Daniel Conde

Influence of hydrology on phytoplankton species composition and life strategies in a subtropical coastal lagoon periodically connected with the Atlantic Ocean

A survey was carried out to investigate the relationship of phytoplankton biovolume, structure, and species life strategies with major abiotic factors in a subtropical choked coastal lagoon (34°33′S, 54°22′W) naturally connecting with the Atlantic Ocean several times a year. Marine and limnetic influence areas were sampled on a monthly basis during two periods, one of low rainfall and high conductivity (August 1996 to February 1998) and a second period with the opposite tendency (December 1998 to March 2000). Photosynthetically active radiation availability was high and reached the bottom (>1% of the incident light), while dissolved inorganic nitrogen (0.6–18.4 μM), soluble reactive phosphorus (<0.3–2.7 μM), and reactive silica (5–386 μM) were highly variable. Life strategies were identified in the phytoplankton as a function of morphology. C-strategists, invasive planktonic and epipelic species of small size, and R-strategists, mixing-dependent species of medium size, characterized this permanently mixed system. High frequency of exchange with the ocean prevented high biomass accumulation. Phytoplankton biomass was lower in the second period of high rainfall (2.3 and 1.1 mm3 1−1 for period 1 and 2 respectively). A canonical correspondence analysis showed that conductivity, nitrogen, phosphorus, and silica were the main environmental variables explaining phytoplankton species composition patterns. During the first period, Bacillariophyceae (mostly pennate species) and the potentially toxicPrococentrum minimum were dominant; during the second period a higher contribution of flagellates (Cryptophyceae, Euglenophyceae, Prasinophyceae, and flagellates <7 μm) was found. Differences of phytoplankton biomass, main taxonomic groups, and strategies were found between periods but not between limnic and marine areas, suggesting that hydrological dynamic is more relevant than seasonal and spatial differences.

Blooms of single bacterial species in a coastal lagoon of the southwestern Atlantic Ocean.

ABSTRACT We investigated seasonal differences in community structure and activity (leucine incorporation) of the planktonic bacterial assemblage in the freshwater and brackish-water zones of a shallow coastal lagoon of the southwestern Atlantic Ocean. Alphaproteobacteria formed the dominant microbial group in both zones throughout the sampling period. After an intrusion of marine water, members of the SAR11 lineage became abundant in the brackish-water zone. These bacteria were apparently distributed over the lagoon during the following months until they constituted almost 30% of all prokaryotic cells at both sampling sites. At the first sampling date (March 2003) a single alphaproteobacterial species unrelated to SAR11, Sphingomonas echinoides, dominated the microbial assemblages in both zones of the lagoon concomitantly with a bloom of filamentous cyanobacteria. Pronounced maxima of leucine incorporation were observed once in each zone of the lagoon. In the freshwater zone, this highly active microbial assemblage was a mix of the typical bacteria lineages expected in aquatic systems. By contrast, a single bacterial genotype with >99% similarity to the facultative pathogen gammaproteobacterial species Stenotrophomonas maltophilia formed >90% of the bacterial assemblage (>107 cell ml−1) in the brackish-water zone at the time point of highest bacterial leucine incorporation. Moreover, these bacteria were equally dominant, albeit less active, in the freshwater zone. Thus, the pelagic zone of the studied lagoon harbored repeated short-term blooms of single bacterial species. This finding may have consequences for environmental protection.

Alteration of chromophoric dissolved organic matter by solar UV radiation causes rapid changes in bacterial community composition

We evaluated the effect of photochemical alterations of chromophoric dissolved organic matter (CDOM) on bacterial abundance, activity and community composition in a coastal lagoon of the Atlantic Ocean with high dissolved organic carbon concentration. On two occasions during the austral summer, bacteria-free water of the lagoon was exposed to different regions of the solar spectrum (full solar radiation, UV-A+PAR, PAR) or kept in the dark. Subsequently, dilution cultures were established with bacterioplankton from the lagoon that were incubated in the pre-exposed water for 5 h in the dark. Cell abundance, activity, and community composition of bacterioplankton were assessed before and after incubation in the different treatments. Changes in absorption, fluorescence, and DOC concentration were used as proxies for CDOM photoalteration. We found a significant CDOM photobleaching signal, DOC loss, as well as a stimulation of bacterial activity in the treatments pre-exposed to UV radiation, suggesting increased bioavailability of DOM. Bacterial community analysis by fluorescence in situ hybridization revealed that this stimulation was mainly accompanied by the specific enrichment of Alpha- and Betaproteobacteria. Thus, our results suggest that CDOM photoalteration not only stimulates bacterioplankton growth, but also induces rapid changes in bacterioplankton composition, which can be of relevance for ecosystem functioning, particularly considering present and future changes in the input of terrestrial CDOM to aquatic systems.

Emergent neutrality drives phytoplankton species coexistence

The mechanisms that drive species coexistence and community dynamics have long puzzled ecologists. Here, we explain species coexistence, size structure and diversity patterns in a phytoplankton community using a combination of four fundamental factors: organism traits, size-based constraints, hydrology and species competition. Using a ‘microscopic’ Lotka–Volterra competition (MLVC) model (i.e. with explicit recipes to compute its parameters), we provide a mechanistic explanation of species coexistence along a niche axis (i.e. organismic volume). We based our model on empirically measured quantities, minimal ecological assumptions and stochastic processes. In nature, we found aggregated patterns of species biovolume (i.e. clumps) along the volume axis and a peak in species richness. Both patterns were reproduced by the MLVC model. Observed clumps corresponded to niche zones (volumes) where species fitness was highest, or where fitness was equal among competing species. The latter implies the action of equalizing processes, which would suggest emergent neutrality as a plausible mechanism to explain community patterns.

Ecophysiological differences of betaproteobacterial populations in two hydrochemically distinct compartments of a subtropical lagoon

We studied the population sizes and substrate incorporation patterns of three phylogenetic groups of Betaproteobacteria in a coastal subtropical lagoon that is characterized by a sharp transition from humic freshwater to turbid brackish water. Various cellular processes were addressed by short-term incubations with four radiolabelled compounds and microautoradiographic assessment of substrate incorporation. Group-specific differences in the abundances and the respective physiological state of the three populations were observed upon transfer from the humic-rich compartment to the main body of the lagoon (estimated at 1-2 days). Members of the clade B of Polynucleobacter (PnecB) experienced only an insignificant change in cell numbers, but displayed a general metabolic downshift, carbon metabolism (glucose incorporation) being most affected. By contrast, bacteria from the closely related Polynucleobacter C clade (PnecC) clearly differed in total abundances and in the numbers of DNA-synthesizing or glucose incorporating cells. At the same time, PnecC bacteria maintained comparable levels of protein synthesis (leucine uptake) in both lagoon compartments, and the proportion of cells incorporating N-acetylglucosamine was even higher in the main body of the lagoon. Members of the R-BT lineage showed little changes in cell numbers, DNA synthesis and carbon metabolism. Altogether, the observed patterns of substrate metabolism suggest that different bacterial populations in the lagoon undergo specific physiological adjustments in response to changing environmental conditions.

Competition Drives Clumpy Species Coexistence in Estuarine Phytoplankton

Understanding the mechanisms that maintain biodiversity is a fundamental problem in ecology. Competition is thought to reduce diversity, but hundreds of microbial aquatic primary producers species coexist and compete for a few essential resources (e.g., nutrients and light). Here, we show that resource competition is a plausible mechanism for explaining clumpy distribution on individual species volume (a proxy for the niche) of estuarine phytoplankton communities ranging from North America to South America and Europe, supporting the Emergent Neutrality hypothesis. Furthermore, such a clumpy distribution was also observed throughout the Holocene in diatoms from a sediment core. A Lotka-Volterra competition model predicted position in the niche axis and functional affiliation of dominant species within and among clumps. Results support the coexistence of functionally equivalent species in ecosystems and indicate that resource competition may be a key process to shape the size structure of estuarine phytoplankton, which in turn drives ecosystem functioning.

The photosynthetic responses of marine phytoplankton, periphyton and epipsammon to the herbicides paraquat and simazine

Short-term toxicity tests using photosynthesis (incorporation of 14C) as a test parameter were performed in order to compare the sensitivities of three marine microalgal communities (phytoplankton, periphyton and epipsammon) to two herbicides, paraquat and simazine. Thirty minutes of pre-exposure to simazine were sufficient to obtain the full effect in all communities, while for paraquat 4 h was required. The bioavailability of paraquat and simazine was not limited by adsorption to sediment in the epipsammon samples. Simazine was more toxic than paraquat for the three communities at similar concentrations. Phytoplankton was slightly more sensitive for both herbicides (EC50 ranges of 9--23 mu m for paraquat and 0.37--0.99 mu m for simazine) than periphyton and epipsammon. These attached communities exhibited different results for each toxicant, periphyton being more sensitive to paraquat (EC50 range 9--21 mu m) and epipsammon to simazine (EC50 range 0.44--1.17 mu m). The three communities presented EC ranges comparable to those found in single species tests, suggesting that different levels of biological organization can exhibit a similar sensibility to toxicants, thus indicating that natural communities are suitable for use in these kinds of toxicity tests

Environmental dynamics as a structuring factor for microbial carbon utilization in a subtropical coastal lagoon

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.