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Abstract:The present study focuses on a description of the biodiversity of subaerial phototrophic biofilms occurring on archaeological surfaces in Maltese hypogean environments, namely St Paul's, St Agatha's and Abbatija tad-Dejr Catacombs, all situated in Rabat and the ancient Ħal Saflieni Hypogeum at Paola, Malta. Direct observation of the biofilms, carried out using light (LM), epifluorescent and confocal laser scanning microscopy (CLSM), allowed the description of the major cyanobacterial and microalgal taxa, and also the associated heterotrophic microorganisms, mainly actinobacteria. Some biofilm microorganisms were able to grow in culture and this allowed the isolation of cyanobacterial, microalgal and chemoorganotrophic bacterial strains. Thylakoid arrangement and cell division were examined using transmission electron microscopy (TEM). The cytomorphology of isolated microorganisms was described. The undisputed phototrophic protagonists in these subaerial biofilms of Maltese hypogean environments were the non-heterocytous (Oscillatorialean) cyanobacteria. In order to increase the limited data available for Leptolyngbya spp. from aerophytic epilithic biofilms in catacombs, the 16S rRNA genes of isolated Leptolyngbya strains were sequenced and compared with those obtained for related strains. Phylogenetic trees of cyanobacterial 16S rRNA sequences were constructed using parsimony and Bayesian analyses. Microorganisms forming biofilms in Maltese hypogea were found to be similar, both cytomorphologically and genetically, to those colonising lithic surfaces of caves and catacombs in other Mediterranean countries.

In this study, we present the effects of environmental factors on phytoplankton in two different oxbow lakes formed from the Vistula River. One of these lakes is under extreme anthropogenic pressure, i.e. impacted by industrial and municipal effluents and characterized by high water temperature and high concentrations of ions. The other oxbow lake is located in a natural forest with some its abiotic factors lower compared to the first oxbow lake. There were significant changes among environmental parameters, which influenced significant differences between the phytoplankton populations. The dominant algae of the impacted oxbow were small centric diatoms, which are able to rapidly absorb nutrients and have a rapid turnover. The results demonstrated that human impacted lake was a habitat, where species with a broad range of ecological requirements survive. However, these habitats are also more vulnerable for the entry and colonization of invasive algal species. In the forest oxbow lake, without human impact, heterogeneity of water quality parameters resulted in different specific habitats. Oxbow lakes are important as biodiversity hotspots for rare and/or endemic species.

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Background and aims -Phenotypic plasticity is the morphological response of a single genotype to produce several different physiological and/or morphological types under changing environmental conditions. Desmodesmus, a green alga, has been well documented for its phenotypic responses (e.g. colony transformation to unicells, and spiny transformation to spineless). Two oxbow lakes (R1 and R2) and one artificial dam reservoir (R3) were used to evaluate the quantity and diversity range of Desmodesmus species occurring during four seasons of one-year. Methods -Water samples were quantified for Desmodesmus species and identified using the Scanning Electron Microscope (SEM). To better understand the morphological responses of Desmodesmus, an axenic culture of Desmodesmus abundans, CCAP 258/299, was cultured in sterile-filtered water from each water body collected from each season. Key results -Not surprisingly, different Desmodesmus species inhabited each water body and exhibited different densities of growth, contributing 77% of the total density in R1, 22% in R2 and only 1% in R3. Summer and autumn were the seasons when Desmodesmus showed the highest density in the shallow and polymictic oxbow lakes (R1 and R2). In the deep and dimictic reservoir (R3), the highest density of Desmodesmus was during autumn. Phenotypic plasticity was observed in the experiments, with unicells being formed from colonies, some with shorter spines. Desmodesmus grown in a high concentration of nitrogen (SE, Medium 7 +3N and BBM) had a higher percentage of unicells compared to the other culture conditions. Conclusions -Our studies are important from both taxonomical and ecological points of view, since our results showed that there may be new possibilities for using the phenotypic plasticity of Desmodesmus to assess water quality and as a potential bioindicator of nutrient availability in natural ecosystems.