SEMINAR: OI Seminar Series - Dr Jesus M Arrieta

Dr Jesús M. Arrieta from Instituto Mediterraneo de Estudios Avanzados (IMEDEA) CSIC-UIB

Title: Experimental evaluation microbial responses to warming and ice melting in the Arctic

Abstract: The Arctic region is warming three times faster than the global mean, resulting in an abrupt reduction in ice cover, exceeding the range of natural variability over the last millennium. Rapid warming is expected to continue in the future, with up to 6 º C warming throughout the 21st century. The metabolic theory of ecology predicts an increase in primary production with increasing temperature. However, respiration rates are expected to show an even stronger response to increasing temperature than photosynthetic rates, resulting in a reduction in the production to respiration ratio with increasing warming. Experimental temperature manipulations in the range of the warming scenarios predicted for the Arctic Ocean along the 21st Century support the hypothesis of a shift towards heterotrophy in planktonic microbial communities of the Arctic Ocean in response to warming. Moreover, we found clear evidence of non-linear responses of different compartments of the Arctic microbial planktonic food web to increasing temperatures.

A different set of experiments was conducted to evaluate the impact of ice melting on Arctic microbial planktonic communities. Apart from the evident impact on salinity, melted ice also releases significant amounts of dissolved and particulate materials into the Arctic Ocean. We hypothesized that heterotrophic prokaryotic communities could utilize these organic and inorganic materials released from sea ice. This hypothesis was tested using mixtures of melted ice and seawater as compared to pure water additions. Our results revealed that the addition of melted ice caused a small, but measurable enhancement of prokaryotic growth in all the samples tested. Moreover, the microcosms containing melted ice showed lower rates of prokaryotic respiration as compared to the controls, resulting in enhanced prokaryotic growth efficiency in response to the materials released by Arctic ice. Prokaryotic community structure was almost identical in all treatments, indicating the higher lability of the materials contained in sea ice as compared to bulk DOM already present in seawater.