first time. Based on pigment analysis, the occurrence of Eustigmatophyceae in Lake Baikal was estimated. Eustigmatophyceae were established to be common members of the phytoplankton community of this large oligotrophic Siberian lake and occurred throughout the year, even under the ice cover during winter. Moreover, they peaked during early summer and in the South Basin. Hence, the widely accepted opinion that Chlorophyceae solely comprise the eukaryotic picoplankton should be changed and the Eustigmatophyceae considered.

RNA/protein and RNA/DNA ratios determined by flow cytometry and their relationship to growth limitation of selected planktonic algae in culture

RNA/protein and RNA/DNA ratios correlate closely to the growth rate for many organisms. However, this has not yet been shown for planktonic algae and could be a basis for a new approach to determine in situ growth rates. Therefore, cultures of Stephanodiscus minutulus, Cyclostephanos invisitatus (Bacillariophyceae), Chlamydomonas sp., a eukaryotic pico-alga, (Chlorophyceae) and Rhodomonas minuta (Cryptophyceae) were grown under limitation by altering daylength, irradiance, and/or nutrients. Specific growth rates were determined using acclimated semi-continuous cultures. Cells of such cultures growing in a quasi-steady state were fixed with glutaraldehyde to conserve the pigment autofluorescence and parallels of the fixed samples were stained by SYTO RNASelect, PicoGreen, or fluorescein-5-ex-succinimidylester for RNA, DNA, and protein, respectively. The staining and the autofluorescence of photosynthetic pigments were analysed by flow cytometry (FACStar PLUS) using blue and green lasers for excitation. The determined ratios of RNA/protein and RNA/DNA correlated well with the specific growth rates of the species regardless of the kind of limitation, but differed between the different taxonomic groups of the species. The kind and strength of limitation resulted in different patterns of pigment autofluorescence, which can easily be evaluated by flow cytometry. Hence, the determination of RNA, DNA, protein, and pigment autofluorescence of planktonic algae delivers information on growth and limitation state which provides a basis for future assessment even in lake samples.

BEITRÄGE ZUR MORPHOLOGIE, ÖKOLOGIE UND ONTOGENIE DER PLANKTISCHEN DIATOMEE CYCLOTELLA COMENSIS GRUNOW. UNTERSUCHUNGEN AN HISTORISCHEM UND REZENTEM MATERIAL

Die planktische Diatomee des Süßwassers Cyclotella comensis wird schalenmorphologisch beschrieben. Dabei wird historisches Material von 1890 und rezentes Material von 2001 aus dem locus typicus Comer See, Norditalien, rezentes Material aus dem Großen Boberowsee, Norddeutschland und Material aus Klonkulturen mit Ursprung aus norddeutschen Seen, verwendet. Die autökologischen Ansprüche der Art auf der Basis von Populationen aus 68 Seen in Norddeutschland werden analysiert. Angaben zur Sexualvermehrung der Art in den drei norddeutschen Seen Großer Boberowsee, Großer Wummsee und Großer Stechlinsee folgen. Der Lebenszyklus von C. comensis im Großen Boberowsee von August 1998 bis Januar 2000 wird beschrieben. The planktonic freshwater diatom Cyclotella comensis will be morphologically described in detail. Therefore both historical (1890) and recently sampled material (2001) from the locus typicus Lake Como in Northern Italy is taken into account, as well as recent samples from the Großer Boberowsee (North Germany) and material from clone cultures originating from North German lakes. The autecological requirements of the species will be analysed on the basis of populations from 68 North German lakes. Information referring to the development of sexual cells of the species in three North German lakes (Großer Boberowsee, Großer Wummsee, Großer Stechlinsee) will be presented. The life-cycle of C. comensis in the lake Großer Boberowsee from August 1998 to January 2000 is described.

Temperature and photoperiod interactions with phosphorus-limited growth and competition of two diatoms.

In lakes, trophic change and climate change shift the relationship between nutrients and physical factors, like temperature and photoperiod, and interactions between these factors should affect the growth of phytoplankton species differently. We therefore determined the relationship between P-limited specific growth rates and P-quota (biovolume basis) of Stephanodiscus minutulus and Nitzschia acicularis (diatoms) at or near light saturation in axenic, semi-continuous culture at 10, 15 and 20 °C and at 6, 9 and 12 h d−1 photoperiod. Photoperiod treatments were performed at constant daily light exposure to allow comparison. Under these conditions, we also performed competition experiments and estimated relative P-uptake rates of the species. Temperature strongly affected P-limited growth rates and relative P uptake rates, whereas photoperiod only affected maximum growth rates. S. minutulus used internal P more efficiently than N. acicularis. N. acicularis was the superior competitor for P due to a higher relative uptake rate and its superiority increased with increasing temperature and photoperiod. S. minutulus conformed to the Droop relationship but N. acicularis did not. A model with a temperature-dependent normalised half-saturation coefficient adequately described the factor interactions of both species. The temperature dependence of the quota model reflected each species’ specific adaptation to its ecological niche. The results demonstrate that increases in temperature or photoperiod can partially compensate for a decrease in P-quota under moderately limiting conditions, like during spring in temperate lakes. Thus warming may counteract de-eutrophication to some degree and a relative shift in growth factors can influence the phytoplankton species composition.

Effects of thermal acclimation and photoacclimation on lipophilic pigments in an invasive and a native cyanobacterium of temperate regions

The freshwater cyanobacterium Cylindrospermopsis raciborskii spreads from tropical to temperate regions worldwide. This entails acclimation to varied light and temperature conditions. We studied the thermal and light acclimation of the photosynthetic machinery of C. raciborskii by monitoring alteration of the chlorophyll a and carotenoid content in German strains of C. raciborskii, in African and Australian strains of C. raciborskii, and in German strains of Aphanizomenon gracile, a native cyanobacterium belonging to the same order (Nostocales). Our results showed that temperate and tropical C. raciborskii strains did not differ in pigment acclimation to light and temperature. In contrast, the ratio of photoprotective carotenoids (namely the carotenoid glycoside 4-hydroxymyxol glycoside [aphanizophyll]) to chlorophyll a increased significantly more in C. raciborskii in comparison with A. gracile (1) with decreasing temperatures from 20 to 10°C and a moderate light intensity of 80 µmol photons m−2 s−1 and (2) with increasing light intensities at a suboptimal temperature of 15°C, compared to 20°C. We conclude that below 20°C photoinhibition is avoided by greater photoprotection in the invasive species C. raciborskii compared to the native species A. gracile.