Environmental Dynamics as a Structuring Factor for Microbial Carbon Utilization in a Subtropical Coastal Lagoon

Abstract

Laguna de Rocha belongs to a series of shallow coastal lagoons located along South America. It is periodically connected to the sea through a sand bar, exhibiting a hydrological cycle where physicochemical and biological gradients are rapidly established and destroyed. Its most frequent state is the separation of a Northern zone with low salinity, high turbidity and nutrient load, and extensive macrophyte growth, and a Southern zone with higher salinity and light penetration, and low nutrient content and macrophyte biomass. This zonation is reflected in microbial assemblages with contrasting abundance, activity, and community composition. The physicochemical conditions exerted a strong influence on community composition, and transplanted assemblages rapidly transformed to resembling the community of the recipient environment. Moreover, the major bacterial groups responded differently to their passage between the zones, being either stimulated or inhibited by the environmental changes, and exhibiting contrasting sensitivities to gradients. Addition of allochthonous carbon sources induced pronounced shifts in the bacterial communities, which in turn affected the microbial trophic web by stimulating heterotrophic flagellates and virus production. By contrast, addition of organic and inorganic nutrient sources (P or N) did not have significant effects. Altogether, our results suggest that (i) the planktonic microbial assemblage of this lagoon is predominantly carbon-limited, (ii) different bacterial groups cope differently with this constraint, and (iii) the hydrological cycle of the lagoon plays a key role for the alleviation or aggravation of bacterial carbon limitation. Based on these findings we propose a model of how hydrology affects the composition of bacterioplankton and of carbon processing in Laguna de Rocha. This might serve as a starting hypothesis for further studies about the microbial ecology of this lagoon, and of comparable transitional systems.

Incidence of phytoplankton and environmental conditions on the bacterial ammonium uptake in a subtropical coastal lagoon

“…However, in this study this may not have been the case. As it has been previously described for this ecosystem, nutrients and DOM concentrations are usually higher in the North zone, although DOM is not always in a bioavailable form (Conde et al, 2002;Piccini et al, 2009;Alonso et al, 2013). In the present study, the North zone also showed a higher concentration of inorganic nitrogen forms and a high C:N ratio, suggesting that although nitrogen was available a great portion of DOM carbon could have not been available for bacterial growth, since it was mainly plant or macrophytederived (allochtonous origin).…”

“…Each microcosm was amended with ( 15 NH 4 ) 2 SO 4 (99% at., Aldrich, St. Louis, MO, USA)) at a final concentration of 50 nmol L -1 , which was used in order to be close to 10% of ambient concentration and avoid over enrichment (Conde et al, 2000;Aubriot et al, 2005). Incubations were run at the in situ temperature (15°C) in a circulating water bath under PAR intensity of 689 µmol m -2 s -1 and subsamples were taken at 0, 0.5, 2 and 4 h. The short incubation times were chosen according to previous studies on site (Piccini et al, 2009;Alonso et al, 2013) considering the bacterial activity and its dynamics as well as to prevent the effect of confinement on bacterial growth as it has been reported (Fergusson et al, 1984;Fuchs et al, 2000). Water samples from LW treatment were size-fractioned post-incubation using 1.2 µm glass fiber filters (GF/C Whatman Inc., Florham Park, NJ, USA) to retain most of the phytoplankton community (phytoplankton-enriched fraction, PHYTO) Vidal et al, 2007); the resulting filtrate was passed through 0.5 µm glass fiber filters (GE Water & Process Technologies, Boulder, CO, USA) to get the bacterioplankton community (bacteria-enriched fraction, BACT).…”

“…DAPI-stained cells were counted manually achieving a minimum of 1000 cells per filter assisted by the free counting software ClickCounter (http://www.biotechnobiology.ch). Phytoplankton biomass was estimated by measuring the in vivo Chl a fluorescence (Aquafluor Turner Designs, USA) (Briand et al, 2004;Alonso et al, 2013). The phytoplankton taxa were not assessed in this study.…”

Photoalteration of macrophyte-derived chromophoric dissolved organic matter induces growth of single bacterial populations in a coastal lagoon

“…However, little is known on how photoaltered CDOM from different macrophyte species affects the bacterial community. Alonso et al (2013) recently showed that the addition of allochthonous carbon sources induces pronounced shifts in the bacterial assemblage of a shallow coastal lagoon, while the amendment of nutrients such as nitrogen (N) or phosphorus (P) does not have a significant effect on it. These results suggest that bacterioplankton growth in this type of aquatic ecosystem would be predominantly limited by carbon and that this limitation would be more or less intense depending on the hydrology of the system (Alonso et al, 2013).…”

“…Alonso et al (2013) recently showed that the addition of allochthonous carbon sources induces pronounced shifts in the bacterial assemblage of a shallow coastal lagoon, while the amendment of nutrients such as nitrogen (N) or phosphorus (P) does not have a significant effect on it. These results suggest that bacterioplankton growth in this type of aquatic ecosystem would be predominantly limited by carbon and that this limitation would be more or less intense depending on the hydrology of the system (Alonso et al, 2013). The reason for this limitation is probably that during some phases of the lagoon hydrology, the main source of organic carbon for bacteria is macrophytes-derived DOM, which due to their chemical composition cannot be readily incorporated by bacteria for growth.…”