TITLE:
Virally-Mediated Versus Grazer-Induced Mortality Rates in a Warm-Temperate Inverse Estuary (Spencer Gulf, South Australia)
AUTHORS:
Laurent Seuront, Mark Doubell, Paul Van Ruth
KEYWORDS:
Lysis, Grazing, Bacteria, Viruses, Prokaryotic and Eukaryotic Picophytoplankton, Dilution Assay
JOURNAL NAME:
Open Journal of Marine Science,
Vol.4 No.4,
October
15,
2014
ABSTRACT: We investigated the seasonal dynamics of flow
cytometrically-defined populations of viruses, heterotrophic bacteria, and the
picoeukaryotic and prokaryotic phytoplankton at three sites in the temperate oligotrophic
inverse estuary of Spencer Gulf (South Australia). We consistently identified
two sub-populations of viruses, three sub-populations of heterotrophic
bacteria, one population of picoeukaryotic phytoplankton and two populations of
prokaryotic phytoplankton (cyanobacteria Prochlorococcus and Synechococcus). Both the
cytometric community composition and the abundance of viruses, heterotrophic
bacteria and both prokaryotic (Synechococcus and Prochlorococcus) and eukaryotic
picophytoplankton were consistent with previous observations conducted in South
Australian continental shelf waters. Noticeably LDNA bacteria (i.e. inactive or dormant cells) were
consistently significantly the most abundant group of heterotrophic bacteria
(totaling from 29% to 68% of total bacterial abundance) and were up to 10-fold
more abundant than that previously reported in South Australian continental
shelf waters, including the nearby Saint Vincent Gulf. These results suggest an
overall low activity of the microbial community, and are consistent with
previous evidence that LDNA cells may play a greater role in heterotrophic
processes than HDNA cells in oligotrophic waters. In an attempt to further
assess the qualitative and quantitative nature of the mortality of these organisms,
we used a specific dilution assay to assess the relative contribution of
viruses and microzooplankton grazers to the mortality rates of heterotrophic
bacteria, and picoeukaryotic and prokaryotic phytoplankton. We consistently reported
site-specific, population specific and sea-son-specific viral lysis and grazing
rates of heterotrophic bacteria and the picoeukaryotic and prokaryotic
(cyanobacteria Prochlorococcus and Synechococcus) phytoplankton across
sites and seasons. Specifically, both viral lysis and micro-zooplankton grazing
rates of heterotrophic bacteria were consistently relatively low across sites
and seasons, even though their seasonality suggested an overall dominance of
grazing over viral lysis in both summer and winter. In contrast, no seasonality
is found in either lysis or grazing rates of prokaryotic and eukaryotic
picophytoplankton, which are comparable to previous observations conducted in
oligotrophic waters, suggesting the mortality dynamics of these populations is
similar to those encountered in other oligotrophic waters. The observed
patterns of mortality rates of heterotrophic bacteria and both prokaryotic and
eukaryotic picophytoplankton are consistent with the low chlorophyll
concentration and production previously observed in the waters of the Spencer
Gulf.