RECRUITMENT OF BENTHIC MICROCYSTIS (CYANOPHYCEAE) TO THE WATER COLUMN: INTERNAL BUOYANCY CHANGES OR RESUSPENSION?1

In some lakes, large amounts of the potentially toxic cyanobacterium Microcystis overwinter in the sediment. This overwintering population might inoculate the water column in spring and promote the development of dense surface blooms of Microcystis during summer. In the Dutch Lake Volkerak, we found photochemically active Microcystis colonies in the sediment throughout the year. The most vital colonies originated from shallow sediments within the euphotic zone. We investigated whether recruitment of Microcystis colonies from the sediment to the water column was an active process, through production of gas vesicles or respiration of carbohydrate ballast. We calculated net buoyancy, as an indication of relative density, using the amounts and densities of the major cell constituents (carbohydrates, proteins, and gas vesicles). Carbohydrate content of benthic Microcystis cells was very low throughout the year. Buoyancy changes of benthic Microcystis were mostly a result of changes in gas vesicle volume. Before the summer bloom, net buoyancy and the amount of buoyant colonies in the sediment did not change. Therefore, recruitment of Microcystis from the sediment does not seem to be an active process regulated by internal buoyancy changes. Instead, our observations indicate that attachment of sediment particles to colonies plays an important part in the buoyancy state of benthic colonies. Therefore, we suggest that recruitment of Microcystis is more likely a passive process resulting from resuspension by wind‐induced mixing or bioturbation. Consequently, shallow areas of the lake probably play a more important role in recruitment of benthic Microcystis than deep areas.     

Transport and accumulation of bloom-forming cyanobacteria in a large, mid-latitude lake: the gyre-Microcystis hypothesis

 Toxic cyanobacterial blooms have occurred in the near-shore waters of the North Basin of Lake Biwa, Japan, since 1994, and have been attributed to deterioration of water quality in the enriched littoral zone of the lake. From 1997 onwards, the bloom-forming cyanobacteria have been observed with increasing frequency in the deep offshore waters of the North Basin. In the present study, we examined the mechanisms responsible for these bloom populations in the main body of the lake. Specifically, we addressed the hypothesis that buoyant, nutrient-replete colonies of cyanobacteria are generated inshore, are advected offshore by large-scale horizontal transport processes, and subsequently accumulate in the downwelling center of large surface gyres that characterize the overall circulation pattern in the epilimnion of the North Basin. Diel variations of Microcystis biomass at the center and the edge of the Lake Biwa gyre were monitored at 6-h intervals on August 23–24, 2000, and the horizontal distribution of buoyant Microcystis was determined on October 6. The hydrodynamic structure of the first gyre was determined over the preceding 2 days by an on-board Acoustic Doppler Current Profiler (ADCP). The gyre was characterized by a counterclockwise horizontal current that could potentially advect material large distances offshore, a downwelling current near the center of the gyre, and an upwelling current at the edge of the gyre, caused by the radial pressure gradients. The biomass of Microcystis near the water surface was greater at the center than at the edge of the gyre, and the biomass at 5 m depth at the edge of the gyre was greater than that at the water surface or at the thermocline near the edge of the gyre. The results are consistent with the gyre-Microcystis hypothesis, and show the potential for accumulation of large concentrations of cyanobacteria in deep offshore lake environments that are normally considered unsuitable for cyanobacterial blooms. Received: July 16, 2001 / Accepted: March 6, 2002     

Diurnal changes in the vertical distribution of phytoplankton in hypertrophic Lake Kasumigaura,Japan

The daily vertical migration of five species;Microcystis aeruginosa (Kütz.) Trevis,Anabaena spiroides Klebahn f.crassa (L.) Elenkin,Aphanizomenon flos-aquae (L.) Ralfs,Melosira granulata (E). Ralfs, andCoscinodiscus lacustris Grun. was studied using a close-interval water sampler on a calm summer day in Lake Kasumigaura. Many colonies ofMicrocystis were observed at the middle of the water column (approx. 1.5 m depth) in the afternoon, and at the surface in the early morning.Anabaena occurred mostly in the upper layer whileAphanizomenon tended to be uniformly distributed. The difference in migration patterns suggests thatMicrocystis is superior toAnabaena andAphanizomenon in obtaining both light and nutrients from this lake. Among diatoms,Melosira remained at the bottom of the water column throughout day and night, but Coscinodiscus was uniformly distributed.     

LOCALIZATION OF MICROCYSTIN SYNTHETASE GENES IN COLONIES OF THE CYANOBACTERIUM MICROCYSTIS USING FLUORESCENCE IN SITU HYBRIDIZATION1

To better understand the production of microcystins (MCs) in Microcystis colonies, fluorescence in situ hybridization (FISH) methods were developed to detect DNA involved in the synthesis of these cyanobacterial hepatotoxins. Using colonies of Microcystis aeruginosa (Kütz.) Kütz. isolated from environmental blooms of cyanobacteria and from a colony‐forming, MC‐producing laboratory strain of Microcystis, amplified PCR products were observed, coincident with positive controls. The total MC content of individual colonies of Microcystis, determined by ELISA, showed a positive correlation with colony cross‐sectional area. FISH analysis of Microcystis colonies gave high fluorescence in comparison to negative controls, indicating the presence of MC synthetase DNA (mcyA) in situ. FISH analysis for MC synthetase genes has the potential to be developed into an effective early warning tool for drinking and recreational water management.     

冲天湖底泥表层微囊藻休眠体复苏与菌群动态

对频繁暴发微囊藻水华的西洞庭冲天湖表层底泥和上覆水取样,检测和分析了底泥表层微囊藻休眠体丰度和菌浓度、上覆水中微囊藻细胞丰度和菌浓度以及部分理化性质,结合室内模拟试验。结果表明:2—6月份冲天湖底泥表层和上覆水中总菌浓度均显著上升(P0.05),底泥表层总菌浓度显著高于上覆水(P0.05),优势菌群均为微小杆菌属(Exiguobacterium)、假单胞菌属(Pseudomonas)和芽孢杆菌属(Bacillus);4月份底泥表层微囊藻休眠体开始复苏且休眠体丰度下降,6月份休眠体丰度显著低于4—5月份(P0.05),而上覆水中微囊藻细胞丰度上升,6月份显著高于4—5月份(P0.05);复苏优势藻为铜绿微囊藻(Microcystis aeruginosa)、水华微囊藻(Microcystis flos-aqua)和惠氏微囊藻(Microcystis wesenbergii);复苏期间促休眠体复苏优势菌群浓度显著上升、"底泥-上覆水"界面溶解氧浓度与TN/TP比显著下降(P0.05)。说明冲天湖底泥表层和上覆水优势菌群可能通过改变底泥表层理化环境影响微囊藻休眠体复苏。     

Determination of Oligopeptide Diversity within a Natural Population of Microcystis spp. (Cyanobacteria) by Typing Single Colonies by Matrix-Assisted Laser Desorption Ionization–Time of Flight Mass Spectrometry

Besides the most prominent peptide toxin, microcystin, the cyanobacteria Microcystis spp. have been shown to produce a large variety of other bioactive oligopeptides. We investigated for the first time the oligopeptide diversity within a natural Microcystis population by analyzing single colonies directly with matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS). The results demonstrate a high diversity of known cyanobacterial peptides such as microcystins, anabaenopeptins, microginins, aeruginosins, and cyanopeptolins, but also many unknown substances in the Microcystis colonies. Oligopeptide patterns were mostly related to specific Microcystis taxa. Microcystis aeruginosa (Kütz.) Kütz. colonies contained mainly microcystins, occasionally accompanied by aeruginosins. In contrast, microcystins were not detected in Microcystis ichthyoblabe Kütz.; instead, colonies of this species contained anabaenopeptins and/or microginins or unknown peptides. Within a third group, Microcystis wesenbergii (Kom.) Kom. in Kondr., chiefly a cyanopeptolin and an unknown peptide were found. Similar patterns, however, were also found in colonies which could not be identified to species level. The significance of oligopeptides as a chemotaxonomic tool within the genus Microcystis is discussed. It could be demonstrated that the typing of single colonies by MALDI-TOF MS may be a valuable tool for ecological studies of the genus Microcystis as well as in early warning of toxic cyanobacterial blooms.     

Overwintering of Microcystis aeruginosa Kutz. in a shallow lake

The standing crop and photosynthetic activity of Microcystisaeruginosa Kütz. in both the plankton and sediment wereinvestigated from November 1979 to May 1982 in Lake Kasumigaura,Japan. The number of planktonic colonies of this species decreasedfrom early autumn to early spring, but increased in the sedimentduring late summer and autumn. The overwintering colonies inthe sediment were –100–1000 times greater per unitarea than those in lake water. No photoinhibition of photosynthesiscould be observed in overwintering Microcystis. The values ofthe initial slopes of photosynthesis-light (P-I) curves weresimilar to those of the summer population, although the maximumphotosynthetic rate (P_max) measured at 20°C was lower thanthat of the summer planktonic population. In winter the valuesof initial slope of the P-I curve, and the ratio of phycobilinto chlorophyll a sorted from sediment were higher than in coloniesfrom the plankton.     

Coupling between benthic biomass of Microcystis and phosphorus release from the sediments of a highly eutrophic lake

Variations in microbial biomass and activity in the sediments of hypereutrophic Lake Vallentunasjön were followed during a period of 5 years. The data were compared to the calculated release of phosphorus from the sediments during the same period. A strong co-variation was found between biomass of Microcystis, heterotrophic bacterial activity in the sediments and internal phosphorus loading. These parameters exhibited mainly a declining trend during the investigation period. A pronounced stability of the sediment chemistry, including the fractional composition of the sediment phosphorus, during the studied period indicates that microbial activity affected the phosphorus release from the sediments. Calculations of the percentage of sediment bacteria that was associated to the mucilage of Microcystis colonies imply, together with the specific bacterial production, that Microcystis in the sediment stimulates bacterial production. In the highly phosphorus-saturated sediments of Lake Vallentunasjön this would ultimately lead to an increased release of phosphorus from the sediment. Lake Vallentunasjön does not follow the common pattern of recovery after reduction of external phosphorus loading. The large biomasses and long survival of Microcystis in the sediment are probably important reasons for the delayed recovery of the lake.     

Effect of nutrient availability on the C, N, and P elemental ratios in the cyanobacterium Microcystis aeruginosa

To clarify whether nutrients limit the growth of Microcystis aeruginosa (Kütz.) Kütz during the growing season in Lake Yogo, we examined the cellular ratios of carbon (C), nitrogen (N), and phosphorus (P) in the populations of M. aeruginosa from August to December 2001. We also measured cellular C, N, and P ratios of M. aeruginosa under batch culture conditions. The cellular levels of N and P of M. aeruginosa in natural population changed more than twofold. The atomic N: C ratio of natural populations of Microcystis fluctuated from 0.11 to 0.26. The atomic P: C ratio fluctuated from 0.0080 to 0.024. The N: C, P: C, and N: P ratios of exponentially growing M. aeruginosa in N-and P-rich medium were 0.19, 0.013, and 15 on average. The growth of M. aeruginosa was suppressed below the N: C ratio of 0.13 under the N-free condition and below the P: C ratio of 0.0026 in the P-free condition. In the natural population, the N: C ratio was low on August 1-2 (0.11) and the P: C ratio was low (less than 0.011) until September. The Microcystis population on August 1-2 was N limited, judging from the results of the culture experiment. In other periods, the population seemed to be supplied with a sufficient amount of N. Although the P: C ratio was low (approximately 0.01) during August and September, it was several times larger than the value of the reduction of growth rate that occurred in culture. P limitation did not occur during the study period. N became more of a limiting factor than P for the formation of blooms of Microcystis. No blooms were observed in August and September, in spite of the increase of cellular levels of N. The formation of Microcystis blooms in Lake Yogo seems to be affected by artificial manipulations such as pumping from Lake Biwa and outflow.     

Microbial activity and phosphorus dynamics in eutrophic lake sediments enriched with Microcystis colonies

1. Sediments from hypereutrophic Lake Vallentunasjön were enriched with Microcystis colonies from the lake water, thereby simulating the conditions after the autumn sedimentation. Release of phosphorus to the overlying lake water was followed during 2–3 weeks in the laboratory. X-ray microanalysis of individual Microcystis and bacterial cells, and chemical phosphorus fractionation, were used to assess the phosphorus pool size in different fractions of the sediment. 2. Benthic Microcystis colonies, most of these having survived within the sediment for 1 year or more, were less susceptible to decomposition, and the specific growth rate of bacteria in their mucilage was lower than for other sediment bacteria. 3. Pelagic Microcystis colonies from late August were resistant to decomposition, when placed on the sediments. When Microcystis colonies from a declining pelagic population in October were added to the sediments, however, a substantial fraction of these colonies was decomposed. The specific growth rate of mucilage bacteria was five times higher than for other sediment bacteria. 4. Release of molybdate-reactive phosphorus to the overlying lake water was larger from sediment cores enriched with Microcystis colonies than from control cores. Chemical phosphorus fractionation showed a decrease in organic-bound phosphorus (residual P). 5. X-ray microanalysis showed that the phosphorus bound in Microcystis cells decreased by -0.300 mg g^?1 DW in the October experiment, due both to a decrease in biomass (i.e. mineralization) and to a decrease in phosphorus content in the remaining cells. Heterotrophic bacteria increased their cellular concentration of phosphorus. The net release of phosphorus from the Microcystis and bacterial pools corresponded to 74% of the decrease of organic-bound phosphorus in the chemical phosphorus fractionation, and to 65% of the decrease of total phosphorus in the upper 0–1 cm of the sediment. 6. Benthic bacteria and cyanobacteria may thus contribute significantly to changes in phosphorus content and turnover of the sediment by changes in their biomass, turnover rate and cellular phosphorus content.     

Evaluation of Microcystis as food for zooplankton in a eutrophic lake

Four experiments were conducted to evaluate Microcystis as food for zooplankton in Lake Kasumigaura, and the following results were obtained. (1) Moina micrura (Cladocera) showed little growth and no reproduction when the animal was reared with Microcystis cultured in the laboratory. The animal did not grow nor reproduce well when Chlorella was mixed with Microcystis as food. (2) Moina micrura assimilated Microcystis much less than Chlorella when the animal fed on single species of Microcystis or a mixture with Chlorella. (3) Microcystis collected from Lake Kasumigaura could not be utilized by Moina micrura even though the colonies were broken up into edible sizes. However, the alga turned into utilizable food when it was decomposed. (4) No inhibitors of Moina micrura population growth could be found in the non-filtered water of Lake Kasumigaura where Microcystis was blooming heavily. Decomposed Microcystis seemed to be utilized by zooplankton as an important food source in Lake Kasumigaura.     

Effect of temperature on growth of the cyanobacterium Aphanizomenon flos-aquae in Lake Biwa and Lake Yogo

The water bloom‐forming cyanobacterium Aphanizomenon flos‐aquae Ralfs ex Bornet et Flahault (Nos‐tocales, Cyanophyceae) appeared in Lake Biwa and Lake Yogo in 1999 for the first time. The morphological characteristics were described using natural samples. In contrast to the other water bloom‐forming cyanobacteria such as Microcystis and Anabaena in Lake Biwa and Lake Yogo, the small summer population of A. flos‐aquae is apt to grow in winter, suggesting the low temperature preference or tolerance of this species. In order to clarify the effect of temperature on the growth, culture experiments were conducted using an axenic strain isolated from Lake Biwa. The strain could grow at above 8°C with an optimum temperature ranging from 23 to 29°C, and survived even at 5°C for at least 25days under low light conditions. Although these results confirmed the ability of the bloom formation during late autumn and winter, it is still unclear why the Aphanizomenon bloom occurred at temperatures of ca 10°C in December and not immediately after the disappearance of Microcystis and/or Anabaena bloom during autumn.     

Taxonomic re-evaluation of Aphanizomenon flos-aquae NH-5 based on morphology and 16S rRNA gene sequences

The taxonomy of Aphanizomenon flos-aquae strain NH-5, a producer of cyanotoxins, was re-evaluated by comparison with six other Aphanizomenon strains using morphological characteristics and 16S rRNA gene sequences. Strain NH-5 was concluded to be improperly identified as Aph. flos-aquae based upon (1) lack of bundle formation in the trichomes, (2) location of akinetes next to heterocytes, (3) lower similarities (less than 97.5%) in the 16S rRNA gene sequences relative to Aph. flos-aquae strains, and (4) comparison within a phylogenetic tree constructed from 16S rRNA gene sequences. The Aphanizomenon strains investigated in this study are classified to four morphological groups as described by the classical taxonomy of Komárek & Kovácik (1989). This classification was supported from the phylogenetic results of 16S rRNA gene sequences. This study also discusses the generic boundaries between Aphanizomenon and Anabaena.     

The budget of dissolved trace metals in Lake Biwa,Japan

A comprehensive study on the dynamics of dissolved elements (Mg, Al, Si, P, Ca, V, Cr, Mn, Fe, Ni, Zn, As, Sr, Y, W, and U) in Lake Biwa was carried out using a clean technique. Lake water samples (n = 523) were collected from six stations in the North Basin and three stations in the South Basin. River water samples (n = 178) were collected from 14 major rivers flowing into the North Basin. Rainwater samples (n = 89) were collected at Otsu. The river water was enriched with Mn, Al, Fe, P, and Zn and the rainwater was enriched with Zn, Al, Fe, and Mn compared to North Basin water during winter mixing. The residence times of dissolved species were estimated on the basis of input through the rivers and rain. The residence times for Ca, Mg, and Sr were about 8 years, the same as that for water. Mn, Al, Fe, and Zn showed the shortest residence times (0.05–0.19 year). A budget calculation suggested that more than 60% of the input of dissolved Si, P, V, Cr, Mn, Fe, Ni, and Zn was scavenged and retained in the lake sediments and/or discharged as suspended particles.     

Horizontal distribution of carbon and nitrogen and their isotopic compositions in the surface sediment of Lake Biwa

The horizontal distribution of the abundance and isotopic composition of carbon and nitrogen was studied on surface sediment samples (0–15 cm) collected from the entire area of Lake Biwa, the largest freshwater lake in Japan. As water depth increased, a marked increase in organic matter content was observed at the sampling sites, especially in the western North Basin, characterized by a steep slope. In the northwestern North Basin, which has no major inflowing streams, the sediments contained large amounts of organic matter, suggesting the possibility of lateral transportation of sedimented matter from other places by lake currents. The total amounts of carbon and nitrogen in the top-2 cm of sediment of the entire area of Lake Biwa were estimated to be 9.2 × 10⁴ tC and 1.0 × 10⁴ tN. The δ¹⁵N values in the littoral sediment were low and close to those in the inflowing river sediment, suggesting selective sedimentation of allochthonous organic matter onto the littoral area. In the North Basin, vertical profiles of organic matter content and δ¹³C values of the sediments in the littoral area showed a smaller downward decrease than in the profundal area, whereas δ¹⁵N values decreased with sediment depth in both areas. It was suggested that the littoral sediments contained abundant amounts of allochthonous and relatively refractory organic matter. Further, it was suggested that the autochthonous organic matter originated from primary production deposited mainly on the profundal zone and was easily decomposed in early diagenesis after sedimentation. Received: July 30, 1999 / Accepted: December 10, 1999     

RECRUITMENT OF MICROCYSTIS (CYANOPHYCEAE) FROM LAKE SEDIMENTS: THE IMPORTANCE OF LITTORAL INOCULA1

Recruitment of Microcystis from sediments to the water column was investigated in shallow (1–2 m) and deep (6–7 m) areas of Lake Limmaren, central Sweden. Recruitment traps attached to the bottom were sampled on a weekly basis throughout the summer season ( June–September). A comparison between the two sites showed that the recruitment from the shallow bay was significantly higher over the entire season for all three Microcystis species present in the lake. Maximum rates of recruitment were found in August, when 2.3 × 10⁵ colonies m ^{? 2}·day ^{? 1} 1 Received 18 April 2002. Accepted 29 October 2002. left the sediments of the shallow area. Calculated over the entire summer, Microcystis colonies corresponding to 50% of the initial abundance in the surface sediments were recruited in the shallow bay, whereas recruitment from the deep area was only 8% of the sediment colonies. From these results we conclude that shallow areas, which to a large extent have been overlooked in studies of recruitment of phytoplankton, may be crucial to the dynamics of these organisms by playing an important role as inoculation sites for pelagic populations.     

MEASUREMENT OF IN SITU SPECIFIC GROWTH RATES OF MICROCYSTIS (CYANOBACTERIA) FROM THE FREQUENCY OF DIVIDING CELLS1

Diel changes in the frequency of dividing cells (FDC) of three Microcystis species were investigated in a small eutrophic pond from July to October 2005. The representative species was M. aeruginosa (Kütz.) Kütz., constituting 57%–86% of the Microcystis population throughout the study period, and the remainder were M. viridis (A. Braun) Lemmerm. and M. wesenbergii (Komárek) Komárek. The FDC of M. aeruginosa and M. wesenbergii increased in the daytime and fell in the nighttime in July and August, but this regular variation was not observed in September or October. The in situ specific growth rates of Microcystis species were estimated based on the assumption that the specific growth rate can be given as an absolute value of the derivative of FDC with respect to time. The calculated values were similar among species—0.15–0.38 · d^?1 for M. aeruginosa, 0.14–0.63 · d^?1 for M. viridis, and 0.18–0.61 · d^?1 for M. wesenbergii. The specific growth rates in July and August slightly exceeded those in September and October. The analysis of the in situ specific growth rate of Microcystis indicated that recruitment of the benthic population or morphological change, rather than massive growth, was at least partly responsible for the dominance of M. aeruginosa in the study pond.     

Depth-related changes in benthic diatom assemblages of a maritime Antarctic lake

Summary The epiphytic diatoms of filamentous matforming algae in Sombre Lake, Signy Island, South Orkneys, Antarctica were examined in a field study from January to March 1987. Results reveal 1) a shallow shelf assemblage, dominated by Fragilaria spp, with high percentages of empty frustules, and low numbers of viable cells and, 2) a mid-depth zone containing the most luxuriant epiphytic growth dominated by Achnanthes minutisima Kütz., Synedra rumpens var. familiaris (Kütz) Hust., and Cymbella minuta Hilse ex Rabenh., and 3) a deep water assemblage with low numbers of actively growing cells, dominated by Stauroneis anceps var. hyalina M. Perag. et Brun. in Hérib. The comparison of methods and their impact on ecological interpretation is discussed together with a consideration of the principal factors potentially influencing community structure.     

Taxonomic status of the species of the green algal genusNeochloris

Komárek has recently reviewed the various species assigned to the green algal genusNeochloris Starr (Chlorococcales, Chlorococcaceae) and removed those with uninucleate vegetative cells to a new genus,Ettlia. Watanabe & Floyd, unaware ofKomárek's work, also reviewed the species ofNeochloris and distributed them among three genera—Neochloris, Chlorococcopsis gen. nov., andParietochloris gen. nov.—on the basis of details of the covering of the zoospore and the arrangement of the basal bodies of the flagellar apparatus. This paper reconciles these two treatments and makes additional recommendations at the ranks of genus, family, order, and class.