Fragilariopsis oceanica

Rautenberger, Ralf, 2024, Germanium dioxide as agent to control the biofouling diatom Fragilariopsis oceanica for the cultivation of Ulva fenestrata (Chlorophyta), Botanica Marina (Warsaw, Poland) 67 (2), pp. 93-100 : 96-97

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

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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

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.