Variability of short-term carbon uptake and transfer within a natural microbial community from Lake Taihu, China

Primary production is usually quantified in terms of total carbon
fixation without considering the organisms involved although we know
they have different growth dynamics. NanoSIMS offers the possibility to
study these differences more systematically. Recently Schreiber et al.
[1] showed that heterogeneity in substrate uptake can increase due to
substrate limitation and may actually be beneficial by promoting
population growth under dynamic environmental conditions (e.g.,
fluctuating availability of nutrients). Our study aimed to quantify the
heterogeneity of short-term carbon uptake and transfer within a mixed
natural microbial community from a freshwater environment (Lake Taihu,
China). After incubating the community in the lake water amended with
13C-enriched dissolved inorganic carbon, we quantified label
incorporation on a single-cell level every 6h during a 24h interval. On
a community level, photosynthetic protists and unidentified coccoid
cyanobacteria had the highest rates of inorganic carbon uptake, followed
by green microalgae, filamentous cyanobacteria, and the dominant
Microcystis sp. Overall, the inorganic carbon uptake rate varied by ~33%
(coefficient of variation) within the autotrophic community (N=5
groups). Variability was relatively low for Microcystis sp. (28%;
N=1440) and unidentified coccoid cyanobacteria (27%; N=130), suggesting
that these groups were not carbon-limited in their natural environment,
whereas variability was larger for green microalgae (46%; N=64),
photosynthetic protists (46%; N=60) and filamentous cyanobacteria (55%;
N=174 cells), likely due to a larger genetic diversity within these
groups or due to carbon-limitation. While the heterotrophic bacterial
community was likely diverse, the relatively low variability in their
assimilation of dissolved organic carbon (27%; N=750) suggested that
their activity was not substrate-limited. Clearly this study represents
only a first step towards understanding and quantifying growth of
individual phytoplankton, but we have to continue this road to further
our knowledge on the relation between community structure and function.