A MODEST review

We present an account of the state of the art in the fields explored by the research community invested in 'Modeling and Observing DEnse STellar systems'. For this purpose, we take as a basis the activities of the MODEST-17 conference, which was held at Charles University, Prague, in September 2017. Reviewed topics include recent advances in fundamental stellar dynamics, numerical methods for the solution of the gravitational N-body problem, formation and evolution of young and old star clusters and galactic nuclei, their elusive stellar populations, planetary systems, and exotic compact objects, with timely attention to black holes of different classes of mass and their role as sources of gravitational waves. Such a breadth of topics reflects the growing role played by collisional stellar dynamics in numerous areas of modern astrophysics. Indeed, in the next decade, many revolutionary instruments will enable the derivation of positions and velocities of individual stars in the Milky Way and its satellites and will detect signals from a range of astrophysical sources in different portions of the electromagnetic and gravitational spectrum, with an unprecedented sensitivity. On the one hand, this wealth of data will allow us to address a number of long-standing open questions in star cluster studies; on the other hand, many unexpected properties of these systems will come to light, stimulating further progress of our understanding of their formation and evolution.Comment: 42 pages; accepted for publication in 'Computational Astrophysics and Cosmology'. We are much grateful to the organisers of the MODEST-17 conference (Charles University, Prague, September 2017). We acknowledge the input provided by all MODEST-17 participants, and, more generally, by the members of the MODEST communit

Cytoskeleton organisation during the infection of three brown algal species, Ectocarpus siliculosus, Ectocarpus crouaniorum and Pylaiella littoralis, by the intracellular marine oomycete Eurychasma dicksonii

Oomycete diseases in seaweeds are probably widespread and of significant ecological and economic impact, but overall still poorly understood. This study investigates the organisation of the cytoskeleton during infection of three brown algal species, Pylaiella littoralis, Ectocarpus siliculosus, and Ectocarpus crouaniorum, by the basal marine oomycete Eurychasma dicksonii. Immunofluorescence staining of tubulin revealed how the development of this intracellular biotrophic pathogen impacts on microtubule (MT) organisation of its algal host. The host MT cytoskeleton remains normal and organised by the centrosome until very late stages of the infection. Additionally, the organisation of the parasite's cytoskeleton was examined. During mitosis of the E. dicksonii nucleus the MT focal point (microtubule organisation centre, MTOC, putative centrosome) duplicates and each daughter MTOC migrates to opposite poles of the nucleus. This similarity in MT organisation between the host and pathogen reflects the relatively close phylogenetic relationship between oomycetes and brown algae. Moreover, actin labelling with rhodamine-phalloidin in E. dicksonii revealed typical images of actin dots connected by fine actin filament bundles in the cortical cytoplasm. The functional and phylogenetic implications of our observations are discussed. © 2013 German Botanical Society and The Royal Botanical Society of the Netherlands

New insights on Laminaria digitata ultrastructure through combined conventional chemical fixation and cryofixation

The objective of the present study is to examine the fine structure of vegetative cells of Laminaria digitata using both chemical fixation and cryofixation. Laminaria digitata was chosen due to its importance as a model organism in a wide range of biological studies, as a keystone species on rocky shores of the North Atlantic, its use of iodide as a unique inorganic antioxidant, and its significance as a raw material for the production of alginate. Details of the fine structural features of vegetative cells are described, with particular emphasis on the differences between the two methods used, i.e. conventional chemical fixation and freeze-fixation. The general structure of the cells was similar to that already described, with minor differences between the different cell types. An intense activity of the Golgi system was found associated with the thick external cell wall, with large dictyosomes from which numerous vesicles and cisternae are released. An interesting type of cisternae was found in the cryofixed material, which was not visible with the chemical fixation. These are elongated structures, in sections appearing tubule-like, close to the external cell wall or to young internal walls. An increased number of these structures was observed near the plasmodesmata of the pit fields. They are similar to the "flat cisternae"found associated with the forming cytokinetic diaphragm of brown algae. Their possible role is discussed. The new findings of this work underline the importance of such combined studies which reveal new data not known until now using the old conventional methods. The main conclusion of the present study is that cryofixation is the method of choice for studying Laminaria cytology by transmission electron microscopy. © 2021 Walter de Gruyter GmbH, Berlin/Boston 2021

Genetic diversity and structure of Cymodocea nodosa meadows in the Aegean sea, eastern Mediterranean

Genetic diversity and structure were investigated among seven meadows of Cymodocea nodosa in the Aegean Sea, eastern Mediterranean basin. Analysis of three genetic loci, corresponding to the nuclear rDNA operon, revealed 12 distinct multilocus RFLP genotypes. The observed FST pairwise values revealed, in most cases, a previously unidentified genetic diversity in Aegean populations. A relative sub-structuring was revealed within Pagassitikos gulf, suggesting a fragmentation possibly due to the significant factor of the previously recorded cyclonic system within the studied gulf. Moreover, individual-based landscape approach analysis supports the latter observation, suggesting the presence of three different sub-populations overall in the studied area. The observed barriers maybe related to angiosperms re-colonization from warmer parts of the eastern Mediterranean, after the last glacial maximum. AMOVA indicated the existence of hierarchical significant genetic variation between meadows within gulfs and among samples within meadows, rather than between the two gulfs, as a potential consideration for unidentified limited dispersal and demographic habits of the species in question. On the other hand, sexual reproduction events may contribute to the maintenance of genetic diversity of C. nodosa structuring in the Aegean Sea, where the species is also exhibiting extensive morphological plasticity. © 2016, ALÖKI Kft., Budapest, Hungary

Macroalgal community response to re-oligotrophication in Saronikos Gulf

The Saronikos Gulf, an embayment of the Aegean Sea (Greece, Eastern Mediterranean), has undergone profound and unprecedented environmental changes due to the rapid urbanization of the Athens-Piraeus metropolitan area since the 1950s. From the mid-1950s until the mid- 1990s, the inner part of Saronikos Gulf was subject to increasing inputs of untreated urban sewage. The resulting long-lasting eutrophication was reflected in the phytobenthos by the presence of low-diversity, mainly ephemeral species communities, largely dominated by the nitrophilous green algae Ulva spp. and Cladophora spp. In the mid-1990s, a wastewater treatment plant started operating on Psittalia islet near the port of Piraeus, capturing the entire central sewage outfall of Athens. Since 1998, the coastal ecosystem of Saronikos Gulf has been under regular monitoring to assess intra-annual and interannual changes in benthic macroalgal communities and nutrient levels. A sharp decline in nutrient and organic loads was soon recorded, leading to a rapid re-oligotrophication of the gulf. Upon this new regime shift, macroalgal communities responded by a general increase in biodiversity and a marked decrease in the abundance of nitrophilous green algae. Within the last years, however, the canopy brown algae (Cystoseira and Sargassum) in the area unexpectedly showed a sharp population decline. Even though this pheno - me non may be still ongoing and thus is not fully described and understood, this study is the first to provide a long-term data set of macroalgal responses to a rapid re-oligotrophication process taking place within a highly urbanized Mediterranean coastal area. © Inter-Research 2013

New confirmed depth limit of Antarctic macroalgae: Palmaria decipiens found at 100 m depth in the Southern Ocean

Living specimens of the macroalga Palmaria decipiens were collected from 100 m depth, representing a new confirmed depth record, considerably exceeding the previous record of 42 m depth. Previous deeper collections (below conventional SCUBA depths) have relied on dredge/grab samples or drop camera surveys. Remote techniques cannot conclusively prove that macroalgae are living at these depths, as algae detach from shallower substrata, e.g., through ice scouring, and drift to depths below their growth limit. This, combined with a low rate of decay of macroalgae around Antarctica, requires validation that algal samples from depth have grown in situ. Estimates of macroalgal biomass, energy fluxes, and the potential energy fixation may need adjusting to consider the deeper growing depths particularly with glacial retreat along the Antarctic Peninsula revealing areas of rocky substrata for macroalgal colonisation. The confirmed extension of depth where macroalgae can grow will have implications for assessments of benthic productivity and food webs in Antarctica. © 2022, The Author(s)

Comparative Immunofluorescence and Ultrastructural Analysis of Microtubule Organization in Uronema sp., Klebsormidium flaccidum, K. subtilissimum, Stichococcus bacillaris and S. chloranthus (Chlorophyta)

A detailed comparative examination of microtubule (MT) organization in interphase and dividing cells of Uronema sp., Klebsormidium flaccidum, K. subtilissimum, Stichococcus bacillaris and S. chloranthus was made using tubulin immunofluorescence and transmission electron microscopy (TEM). During interphase all the species bear a well-organized cortical MT system, consisting of parallel bundles with different orientations. In Uronema sp. the cortical MT bundles are longitudinally oriented, whereas in the other species they are in transverse orientation to the axis of the cells. Considerable differences in MT organization were also observed during stages of mitosis, mainly preprophase, as well as cytokinesis. In Uronema sp., a particular radial MT assembly is organized during preprophase-early prophase, which was not observed in the other species. In Stichococcus a fine MT ring surrounded the nucleus during preprophase and prophase. An MT ring, together with single cytoplasmic MTs, was also found associated with the developing diaphragm during cytokinesis in Stichococcus. A phycoplast participates in cytokinesis in Uronema sp., but not in the other species. In Uronema sp. the centrosome functions as a microtubule organizing center (MTOC) during mitosis, but not during interphase and cytokinesis. The phylogenetic significance of these differences is discussed in combination with SSU/ITS sequencing and other, existing molecular data. © 2010 Elsevier GmbH

Biogeography pattern of the marine angiosperm Cymodocea nodosa in the eastern Mediterranean Sea related to the quaternary climatic changes

We investigated the population dynamics of a highly clonal marine angiosperm, Cymodocea nodosa, in the eastern Mediterranean Sea, to identify the historical dynamics, demography, and connectivity of the species in the area. Eighteen microsatellite loci were used in conjunction with coalescent methods to investigate the genetic structure and demographic history of C. nodosa meadows. Approximate Bayesian computation (ABC) modeling was used to examine the pattern of divergence over time in the context of environmental change over the course of the Quaternary period. ABC analysis revealed an initial split of the C. nodosa populations between the north-western, northern, and north-eastern Aegean Sea during the Pleistocene epoch, followed by a more recent divergence of the north-western population and the central-western part of the Aegean Sea. According to the results, the most parsimonious historical scenario is that of a pervasive genetic signature of the effects of the drop in sea level during the Pleistocene epoch. This scenario supports the isolation of the north-western, north, and north-eastern area, and the subsequent recolonization after post-glaciation sea level rise that may explain the north-western differentiation as well present-day detected dispersion of C. nodosa. © 2022 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd