2016-06, Buhmann, Matthias T., Schulze, Birgit, Förderer, Alexander, Schleheck, David, Kroth, Peter G.

Benthic diatoms live in photoautotrophic/heterotrophic biofilm communities embedded in a matrix of secreted extracellular polymeric substances. Closely associated bacteria influence their growth, aggregation, and secretion of exopolymers. We have studied a diatom/bacteria model community, in which a marine Roseobacter strain is able to grow with secreted diatom exopolymers as sole source of carbon. The strain influences the aggregation of Phaeodactylum tricornutum by inducing a morphotypic transition from planktonic, fusiform cells to benthic, oval cells. Analysis of the extracellular soluble proteome of P. tricornutum in the presence and absence of bacteria revealed constitutively expressed newly identified proteins with mucin-like domains that appear to be typical for extracellular diatom proteins. In contrast to mucins, the proline-, serine-, threonine-rich (PST) domains in these proteins were also found in combination with protease-, glucosidase and leucine-rich repeat (LRR-) domains. Bioinformatic functional predictions indicate that several of these newly identified diatom-specific proteins may be involved in algal defense, intercellular signaling, and aggregation This article is protected by copyright. All rights reserved.

2012-02, Buhmann, Matthias T., Kroth, Peter G., Schleheck, David

Biofilm communities in the euphotic zone of aquatic habitats comprise photoautotrophic microorganisms, such as diatoms, green algae and cyanobacteria, which produce the organic carbon that fuels the life of a heterotrophic contingent of microorganisms, mostly bacteria. Such photoautotrophic–heterotrophic mixed-species biofilms have received little attention in biofilm research due to a lack of suitable pure-culture laboratory model systems. However, they offer important insight into microbial population dynamics and community interactions during a biofilm-developmental process that shapes highly structured, extremely well-adapted microbial landscapes. Here, we report on the development of a sterile incubation chamber for growing and monitoring axenic phototrophic biofilms, i.e. a sterilizable, illuminated, continuous-flow system for a routine work with pure cultures. The system has been designed to simulate the growth conditions in the shallow, littoral zone of aquatic habitats (horizontal surface, submerged in water, illuminated, aerated). Additional features of the concept include automated photometrical monitoring of biofilm density (as biofilm turbidity), analysis via confocal microscopy, direct harvesting of cells, and options to control illumination, flow velocity, and composition of culture fluid. The application of the system was demonstrated in growth experiments using axenic diatom biofilms, or axenic diatom biofilms co-cultivated with different bacterial strains isolated from epilithic biofilms of an oligotrophic freshwater lake.

2015, Schulze, Birgit, Buhmann, Matthias T., Río Bártulos, Carolina, Kroth, Peter G.

We have screened the genome of the marine diatom Phaeodactylum tricornutum for gene models encoding proteins exhibiting leucine-rich repeat (LRR) structures. In order to reveal the functionality of these proteins, their amino acid sequences were scanned for known domains and for homologies to other proteins. Additionally, proteins were categorized into different LRR-families according to the variable sequence part of their LRR. This approach enabled us to group proteins with potentially similar functionality and to classify also LRR proteins where no characterized homologues in other organisms exist. Most interestingly, we were able to indentify several transmembrane LRR-proteins, which are likely to function as receptor-like molecules. However, none of them carry additional domains that are typical for mammalian or plant-like receptors. Thus, the respective signal recognition pathways seem to be substantially different in diatoms. Moreover, P. tricornutum encodes a family of secreted LRR proteins likely to function as adhesion or binding proteins as part of the extracellular matrix. Additionally, intracellular LRR-only proteins were divided into proteins similar to RasGTPase activators, regulators of nuclear transport, and mitotic regulation. Our approach allowed us to draw a detailed picture of the conservation and diversification of LRR proteins in the marine diatom P. tricornutum.

2013-08, Buhmann, Matthias T., Day, John G., Kroth, Peter G.

Over recent years, several planktonic and benthic freshwater diatom taxa have been established as laboratory model strains. In common with most freshwater diatoms the pennate diatom Planothidium frequentissimum suffers irreversible cell shrinkage on prolonged maintenance by serial transfers, without induction of the sexual cycle. Therefore, alternative strategies are required for the long-term maintenance of this strain. Conventional colligative cryopreservation approaches have previously proven unsuccessful with no regrowth. However, in this study using 5% dimethyl sulfoxide (Me₂SO), controlled cooling at 1 °C min⁻¹, automated ice seeding and cooling to −40 °C with a final plunge into liquid nitrogen, viability levels were enhanced from 0.3 ± 0.4% to 80 ± 3%, by incorporating a 48 h dark-recovery phase after rewarming. Omission, or reduction, of this recovery step resulted in obvious cell damage with photo-bleaching of pigments, indicative of oxidative-stress induced cell damage, with subsequent deterioration of cellular architecture.