Impairment of benthic diatom adhesion and photosynthetic activity by allelopathic compounds from a green alga: involvement of free fatty acids?

Abstract

The role of chemical interactions in shaping microbial communities has raised increasing interest over the last decade. Many benthic microorganisms are known to develop chemical strategies to overcome competitors, but the real importance of chemical interactions within freshwater biofilm remains unknown. This study focused on the biological and chemical mechanisms of an interaction involving two benthic microorganisms, an allelopathic filamentous green alga, Uronema confervicolum, and a common diatom, Fistulifera saprophila. Our results showed that functions critical for benthic phototrophic microorganisms were inhibited by U. confervicolum extracts. Growth, cell motility, adhesion, and photosynthetic activity were impaired at extract concentrations ranging between 5 and 20 μg ml(-1). The adhesion inhibition was mediated by intracellular nitric oxide (NO) induction. A bioassay-guided fractionation of the extract with HPLC helped to identify two C18 fatty acids present in the growth-inhibiting fractions: linoleic (LA) and α-linolenic (LNA) acids. These compounds represented 77% of the total free fatty acids of U. confervicolum and were present in the culture medium (1.45 μg l(-1) in total). Both could inhibit the diatom growth at concentrations higher than 0.25 μg ml(-1), but had no effect on cell adhesion. The discrepancy between the effective concentrations of fatty acids and the concentration found in culture medium may be explained by the presence of high-concentration microenvironments. The compounds involved in adhesion inhibition remain to be identified. Though further experiments with complex biofilms are needed, our results suggest that U. confervicolum may participate to the control of biofilm composition by inhibiting diatom adhesion.

New Methods, New Concepts: What Can Be Applied to Freshwater Periphyton?

“…For example, synthesis of allelopathic substances, e.g., antibiotics and other inhibitors, by members of the biofilm community plays an essential role in determining microbial diversity and structure (Lenski and Riley, 2002;Burmolle et al, 2006). Studies on biofilms of different origin and bacterial associations with eukaryotic hosts, e.g., human pathogens (Harriott and Noverr, 2009;Lopes et al, 2012;Korgaonkar et al, 2013), as well as aquatic microbial and epiphytic communities (Burmolle et al, 2006;Allen et al, 2015;Kouzuma and Watanabe, 2015;Dittami et al, 2016), have revealed both synergistic and antagonistic inter-species interactions. It is well-known that bacteria are able to stimulate growth, morphogenesis, and zoosporogenesis of algae (Marshall et al, 2006;Goecke et al, 2013) as well as the synthesis of algal extracellular compounds (Bruckner et al, 2011).…”

“…For example, studies on cell cultures of the brown alga Ectocarpus had shown that presence of specific associated bacteria increase the tolerance of the algal host to low salinity (Dittami et al, 2016). Besides synergistic interactions, a multispecies community inevitably faces competitive antagonistic interactions among species (Freilich et al, 2011;Allen et al, 2015;Yuan and Meng, 2020). Last, studies have shown that in the absence of carbon limitation, heterotrophs can out-compete algae for nutrients and thus suppress algal growth (Wyatt et al, 2019).…”

“…These substances include antibiotics and enzymes, which can be transported to competitors with extracellular vesicles and affect cellular differentiation, protein synthesis, and inhibit cell-to-cell signaling (Stubbendieck and Straight, 2016). Studies on macro-and microalgae indicate that several specific algal compounds such as fatty acids including α-linolenic (18:3 n-3) and eicosapentaenoic acids (22:5 n-3) have a strong antibacterial activity as well (Debois et al, 2010;Smith et al, 2010;Allen et al, 2015). Inhibiting the growth of both bacteria and other algae may control and shape the associated microbial communities (Allen et al, 2015).…”

“…These studies revealed that microbial coupling depends to a large extent on environmental variables, such as nutrient fluxes ( Tank and Dodds, 2003 ; Carr et al, 2005 ; Rusanov et al, 2009 ), light intensity ( Roeselers et al, 2007 ), and availability of dissolved organic carbon (DOC; Sobczak, 1996 ; Rier and Stevenson, 2002 ), etc. During the past decades some physiological aspects of interactions between auto- and heterotrophs in freshwater periphytic biofilms have been studied ( Bruckner et al, 2008 , 2011 ; Allen et al, 2015 ), however, many mechanistic aspects of interactions in freshwater periphyton communities are still unknown and need to be examined in more details ( Allen et al, 2015 ).…”

Allelopathic interactions involving benthic phototrophic microorganisms

“…However, several studies have demonstrated that filamentous green algae (FGA) use chemicals to ensure their dominance (Leflaive et al, 2008b;Trochine et al, 2011). Biofilm forming green algal allelochemicals have rarely been identified, but it has been shown that one species, Uronema confervicolum, probably produces an array of different allelopathic compounds (Allen et al, 2015).…”