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Karel Castro-Morales

and 14 more

Arctic rivers are intricate water networks that chemically and biologically process carbon before releasing it as carbon dioxide (CO2) into the atmosphere or carrying it to the ocean. Primary producers use inorganic carbon to build biomass at the basis of the trophic chain. Little is known about how Arctic rivers adapt to climate warming, changes in hydrology and biogeochemical properties. To quantify net and gross biological productivity we measured the dissolved oxygen-to-argon (O2/Ar) ratios and O2 triple isotopologue composition in the river Kolyma and in its tributary Ambolikha during late freshet (June) and base-flow conditions (August) in 2019. We found that hydrological factors restricted river productivity. The river system released CO2 into the atmosphere in June and August, however August emissions were only 6 % of late freshet emissions. Also, the Ambolikha tributary emitted twice as much CO2 per area than the main Kolyma channel in June. Due to higher river flow and turbidity in June, river production was reduced, while lower flows in August permitted more light penetration and a phytoplankton bloom at the confluence of tributary and main Kolyma channel. Total CO2 emissions per area during June and August amounted to (5±11) % of the gross carbon uptake estimated at the bloom site. Thus, in-stream metabolism can exceed riverine CO2 emissions under specific flow and light conditions. Arctic climate change may promote biological productivity in particular locations and increase its contribution to carbon budgets in Arctic rivers as flow slows during longer open water periods.
Cheating in microbial communities is often regarded as a precursor to a “tragedy of the commons”, ultimately leading to over-exploitation by a few species, and destabilisation of the community. However, this view does not explain the ubiquity of cheaters in nature. Indeed, existing evidence suggests that cheaters are not only evolutionarily and ecologically inevitable, but also play important roles in communities, like promoting cooperative behaviour. We developed a chemostat model with two microbial species and a single, complex nutrient substrate. One of the organisms, an enzyme producer, degrades the substrate, releasing an essential and limiting resource that it can use both to grow and produce more enzymes, but at a cost. The second organism, a cheater, does not produce the enzyme but benefits from the diffused resource produced by the other species, allowing it to benefit from the public good, without contributing to it. We investigated evolutionarily stable states of coexistence between the two organisms and described how enzyme production rates and resource diffusion influence organism abundances. We found that, in the long-term evolutionary scale, monocultures of the producer drive themselves extinct because selection always favours mutant invaders that invest less in enzyme production. However, the presence of a cheater buffers this runaway selection process, preventing extinction of the producer and allowing coexistence. Resource diffusion rate controls cheater growth, preventing it from outcompeting the producer. These results show that competition from cheaters can force producers to maintain adequate enzyme production to sustain both itself and the cheater. This is known in evolutionary game theory as a “snowdrift game” – a metaphor describing a snow shoveler and a cheater following in their clean tracks. We move further to show that cheating can stabilise communities and possibly be a precursor to cooperation, rather than extinction.