Germanium dioxide as agent to control the biofouling diatom Fragilariopsis oceanica for the cultivation of Ulva fenestrata (Chlorophyta) Author Rautenberger, Ralf text Botanica Marina 2024 Warsaw, Poland 2024-02-26 67 2 93 100 http://dx.doi.org/10.1515/bot-2023-0075 journal article 298376 10.1515/bot-2023-0075 07feacfc-bfbc-4cf1-92b6-f6ede7497c27 1437-4323 11360456 Fragilariopsis oceanica GeO 2 is widely used as an effective growth inhibitor for diatoms and other silicifying microalgae (e.g. chrysophytes). It competitively inhibits the uptake of Si through Si-transporters (SITs), which eventually interrupts cell wall formation in the algae ( Thamatrakoln and Hildebrand 2008 ).) 20 photons 0.4 1 ) 15 1 − 0.3 − d % ETR μmol (10 0.2 RGR 5 α electrons 0.1 0 μmol 0.0 0 0.223 2.235 (0 0.223 2.235 Figure 4: Physiological parameters of the large-scale cultivation of Ulva fenestrata with GeO 2 . 100 1 ) 300 ( A ) Relative growth rates ( RGR ), − 1 ) − s 2s 250 ( B ) photosynthetic electron transport 802 − mmefficiencies ( α ETR max − electrons 40 60 photons 100 200 150 transport fenestrata saturation rates after points ETR large-scale ( ETR ), of ( C max photosynthesis) maximum) and cultivation (D) electron light ( E k (14 ) of days Ulva ) μmol ( 20 E ( μmol k 50 0 in concentrations 100-l Plexiglass at 140 water µmol tanks photons with three m −2 s GeO −1 2 0 and 12 ° C . Data are means of three replicates 0 0.223 2.235 0 0.223 2.235 per treatment ( n = 3) and error bars represent GeO 2 concentration (mg L− 1 ) GeO 2 concentration (mg L− 1 ) standard deviations. 2 Figure 5: GeO 2 -dependent total diatom density present on the surfaces of Plexiglass water tanks after 14 days of large-scale cultivation of Ulva fenestrata . Data are means of three replicates per treatment ( n = 3) and error bars represent standard deviations. Lowercase letters above columns indicate statistically significant differences between the treatments ( P <0.001, 1-way ANOVA, Tukey-Kramer HSD post-hoc test). Toxicological studies have shown species- and strain-specific responses of diatoms to GeO 2 . While concentrations of up to 1 mg GeO 2 l −1 inhibited the growth of highly silicified diatom species (e.g. Amphiphora paludosa , Cylindrotheca fusiformis ), diatoms with a low degree of silicified cell walls (e.g. Phaeodactylum tricornutum ) were insensitive even to 10 GeO 2 mg l −1 ( Lewin 1966 ; Markham and Hagmeier 1982 ; Tatewaki and Mizuno 1979 ). By using these results as benchmark for the present study, F. oceanica seems to be highly sensitive to GeO 2 because its growth was inhibited at a low concentration of 0.014 mg GeO 2 l −1 . Assuming that Si uptake in F. oceanica is mediated by SITs at low Si concentrations in seawater (<30 µM) as shown for Thalassiosira pseudonana ( Thamatrakoln and Hildebrand 2008 ) , a growth inhibition by GeO 2 could have been expected because the seawater Si concentration used was approx. 2.1 µM ( Busch et al. 2014 ). However, if Si was added to the seawater according to formulation of the ESNW medium with a final concentration of 106 µM, a different result would have been observed because diffusive Si uptake predominates at higher Si concentrations ( Thamatrakoln and Hildebrand 2008 ). Interestingly, the use of GeO 2 in the glass beakers and Plexiglass water tanks showed different effects on F. oceanica . While low GeO 2 concentration inhibited the growth of F. oceanica , the colonisation of the Plexiglass walls by F. oceanica could not be fully prevented even by the use of 2.235 mg GeO 2 l −1 . This could be possibly ascribed to the different physical surface properties between glass and Plexiglass. Insoluble extracellular polymeric substances ( EPS ) secreted by diatoms allow them to adsorb better on hydrophobic surfaces such as Plexiglass than on the hydrophilic surfaces of the glass beakers ( Finlay et al. 2013 ; Holland et al. 2004 ; Krishnan et al. 2006 ; reviewed by Thompson and Coates 2017 ). In addition, EPS-rich biofilms from Roseobacter and S ul fi tobacter , which are associated with Ulva , could have enhanced the attachment of F. oceanica on the Plexiglass walls ( Bruckner et al. 2011 ; Buhmann et al. 2016 ; Spoerner et al. 2012 ). Thus, the bacterial biofilms could help F. oceanica to overcome the negative effects of GeO 2 . Other factors such as the different treatments of the glass beakers (acid-washed) and water tanks (hand washing and sodium hypochlorite) prior to the experiments could also have contributed to the different outcomes of the two experiments. Nevertheless, since the large-scale experiment showed a considerable reduction in diatom density on the Plexiglass surfaces by 0.223 mg GeO 2 l −1 (f.c.), the costs for the biomass production of U. fenestrata are expected to be lower than without employing any GeO 2 at all.