Jacqueline Rücker

Cyanobacteria and Cyanotoxins: The Influence of Nitrogen versus Phosphorus

The importance of nitrogen (N) versus phosphorus (P) in explaining total cyanobacterial biovolume, the biovolume of specific cyanobacterial taxa, and the incidence of cyanotoxins was determined for 102 north German lakes, using methods to separate the effects of joint variation in N and P concentration from those of differential variation in N versus P. While the positive relationship between total cyanobacteria biovolume and P concentration disappeared at high P concentrations, cyanobacteria biovolume increased continually with N concentration, indicating potential N limitation in highly P enriched lakes. The biovolumes of all cyanobacterial taxa were higher in lakes with above average joint NP concentrations, although the relative biovolumes of some Nostocales were higher in less enriched lakes. Taxa were found to have diverse responses to differential N versus P concentration, and the differences between taxa were not consistent with the hypothesis that potentially N2-fixing Nostocales taxa would be favoured in low N relative to P conditions. In particular Aphanizomenon gracile and the subtropical invasive species Cylindrospermopsis raciborskii often reached their highest biovolumes in lakes with high nitrogen relative to phosphorus concentration. Concentrations of all cyanotoxin groups increased with increasing TP and TN, congruent with the biovolumes of their likely producers. Microcystin concentration was strongly correlated with the biovolume of Planktothrix agardhii but concentrations of anatoxin, cylindrospermopsin and paralytic shellfish poison were not strongly related to any individual taxa. Cyanobacteria should not be treated as a single group when considering the potential effects of changes in nutrient loading on phytoplankton community structure and neither should the N2-fixing Nostocales. This is of particular importance when considering the occurrence of cyanotoxins, as the two most abundant potentially toxin producing Nostocales in our study were found in lakes with high N relative to P enrichment.

Phytoplankton assemblages and steady state in deep and shallow eutrophic lakes - an approach to differentiate the habitat properties of Oscillatoriales

Ecological conditions and phytoplankton succession in two shallow hypertrophic lakes (Langer See and Melangsee) and a dimictic, eutrophic lake (Scharmützelsee) in a lake chain in Eastern Germany were analyzed from 1999 to 2001 in order to find situations of phytoplankton steady state assemblages and variables controlling the phytoplankton composition according to Reynolds et al. (2002). Long term background data from 1993 to 2001 suggest steady state conditions in shallow lakes, whereas the deep lake exhibited irregular fluctuations between various phytoplankton stages. Since the phytoplankton composition in the shallow lakes was similar in all the 3 years, it was highly predictable. Steady state conditions dominated by different species of Oscillatoriales were detected during the summer period 1999 and 2000 in Langer See and in Melangsee (see Mischke & Nixdorf, this volume). This dominant assemblage found in both lakes (group S1 acc. to Reynolds et al., 2002): Planktothrix agardhii (Gom.) Anagn. et Kom., Limnothrix redekei (Van Goor) Meffert, Pseudanabaena (Lauterb.) is typical in turbid mixed layers with highly light deficient conditions, but it is also regularly dominant in the dimictic lake Scharmützelsee as observed in 1999 and 2001 (Pseudanabaena limnetica (Lemm.) Kom. The Nostocales Cylindrospermopsis raciborskii (Wolz.) Seenayya et Subba Raju and Aphanizomenon gracile (Lemmerm.) Lemmerm. were important in the shallow lakes as well as in lake Scharmützelsee. Nevertheless, the occurrence of filamentous cyanobacteria in the dimictic lake was not regular and an unpredictable change in phytoplankton development was observed in 2000. It is discussed, whether this phenomenon of regular succession in shallow hypertrophic lakes is caused by adaptation to a resilient and an extreme environment or by the pool of species that can live or survive in that environment. This was checked through comparison of the depth of the mixed layer, the mean daily irradiance within this layer and the nutrient resources. Although the nutrient resources in both types of lake are near threshold levels, indicating growth inhibition by dissolved nutrients (DIP, DIN, TIC, DSi), the under water light supply seems to be the key factor favoring the dominance of filamentous cyanobacteria belonging to the functional group S1.

Distribution of three alien cyanobacterial species (Nostocales) in northeast Germany: Cylindrospermopsis raciborskii, Anabaena bergii and Aphanizomenon aphanizomenoides

A. Stüken, J. Rücker, T. Endrulat, K. Preussel, M. Hemm, B. Nixdorf, U. Karsten and C. Wiedner. 2006. Distribution of three alien cyanobacterial species (Nostocales) in northeast Germany: Cylindrospermopsis raciborskii, Anabaena bergii and Aphanizomenon aphanizomenoides. Phycologia 45: 696–703. DOI: 10.2216/05-58.1 Cylindrospermopsis raciborskii is considered a cyanobacterium of tropical origin and an alien species to temperate waters. However, it has been detected as far north as northern Germany. While previous studies have shown that all isolated German C. raciborskii strains are hepatotoxic, little is known about the spatial occurrence and relative frequency of this species in temperate Germany. The aim of this study was to investigate the spatial distribution and relative frequency of C. raciborskii close to its northernmost distribution limit, to characterise the habitat in which it is most likely to occur in this climatic zone and to search for any other neocyanobacterial species that might be present in German waters but has so far been overlooked. One hundred forty-two water bodies in northeast Germany were sampled from June until September 2004. All cyanobacteria species were analysed qualitatively and semiquantitatively. Besides C. raciborskii, two additional neocyano-bacterial species were detected: Anabaena bergii and Aphanizomenon aphanizomenoides. For both taxa, these findings represent their northernmost occurrence and their first report from German waters. Cylindrospermopsis raciborskii was present in 27%, Anabaena bergii in 9% and Aphanizomenon aphanizomenoides in 7% of the samples. The occurrence of each species was analysed in relation to maximum lake depth, Secchi depth, lake volume and lake surface area. All three species were present in a wide range of habitats, but C. raciborskii and Anabaena bergii occurred significantly more often in shallow, turbid waters than in deep, transparent water bodies. None of the parameters investigated were significantly correlated with the occurrence of Aphanizomenon aphanizomenoides. In conclusion, alien thermophilic cyanobacterial species are much more widely distributed in temperate Germany than previously known. The results are discussed with respect to the possible mechanisms that enable these organisms to expand northwards.

Factors controlling the dominance of Planktothrix agardhii and Limnothrix redekei in eutrophic shallow lakes

Lakes in the Scharmutzelsee-region (East-Brandenburg, Germany) were found to be dominated either by Limnothrix redekei or Planktothrix agardhii. These dominance regimes were stable over the whole investigation period from 1993 to 1995. Six lakes (maximum depth between 2 and 12 m) were compared in regard to physical and chemical conditions to find reasons for the superiority of the first or the second species. All investigated lakes are polymictic but show, nevertheless differences in their mixing-behaviour. In lakes dominated by Limnothrix redekeistratification events occur more often than in the lakes dominated by Planktothrix agardhii. The latter are more windexposed due to their greater size and/or their surroundings. In combination with polymixis light and nutrient supply are discussed as the key factors for the species composition. A lower content of soluble reactive phosphorus and lower light supply are considered as possible advantages for Limnothrix redekeiover Planktothrix agardhiiin three lakes. A lower total nitrogen/total phosphorus ratio may be a reason for the dominance of Planktothrix agardhiiin the other three investigated lakes.

Akinetes: Dormant Cells of Cyanobacteria

Cyanobacteria are an ancient and morphologically diverse group of photosynthetic prokaryotes, which were the first to evolve oxygenic photosynthesis. Cyanobacteria are widely distributed in diversed environments. In the case of members of the orders Nostocales and Stigonematales, their persistence and success were attributed to their ability to form specialized cells: heterocysts, capable of fixing atmospheric nitrogen and spore-like cells, the akinetes. This review focuses on akinetes of Nostocales, emphasizing environmental triggers and cellular responses involved in differentiation, maturation, dormancy, and germination of these resting cells. Morphological and structural changes, variation in akinete composition, and metabolism are summarized. Special attention is given to the genetic regulation of the differentiation process in an attempt to close gaps in our understanding of the dormancy phenomenon in cyanobacteria and to identify open questions for future research.

Seasonal Patterns of Nitrogen and Phosphorus Limitation in Four German Lakes and the Predictability of Limitation Status from Ambient Nutrient Concentrations

To identify the seasonal pattern of nitrogen (N) and phosphorus (P) limitation of phytoplankton in four different lakes, biweekly experiments were conducted from the end of March to September 2011. Lake water samples were enriched with N, P or both nutrients and incubated under two different light intensities. Chlorophyll a fluorescence (Chla) was measured and a model selection procedure was used to assign bioassay outcomes to different limitation categories. N and P were both limiting at some point. For the shallow lakes there was a trend from P limitation in spring to N or light limitation later in the year, while the deep lake remained predominantly P limited. To determine the ability of in-lake N:P ratios to predict the relative strength of N vs. P limitation, three separate regression models were fit with the log-transformed ratio of Chla of the P and N treatments (Response ratio = RR) as the response variable and those of ambient total phosphorus:total nitrogen (TN:TP), dissolved inorganic nitrogen:soluble reactive phosphorus (DIN:SRP), TN:SRP and DIN:TP mass ratios as predictors. All four N:P ratios had significant positive relationships with RR, such that high N:P ratios were associated with P limitation and low N:P ratios with N limitation. The TN:TP and DIN:TP ratios performed better than the DIN:SRP and TN:SRP in terms of misclassification rate and the DIN:TP ratio had the highest R2 value. Nitrogen limitation was predictable, frequent and persistent, suggesting that nitrogen reduction could play a role in water quality management. However, there is still uncertainty about the efficacy of N restriction to control populations of N2 fixing cyanobacteria.

Seasonal dynamics of cylindrospermopsin and cyanobacteria in two German lakes

Cylindrospermopsin (CYN) is a potent hepatotoxin produced by different cyanobacteria of the order Nostocales. Questions of major concern are: which species produce CYN, which are the seasonal patterns of CYN dynamics and how are they regulated? Therefore, we studied for the first time the seasonal dynamics of particulate and dissolved CYN concentrations, cyanobacterial abundance and environmental factors in two German lakes over 2 years. Total CYN reached maximum concentrations of 0.34 and 1.80 microg L(-1) in Melangsee and Langer See, respectively. In both lakes, maxima of the dissolved CYN fraction occurred later in the season than those of the particulate fraction, and it reached higher concentrations. This indicates that CYN is poorly decomposed and accumulates in the water. The cyanobacterial community in both lakes included several potentially CYN-producing species that did not correlate with CYN concentrations. Significant correlations between the particulate CYN concentrations and species biovolume were only found for Aphanizomenon gracile (r(s)=0.803) in Langer See indicating that this species is a CYN producer. Different correlations of CYN with abiotic factors in the two lakes indicate the presence of further undetected CYN producers as well as different regulation mechanisms of their dynamics and the variability of CYN.

Can submerged macrophytes influence turbidity and trophic state in deep lakes? Suggestions from a case study.

Feedback between submerged macrophytes and water transparency stabilizing the clear, macrophyte-dominated regime has been described so far for shallow lakes. Based on data of total phosphorus (TP) concentrations, underwater light supply, phytoplankton and submerged macrophyte abundance from narrow, stratified Lake Scharmützelsee (mean depth: 9 m, retention time: 16 yr) of the period 1994-2006 we hypothesize that submerged macrophytes may influence transparency and trophic state in deep lakes. The lake was characterized by summer epilimnion TP concentrations of 38 to 57 mug L(-1), turbid water due to mass development of cyanobacteria, and low abundance of few submerged macrophyte species until 2003. Thereafter, a sudden increase in water transparency was followed by a rapid submerged macrophyte colonization of the littoral down to about 5 m depth corresponding to the depth of a light supply of 3 E m(-2) d(-1). Initially, this recolonization was probably a consequence of decreased turbidity. We argue that the increase of submerged macrophyte coverage from < 10% in 1994 to 2003 to about 24% in 2005-2006 has contributed to the stabilization of the clear-water regime during the subsequent years. This is supported by the fact that earlier shifts to clear-water regimes in 1994 and 2000 without a significant spread of submerged macrophytes were not stable. We discuss potential mechanisms that may have resulted in a positive effect of plants on transparency such as P uptake and immobilization by the dominant rootless macrophyte species Nitellopsis obtusa and Ceratophyllum demersum and other macrophyte-related mechanisms such as increased zooplankton grazing and allelopathy.

Extending one-dimensional models for deep lakes to simulate the impact of submerged macrophytes on water quality

Submerged macrophytes can stabilise clear water conditions in shallow lakes. However, many existing models for deep lakes neglect their impact. Here, we tested the hypothesis that submerged macrophytes can affect the water clarity in deep lakes. A one-dimensional, vertically resolved macrophyte model was developed based on PCLake and coupled to SALMO-1D and GOTM hydrophysics and validated against field data. Validation showed good coherence in dynamic growth patterns and colonisation depths. In our simulations the presence of submerged macrophytes resulted in up to 50% less phytoplankton biomass in the shallowest simulated lake (11?m) and still 15% less phytoplankton was predicted in 100?m deep oligotrophic lakes. Nutrient loading, lake depth, and lake shape had a strong influence on macrophyte effects. Nutrient competition was found to be the strongest biological interaction. Despite a number of limitations, the derived dynamic lake model suggests significant effects of submerged macrophytes on deep lake water quality. Existing models were innovatively combined to study macrophyte effects in deep lakes.Submerged macrophytes can significantly affect the water clarity in deep lakes.This effect depends on lake geometry, depth, and nutrient loading.

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.