Distribution of picophytoplankton communities from brackish to hypersaline waters in a South Australian coastal lagoon

Mathilde Schapira,Marie-Jeanne Buscot,Thomas Pollet,Laurent Seuront,Sophie C Leterme

doi:10.1186/1746-1448-6-2

Abstract

BackgroundPicophytoplankton (i.e. cyanobacteria and pico-eukaryotes) are abundant and ecologically critical components of the autotrophic communities in the pelagic realm. These micro-organisms colonized a variety of extreme environments including high salinity waters. However, the distribution of these organisms along strong salinity gradient has barely been investigated. The abundance and community structure of cyanobacteria and pico-eukaryotes were investigated along a natural continuous salinity gradient (1.8% to 15.5%) using flow cytometry.ResultsHighest picophytoplankton abundances were recorded under salinity conditions ranging between 8.0% and 11.0% (1.3 × 106 to 1.4 × 106 cells ml-1). Two populations of picocyanobacteria (likely Synechococcus and Prochlorococcus) and 5 distinct populations of pico-eukaryotes were identified along the salinity gradient. The picophytoplankton cytometric-richness decreased with salinity and the most cytometrically diversified community (4 to 7 populations) was observed in the brackish-marine part of the lagoon (i.e. salinity below 3.5%). One population of pico-eukaryote dominated the community throughout the salinity gradient and was responsible for the bloom observed between 8.0% and 11.0%. Finally only this halotolerant population and Prochlorococcus-like picocyanobacteria were identified in hypersaline waters (i.e. above 14.0%). Salinity was identified as the main factor structuring the distribution of picophytoplankton along the lagoon. However, nutritive conditions, viral lysis and microzooplankton grazing are also suggested as potentially important players in controlling the abundance and diversity of picophytoplankton along the lagoon.ConclusionsThe complex patterns described here represent the first observation of picophytoplankton dynamics along a continuous gradient where salinity increases from 1.8% to 15.5%. This result provides new insight into the distribution of pico-autotrophic organisms along strong salinity gradients and allows for a better understanding of the overall pelagic functioning in saline systems which is critical for the management of these precious and climatically-stress ecosystems.

Highlights

Picophytoplankton are abundant and ecologically critical components of the autotrophic communities in the pelagic realm
Salinity was identified as the main factor controlling the picophytoplankton dynamic along the salinity gradient
Our results highlight the dominance of one cytometricallydefined population of pico-eukaryote throughout the salinity gradient, which was able to form a large bloom under relatively high salinity conditions (i.e. 8.0-11.0%)

Summary

Introduction

Picophytoplankton (i.e. cyanobacteria and pico-eukaryotes) are abundant and ecologically critical components of the autotrophic communities in the pelagic realm. Environmental factors controlling the distribution and composition of these distinct communities have been extensively reviewed, such as light requirement [5,7], water temperature [8,9], nutrient levels [1,10], grazing [11,12] and viral lysis [13,14]. Most of these investigations concerned pelagic ecosystems and the picophytoplankton communities in coastal waters have still received little attention. Little is still known about the effect of salinity on the distribution and community composition of picophytoplankton [17]

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