The ecology of freshwater epipelic algae: an update

Epipelic algae perform a range of ecosystem functions including biostabilisation of sediments, regulation of benthic–pelagic nutrient cycling, and primary production. There is a growing need to understand their ecological role in light of current and future alterations in sediment loading resulting from land-use change and land management practices. Although the majority of recent work on epipelic algal ecology has been conducted within estuarine ecosystems, significant advances have also been made in freshwaters. We review these recent studies in combination with more classical freshwater approaches to highlight the importance of freshwater epipelic algal ecology and to aid discussions regarding future research. A summary of benthic algal groups is given with particular emphasis on substratum preference and habitat boundaries. No standard methodology exists for sampling freshwater epipelon, and we discuss the advantages and disadvantages of a suite of currently employed methodologies. Spatial variability in epipelic community biodiversity is discussed from the micro-scale (i.e. vertical migration in the sediment surface) to the ecosystem scale (i.e. lake vs river habitats), and finally at the geographic scale (i.e. the ‘ubiquity’ of epipelic species). Factors regulating epipelon community composition and biomass (e.g. reproductive strategies, habitat disturbance, grazing pressures, resource limitation, resilience, symbiosis, and parasitism) are also reviewed. Finally, examples of specific epipelic ecosystem functions (e.g. primary production, biostabilisation, and regulation of biogeochemical cycling) are given and areas of research requiring particular focus in the future are outlined

A time-calibrated multi-gene phylogeny provides insights into the evolution, taxonomy and DNA barcoding of the Pinnularia gibba group (Bacillariophyta )

Many diatom groups are known for widespread (pseudo)cryptic species diversity and Pinnularia gibba group is one of them. Recently, KoLLAR et al. (2019) delimited species within the group by means of a polyphasic approach, providing an evidence for the existence of fifteen species. In order to further guide the systematic revision of the group, the present study focuses on the evolution and morphology of the P. gibba group. Using representatives of the fifteen species we used fossils to constrain and calibrate a multi-gene species-level phylogeny. Although many species are morphologically highly similar, significant differences in cell size were detected in different sections of the tree, suggestive of ongoing morphological differentiation. We further used the phylogenetic analysis to assess marker resolution for DNA barcoding, showing that the proposed diatom barcode markers rbcL and V4 SSU rDNA can distinguish closely related (pseudo)cryptic species of the group

Heterothallic auxosporulation, incunabula and perizonium in Pinnularia (Bacillariophyceae)

The cytology and life cycle of Pinnularia cf. gibba was examined in nine clones from three Scottish localities. This freshwater epipelic diatom is heterothallic and produces two isogametes per gametangium in type IC auxosporulation (Geitler's classification [1973]). The zygote undergoes a highly unusual metamorphosis before beginning expansion, becoming shortly linear-lanceolate; this is accompanied by formation of a complete covering of thin, oxidation-resistant strips and scale-like structures (at the poles), which are quite separate from the perizonium formed during auxospore expansion. Observations of similar incunabular structures in P. acidojaponica show that these elements are siliceous. The P. cf. gibba perizonium also has unusual features, including a remarkably wide primary band. Trikaryotic and haploid auxospores are sometimes formed and haploid 'zygotes' mature and expand like diploids, but do not develop into mature initial cells. Several phases of mucilage secretion take place, from the gametangia, zygotes and auxospores. Triplets of gametangia and polyspermy occurred with high frequency; this and the systematic significance of variation in auxospore, incunabula and perizonium structure, are discussed. Aspects of the taxonomy of the P. gibba group are treated in supplementary material provided on the European Journal of Phycology website