High-throughput sequencing (HTS) was used to analyze the seasonal variations in the bacterioplankton community composition (BCC) in the euphotic layer of a large and deep lake south of the Alps (Lake ...Garda). The BCC was analyzed throughout two annual cycles by monthly samplings using the amplification and sequencing of the V3–V4 hypervariable region of the 16S rRNA gene by the MiSeq Illumina platform. The dominant and most diverse bacterioplankton phyla were among the more frequently reported in freshwater ecosystems, including the Proteobacteria, Cyanobacteria, Bacteroidetes, Verrucomicrobia, Actinobacteria, and Planctomycetes. As a distinctive feature, the development of the BCC showed a cyclical temporal pattern in the two analyzed years and throughout the euphotic layer. The recurring temporal development was controlled by the strong seasonality in water temperature and thermal stratification, and by cyclical temporal changes in nutrients and, possibly, by the remarkable annual cyclical development of cyanobacteria and eukaryotic phytoplankton hosting bacterioplankton that characterizes Lake Garda. Further downstream analyses of operational taxonomic units associated to cyanobacteria allowed confirming the presence of the most abundant taxa previously identified by microscopy and/or phylogenetic analyses, as well as the presence of other small Synechococcales/Chroococcales and rare Nostocales never identified so far in the deep lakes south of the Alps. The implications of the high diversity and strong seasonality are relevant, opening perspectives for the definition of common and discriminating patterns characterizing the temporal and spatial distribution in the BCC, and for the application of the new sequencing technologies in the monitoring of water quality in large and deep lakes.
The structure of microbial communities, microalgae, heterotrophic protozoa and fungi contributes to characterize food webs and productivity and, from an anthropogenic point of view, the qualitative ...characteristics of water bodies. Traditionally, in freshwater environments many investigations have been directed to the study of pelagic microalgae ("phytoplankton") and periphyton (i.e., photosynthetic and mixotrophic protists) through the use of light microscopy (LM). While the number of studies on bacterioplankton communities have shown a substantial increase after the advent of high-throughput sequencing (HTS) approaches, the study of the composition, structure, and spatio-temporal patterns of microbial eukaryotes in freshwater environments was much less widespread. Moreover, the understanding of the correspondence between the relative phytoplankton abundances estimated by HTS and LM is still incomplete. Taking into account these limitations, this study examined the biodiversity and seasonality of the community of eukaryotic microplankton in the epilimnetic layer of a large and deep perialpine lake (Lake Garda) using HTS. The analyses were carried out at monthly frequency during 2014 and 2015. The results highlighted the existence of a rich and well diversified community and the presence of numerous phytoplankton taxa that were never identified by LM in previous investigations. Furthermore, the relative abundances of phytoplankton estimated by HTS and LM showed a significant relationship at different taxonomic ranks. In the 2 years of investigation, the temporal development of the whole micro-eukaryotic community showed a clear non-random and comparable distribution pattern, with the main taxonomic groups coherently distributed in the individual seasons. In perspective, the results obtained in this study highlight the importance of HTS approaches in assessing biodiversity and the relative importance of the main protist groups along environmental gradients, including those caused by anthropogenic impacts (e.g., eutrophication and climate change).
The seasonal development of planktonic diatoms in Lake Tovel has been studied from 2002 to 2004 within the SALTO research project. The objective was to investigate the role of weather conditions and ...lake hydrology in regulating the development of Cyclotella and Fragilaria functional species groups. The different responses of the two species to the environmental variability and their reciprocal relationships were considered. Cyclotella showed a weak relationship with the selected environmental variables and its seasonal development was difficult to interpret, also in relation to its opportunistic behaviour. On the contrary, Fragilaria complex showed a prompt response to environmental variability and its seasonal development appeared to be mainly regulated by hydrological conditions, through nitrate nitrogen and, secondarily, by silica concentrations. Water temperature and thermal stability affected only Fragilaria species, while different nutrient requirements allowed Cyclotella species to grow under conditions of limited resources for Fragilaria. Since Lake Tovel has scarce internal nutrient reserves and its hydrology is strongly affected by precipitation both in winter and summer, diatom phenology seems to be indirectly regulated by weather conditions.
We describe here the draft genome sequence of the cyanobacterium
, assembled from a metagenome of a nonaxenic culture. The strain (FEM_GT703) was isolated from a freshwater sample taken from Lake ...Garda, Italy. The draft genome sequence represents the first assembled
strain.
Among cyanobacteria, Planktothrix rubescens (De Candolle ex Gomont) Anagnostidis & Komárek is a species that is well adapted to develop in moderately nutrient rich and deep lakes. In this typology of ...waterbodies, the competitive abilities of this species rely in its capacity to stand and growth in the dimly illuminated metalimnetic layer during the warmer months. I this work, we have studied the seasonal development and distribution of this species in Lake Ledro, a meso-oligotrophic reservoir located in the Eastern Alps. During the last decade, this species has given rise to numerous and extended surface bloom episodes, causing the reddening of vaste areas of the lake. In summer, the light intensities in the zone of the greater development of this cyanobacterium (in the metalimnion, between the euphotic depth and the layer of maximum development of the species) were bewteen 2 and 20 µmol m-2 s-1, i.e. values that were well within the light intensities required to sustain the optimal growth of filaments. The formation of the autumn and winter blooms was triggered by the cooling of surface waters and increase of the mixed layer, which, eroding the metalimnion, entrained the filaments of Planktothrix in the surface mixed layers. The formation of the surface blooms was associated with the presence of high amounts of microcistins, which in a few occasions reached concentrations between 10 and 22 µg L-1, posing potential problems for the exploitation of water resources.
ABSTRACT
Freshwater ecosystems represent 0.01% of the water on Earth, but they support 6% of global biodiversity that is still mostly uncharacterized. Here, we describe the genome sequences of three ...strains belonging to novel species in the
Pseudomonas
,
Flavobacterium
, and
Sediminibacterium
genera recovered from a water sample of Lake Garda, Italy.
The long-term research carried out since the 1970s in the deep perialpine Lake Garda documented more than a threefold increase of total phosphorus (TP) in the whole water column until mid-2000s and a ...continuous warming. We evaluated the impact of these changes on phytoplankton, assessing similarities and peculiarities compared with other lakes. The increase of TP favoured cyanobacteria (microcystins producer
Planktothrix rubescens
(De Candolle ex Gomont) Anagnostidis & Komárek) and large diatoms. The warming of the lake caused a decrease in the frequency of full mixing episodes, which stopped completely after 2006, and a lower supply of nutrients to the upper layers. This “climate warming-induced oligotrophication” was mediated by the physiographic characteristics typical of deep lakes. The decrease of nutrients in the upper layers induced by the long period of incomplete mixing caused a decline of the mesotrophic
P. rubescens
, which was partially replaced by anatoxins producer
Tychonema bourrellyi
(J.W.G. Lund) Anagnostidis & Komárek. Lake warming favoured a higher development of mixotrophic dinoflagellates and cryptophytes. Many dinoflagellates are adapted to grow in warm and low nutrient lakes, therefore it is likely that the development of selected species will increase in large (meso-)oligotrophic lakes because of climate change.
This work allowed assessing a widespread occurrence of Tychonema bourrellyi in the largest lakes south of the Alps (Garda, Iseo, Como and Maggiore). The taxonomy of the species was confirmed adopting ...a polyphasic approach, which included microscopic examinations, molecular (16S rRNA and rbcLX sequences) and (Lake Garda) ecological characterisations. Over 70% of the 36 isolates of Tychonema sampled from the four lakes tested positive for the presence of genes implicated in the biosynthesis of anatoxins (anaF and/or anaC) and for the production of anatoxin-a (ATX) and homoanatoxin-a (HTX). A detailed analysis carried out in Lake Garda showed strong ongoing changes in the cyanobacterial community, with populations of Tychonema developing with higher biovolumes compared to the microcystins (MCs) producer Planktothrix rubescens. Moreover, the time × depth distribution of Tychonema was paralleled by a comparable distribution of ATX and HTX. The increasing importance of Tychonema in Lake Garda was also suggested by the opposite trends of ATX and MCs observed since 2009. These results suggest that radical changes are occurring in the largest lakes south of the Alps. Their verification and implications will require to be assessed by extending a complete experimental work to the other large perialpine lakes.
This work highlights and discusses the implications of the ongoing replacement of Planktothrix and microcystins (hepatotoxins) by Tychonema and anatoxin-a (and homoanatoxin-a) (neurotoxins) in the large lakes south of the Alps.
Since the beginning of the 1990s, Lake Garda showed the appearance of extended surface water blooms of Dolichospermum lemmermannii (Nostocales, Cyanobacteria). Between the 1990s and the 2000s, the ...blooms appeared also in the other large lakes south of the Alps (Iseo, Como and Maggiore). Despite the sudden appearance of the blooms, the correct identification of the establishment time of the populations of Dolichospermum in the southern subalpine lake district remained unclear. In this work, the establishment of the populations of D. lemmermannii in Lake Garda has been evaluated by the direct counting of sub-fossil akinetes extracted from sediment cores, and by estimating the abundance of filaments germinated from sub-fossil viable akinetes. The two techniques provided comparable results, allowing locating the beginning of the establishment of Dolichospermum around the middle of the 1960s. Four strains of Dolichospermum germinated from akinetes isolated from the core sediments between around the 1989 and 2012 did not show any mutation or recombination signal in the rpob gene sequences, suggesting a strong founder effect. The establishment of Dolichospermum coincided with the beginning of the rapid increase of total phosphorus as inferred from the distribution of sub-fossil diatoms in the sediment core. These results supported the hypothesis of a strong link between the shift of Lake Garda from ultraoligotrophy/oligotrophy to oligo-mesotrophy and the development of Dolichospermum. This colonisation pattern was possibly reinforced by the increase in the water temperatures in the subalpine lake district during the last 3 decades. In warmer lakes, gas-vacuolated Nostocales are favoured by high replication rates and, in particular, by their ability to control vertical movements in stratified water columns. This allows these species to exploit the gradients of light and nutrients, giving them a competitive advantage compared to other species. From a management point of view, the control and decrease of Dolichospermum should be obtained through the reduction and control of nutrient loads to the lake.
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