6. COMPENSACIÓN EQUITATIVA POR COPIA PRIVADA
6.4 EL CRITERIO DE EQUIDAD DE LA COMPENSACIÓN
Salinity has been identified as the main factor triggering the succession of pico- autotrophs along the salinity gradient (Table 2). Salinity could act directly on picophytoplankton assemblages by selecting groups adapted to life at a particular salt concentration. Cyanobacteria are known to tolerate and acclimate to high salt concentrations [92]. However, the different groups of cyanobacteria do not exhibit the same tolerance to salinity stress and have been consequently classified into 3 groups, i.e. stenohaline, halotolerant and extremely halotolerant [93]. The decrease in cyanobacteria abundances for salinity higher than 35‰, suggests that the populations inhabiting the brackish-marine part of the Coorong may belong to the stenohaline group with a salinity tolerance range characteristic of estuarine and marine populations. Synechococcus species are known to be abundant in transitional and freshwater areas [94] whereas Prochlorococcus species are thought to be restricted to marine waters [95]. However, the observations of Prochlorococcus-like populations in the Rhône River [96], in the Changjiang river estuaries [28] and in the present work in the low-salinity part of the Coorong tend to challenge this hypothesis.
In contrast, the occurrence and predominance of the pico-eukaryote P-Eu1 throughout the
salinity gradient, suggest that this population may represent halotolerant organisms. The dominance of halotolerant pico-eukaryotes has been previously described for the same salinity range [33-35]. In particular, eukaryotic pico-autotrophs have been shown to be responsible for dense blooms in surface water of Mono Lake in California where salinity was around 85‰ [35]. The organism isolated from this lake and identified as a Picocystis spp. has been shown to exhibit high growth efficiency over the 2.0‰-150‰ salinity range, which is consistent with the range of salinity where the population P-Eu1 was found in the present
work. However, this population was identified here through flow cytometry and further work is needed to unambiguously conclude on the identity of this population.
Independent of their intrinsic salinity tolerance, the succession of picophytoplankton organisms along the salinity gradient could be indirectly controlled by salinity. In particular, by controlling the diversity and abundance of microzooplankton and VLP, salinity could exert
a control on the top-down processes including grazing and viral lysis. The presence of a large population of phytoplankton virus up to the highest salinity observed in the present work highlights the necessity to investigate the role of viral infection in regulating the community structure of picophytoplankton along this salinity gradient.
C
ONCLUSIONIn the literature, microbial community dynamics has been mainly described along discontinuous (i.e. solar salterns, hypersaline lakes) or weak salinity gradients (i.e. estuaries). The present chapter then constitutes the first observation of the joint dynamics of viral, microbial and picophytoplankton communities in a system where salinity continuously ranged from brackish to hypersaline.
High abundances of virus-like particles and heterotrophic bacteria were observed in the hypersaline part of the lagoon. In contrast, the highest picophytoplankton abundances were recorded for salinity ranging from 80‰ to 110‰. This shift from autotrophy to heterotrophy with increasing salinity may have considerable consequences on food web structure and biogeochemical cycles. Salinity was identified as the main factor controlling the picophytoplankton dynamic along the salinity gradient. However, the variability in nutrients availability as well as the intensity of viral lysis and microzooplankton grazing may have also played an important role in structuring the succession of picophytoplankton communities along the lagoon.
As previously shown, important ecological changes such as decreasing biodiversity and increasing dominance of prokaryotes occurred along salinity gradients (e.g. [97,98]). Our findings highlight a significant decrease in the number of cytometrically-defined sub- populations of heterotrophic bacteria and VLP with increasing salinity.
These changes could reflect both a modification of phylogenetic composition and activity level within the bacterial community. In addition, the number of cytometrically-defined picophytoplankton populations also decreased with increasing salinity, affecting both prokaryotic and eukaryotic organisms. Moreover, our results highlight the ability of cyanobacteria to tolerate a wide range of salinity.
The existence of a Prochlorococcus-like population in both extremes of the salinity gradient (i.e. low salinity and the hypersaline water) highlights the ability of cyanobacteria to tolerate and acclimate to a wide range of salinity and therefore to colonize extreme environments. However, even if the Synechococcus-like and Prochlorococcus-like populations identified in the present work exhibited the archetypical flow cytometric signature of Synechococcus sp. and Prochlorococcus sp., further work is needed (e.g. genetic fingerprinting) to unambiguously identify these populations.
The results obtained in this study nevertheless provide new insight into the potential effects of salinity gradient and perturbations on phytoplankton community shifts.
As these shifts in abundances and richness may ultimately have considerable implications in biogeochemical cycling dynamics along the salinity gradient, the signification of this decrease in cytometrically-defined richness of heterotrophic bacteria and picophytoplankton with increasing salinity warrant further considerations.
A
CKNOWLEDGMENTSWe thank M.J. Buscot, S. Leterme and T. Pollet for their assistance in the field. Aloysio da Silva Ferrão Filho and one anonymous reviewer are acknowledged for their constructive comments and criticisms on an earlier version of this work.
This research was supported under Australian Research Council’s Discovery Projects funding scheme (projects number DP0664681 and DP0988554), and by the South Australian Government (through the Marine Innovation South Australia initiative). Professor Seuront is the recipient of an Australian Professorial Fellowship (project number DP0988554).
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