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

Abstract

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.

Different tolerances and responses to low temperature and darkness between waterbloom forming cyanobacterium Microcystis and a green alga Scenedesmus

“…Compared to colonial Microcystis, the cell number of unicellular Microcystis had a significant decrease, suggesting that colonial Microcystis can survive a longer period under the stress of darkness and low temperature than unicellular Microcystis. This may be an explanation why Microcystis colonies were found in winter at the bottom of freshwater lakes (Reynolds et al, 1981;Tsujimura et al, 2000;Brunberg & Blomqvist, 2002;Latour et al, 2004a;Verspagen et al, 2004Verspagen et al, , 2005. Our previous study also indicated that colonial Microcystis exhibited more tolerance to copper than unicellular Microcystis (Wu et al, 2007), supporting that higher endurance to environmental stresses might play an important role in survivals of colonial Microcystis.…”

“…However, unlike some algae with capacity to form resting spores or stages (Anderson, 1976), cyst stages (Dale, 1983;Lewis et al, 1999), and akinetes (Li et al, 1997), Microcystis remains in a vegetative cell state at the sediment for the whole winter (Fallon & Brock, 1981;Reynolds et al, 1981;Tsujimura et al, 2000). Verspagen et al (2004) found that there were photochemically viable Microcystis cells in the water columns and sediments of Lake Volkerak throughout the year. Previous studies have been conducted by dealing with responses of Microcystis and other phytoplankton to only one factor of darkness or low temperature (Dehning & Tilzer, 1989;Furusato et al, 2004;Chu et al, 2007;Zhang et al, 2007).…”

Rapid adaptation of harmful cyanobacteria to rising CO 2

“…Furthermore, the chemostats applied homogeneous mixing to single-celled Microcystis populations, whereas Microcystis often develops multicellular colonies migrating vertically in stagnant lakes (37). Microcystis is also known to survive prolonged burial in lake sediments, after which it can be resuspended in the water column (38). Both spatiotemporal heterogeneity and reseeding from the sediments tend to promote diversity, and may explain the observed co-occurrence of all three C i uptake genotypes in the lake.…”

Vertical Heterogeneity of Genotypic Structure and Toxic Potential within Populations of the Harmful Cyanobacterium Microcystis aeruginosa

“…According to the literature, several factors could potentially drive such a vertical heterogeneity by influenc-ing the distribution of the colonies of M. aeruginosa into the water column: 1) interactions with the benthic compartment, by recruiting benthic colonies or by the continuous sedimentation of planktonic ones [29,40]; 2) vertical migration of the colonies, which can regulate their position in the water column by balancing their internal stock of gas vacuoles and carbon reserves [13,30,41,42]; and 3) fitness-related natural selection, which could allow some particular genotypes or phenotypes to better survive or develop at a specific depth because of particular conditions such as light intensity, nutrients availability or benefits of microcystin production [10,23,[43][44][45]. Since the qualitative shifts we observed were mainly restricted to the surface water layers of the deep reservoirs we studied, the influence of the interactions with the benthic compartment through recruitment or sedimentation can be minimized.…”

“…Many comprehensive studies have therefore been realized in order to explain the ecological success of this cyanobacterium and to unravel the complex set of factors driving both its dynamics and its toxicity. Such previous studies mainly dealt with the genetic diversity of M. aeruginosa's populations [4][5][6][7], the toxic potential of M. aeruginosa [8][9][10][11][12], and the determinism of the different steps of M. aeruginosa's life cycle [13][14][15][16].…”