Abstract
Increase of atmospheric CO 2 on ocean ecosystems are of major environmental concern.
Despite the importance of microbial communities in global carbon cycling, our knowledge of their
community dynamics is still rudimentary, and their responses to ocean acidification and ocean
warming, remains poorly understood.
Effects of elevated pCO 2 , and subsequent acidification of the ocean as well as increasing ocean
temperatures on microbial communities and marine aggregate formation have previously been
simulated using mesocosm experiments. Nevertheless, laboratory experiments have limitations
when attempting to scale-up the effects in the oceans and underestimate natural conditions.
Three locations in the Galapagos Archipelago were selected because of their unique oceanographic
features to test various hypotheses in situ: A submarine volcano, an upwelling site and a control site,
which presented typical ocean conditions. The volcanic vents and the upwelling site, which was
affected by El Niño conditions during sampling, simulate the forecasted characteristics of ocean
acidification and ocean warming. Hence these oceanographic settings were used as natural
treatments to investigate their effects on the microbial communities as well as on organic matter
dynamics, including in situ particle formation. A set of environmental parameters was recorded at
the three study sites and samples of the water column were collected to quantify the concentration
of three types of carbon pool and reveal bacterial community composition through 16S rRNA gene
amplicon sequencing on the MiSeq Illumina platform. Overall the volcanic site showed the lowest pH
values, nitrate, chlorophyll concentrations and the highest DOC concentration. The highest
temperature was recorded at the upwelling site, which represents temperatures typical for weak El-
Niño conditions, but not as warm as registered during strong El Niño events.
Despite the volcanic site presenting the highest DOC and TEP concentrations, POC was
markedly lower at this site, compared to the control and the upwelling site. The main pathways,through which DOC is transformed into POC, are bacterial uptake and TEP formation. Therefore, the
low particle formation at this site may be attributed to the inability of heterotrophic microorganisms
to re-mineralize the dissolved carbon source, either because of the DOC having a refractory nature
or because of the carbon consumption being limited by the low concentration of inorganic nutrients.
Moreover, low pH levels might be affecting the stickiness of TEP, thus preventing the formation of
particulate material or the formation of bigger aggregates, which causes a decrease in the downward
fluxes of carbon.
The microbial communities were marked by a clear difference among sites and size fractions.
The upwelling site’s community showed less richness, diversity and evenness in the attached portion,
compared to the other two sites. Despite, sharing similar environmental parameters (nutrients,
chlorophyll concentration and pH) the control and the upwelling site revealed a significant
dissimilarity in microbial community composition in both size fractions. Furthermore, the highest
proportion of the genus Vibrio was found at the upwelling followed by the volcanic site. This increase
in abundance of the pathogenic and disease-associated genus seemed to be tied to the magnitude of
sea surface temperature anomaly.
After 24 hour incubation all experimental treatments showed a marked loss of diversity, and
shifts of the microbial community composition. At 48 hour incubation all the treatments showed a
highly similar community. Because the number of replicates was not possible to determine
statistically the environmental factor which drives this changes.
The results of this project are the first to report the in situ effects of climate change on multiple
components of the marine carbon cycle and microbial communities, offering a better approximation
future scenarios. The information provided in this study might be further extrapolated in order to
predict changes in biogeochemical cycles in the ocean.
