3.1. Fatty Acid Compositions of P. salina in Relation to Experimental Conditions
The cosmopolitan fungi P. salina
is widely spread in all type of marine ecosystems, which clearly demonstrate its effective capacity to adapt to diverse temperature and salinity conditions [5
]. In the present study, salinity levels (including the extreme 70 PSU) did not impact drastically total fatty acid concentrations, except a noticeable decrease at 10
C and 70 PSU (Supplementary Materials Figure S2
), which further illustrate the capacity of the fungi to thrive at salinity conditions well beyond the growth capacity of both host-algae.
Several studies have documented the effect of environmental variables on recruitment, survival, growth, size, biomass and density of kelps, nutrient and light being key factors [16
]. Along the NE Atlantic (Norwegian) coasts, L. hyperborea
abundance is, for instance, primarily driven by the interaction between wave exposure and either depth or ocean currents, implying depth-specific effects of wave exposure and wave-specific effects of current speed [18
]. In terms of salinity tolerance, L. digitata
exhibit optimal growth between salinity of 23 and 31 PSU, with a strong reduction of growth at 16 PSU and high mortality below 8 PSU [1
]. In a study on Artic kelps, Karsten et al. [19
] showed that, on a gradient from 5 to 60 PSU, maximum effective quantum yields (a proxy for photosynthetic efficiency) were measured between 20 and 55 PSU for L. digitata
and S. latissima
. Thus, in the present study, while 50 PSU is already a challenging condition; 70 PSU is clearly extreme for these species. Interestingly, despite the recognised phenotypic plasticity of P. salina
and that the fungi was able to grow at the highest studied salinity without significant loss in total lipid mass, the observed fatty acids trajectories as well as
relationships were no longer observed at 70 PSU. This indicated a dynamic relationship of the fatty acid metabolism between P. salina
and its host-algae and is emphasized by the opposed response to temperature increase between both algal host species.
3.2. Divergent Fatty Acid Trajectories in P. salina Revealed Adaptive Strategies to Temperature Changes
Kelp forests are found on rocky seabeds from temperate to Arctic ecosystems and many species, such as Laminaria
sp., have an important adaptive capacity to temperature changes [1
]. For instance, endemic Arctic L. solidungula
grow at temperatures between 5 and 16
C, and cold-temperate NE Pacific species grow between 0 and 18
C with optima between 5 and 15
C. The growth range of cold-temperate N Atlantic species extends from 0 to 20
C with optima between 5 and 15
C while warm-temperate Atlantic species grow at up to 23–24
C and have slightly elevated optima [1
]. The temperature gradient investigated in the present study is thus within the range of natural temperature conditions.
When submitted to this gradient, endophytic P. salina showed divergent fatty acids trajectories as well as relationships depending on the host. At salinities 23.5 and 50 PSU the effect of host algae on was significant. The host effect was more pronounced at 23.5 than at 50 PSU as shown by the -value and disappeared at the extreme 70 PSU which indicated that the opposition in lipid metabolism and C18 trajectories between LD and SL are conserved throughout the salinity gradient although severe (50 PSU) and extreme (70 PSU) salinities did impact P. salina fatty acid metabolism.
In L. digitata
C18 fatty acids and especially linoleic acid (
) are essential in the response of the algae against stressful conditions such as the perception of pathogenic metabolites [20
] or against grazing by specialised herbivorous species [21
]. The response, in all cases, imply an oxidative stress and the activation of fatty acid oxidation cascades [22
]. For instance, early events in the perception of pathogens lipopolysaccharides in this brown alga include the production of 13-hydroxyoctadecadienoic acid (13-HODE) as a result of the oxydation of
by lipoxygenase activity [20
]. A decrease in fatty acid occurs in S. latissima
during the early development from gametes to gametophytes. The decrease was significant for
, from 45 to 30% of total fatty acids, suggesting that it might be important in the transition from storage lipids to photo-autotrophic strategies [23
]. Thus, an increase in
in laminariales is likely associated to the redirection of the algal lipid metabolism toward photosynthesis or defence to the detriment of storage lipids.
Homeoviscous and homeophasic adaptations, which is the process of keeping adequate membrane fluidity, as a response to temperature changes are well documented for microorganisms. Degree of unsaturation, variation in chain length, branching and cyclization of fatty acids are known adaptative strategies to enhance membrane fluidity. A considerable decrease in
and the marked increase in
with lower temperatures have already been observed in bacteria, fungus and yeast [24
]. In the present study, any decrease in temperature is thus expected to induce an increase in
as a response. However, this expected relationship was noticed only when the fungal endophyte was isolated from S. latissima
and, intriguingly, it exhibited an opposite trend when isolated from L. digitata
In absence of dedicated temperature experiments on both L. digitata and S. latissima, it is difficult to conclude on whether P. salina lipid metabolism was fully aligned with its host requirements. However, the observed opposed trend in lipid trajectories between the endophytic fungi of the two hosts revealed a temperature-response that was clearly host dependant.
Host species originated from separate areas (Roscoff-FR and Oban-UK for LD and SL respectively) which, despite being slightly warmer in average (2.6 ± 0.4) in Roscoff, are relatively similar in terms of sea surface temperature and salinity (SST NOAA). It is thus very likely that the two fungal strains originated from two different populations that were each adapted to their Laminariale host. Unfortunately, we do not have precise genomics information about the two endophytic strains (other than ITS barcode sequencing) to validate this hypothesis.
However, previous comparative metabolomics on the same endophytic strains, and seven additional P. salina
isolates from various brown algae, have demonstrated a clearly divergent metabolome between algal species as well as orders (i.e., Fucales vs. Laminariales) [8
]. Altogether, the present findings highlight the plasticity of the fungus to adapt to a new environment (i.e., the hosting algae). The fact that the host influenced the expression of P. salina
metabolome may reflect epigenetic mechanisms as changes in metabolome expression [8
] and lipid trajectories (this study) might be conserved across multiple generations.