Irreversibly increased nitrogen fixation in Trichodesmium experimentally adapted to elevated carbon dioxide

Abstract

Nitrogen fixation rates of the globally distributed, biogeochemically important marine cyanobacterium Trichodesmium increase under high carbon dioxide (CO2) levels in short-term studies due to physiological plasticity. However, its long-term adaptive responses to ongoing anthropogenic CO2 increases are unknown. Here we show that experimental evolution under extended selection at projected future elevated CO2 levels results in irreversible, large increases in nitrogen fixation and growth rates, even after being moved back to lower present day CO2 levels for hundreds of generations. This represents an unprecedented microbial evolutionary response, as reproductive fitness increases acquired in the selection environment are maintained after returning to the ancestral environment. Constitutive rate increases are accompanied by irreversible shifts in diel nitrogen fixation patterns, and increased activity of a potentially regulatory DNA methyltransferase enzyme. High CO2-selected cell lines also exhibit increased phosphorus-limited growth rates, suggesting a potential advantage for this keystone organism in a more nutrient-limited, acidified future ocean.

Rapid adaptation of harmful cyanobacteria to rising CO 2

“…Our results add to the growing literature showing that plankton species are capable of rapid evolutionary adaptation to changing conditions; for example, in response to rising CO 2 levels (6,8,9), increasing temperature (42,43), and changes in predation pressure (44,45). A key advance of the present work is that the adaptive changes could be linked to specific genetic and molecular traits, which enabled monitoring of natural selection not only in confined laboratory experiments but also in lakes.…”

Effects of ultraviolet radiation on photosynthetic performance and N<sub>2</sub> fixation in <i>Trichodesmium erythraeum</i> IMS 101

“…This global importance of Trichodesmium has motivated numerous studies regarding the physiological responses of Trichodesmium to environmental factors, including visible light, phosphorus, iron, temperature, and CO 2 (Kranz et al, 2010;Shi et al, 2012;Fu et al, 2014;Spungin et al, 2014;Hutchins et al, 2015). However, to the best of our knowledge, there have been no reports on how UV exposure may affect Trichodesmium.…”

Comment on “The complex effects of ocean acidification on the prominent N 2 -fixing cyanobacterium Trichodesmium ”

“…Although this is a relatively basic analysis, in the case of Trichodesmium the protocol chosen is critical. Nitrogen and carbon fixation and growth in this species follow a pronounced diel rhythm [3,4,6], so most cell division occurs in the afternoon. One can thus calculate anomalously elevated growth rates similar to those reported by Hong et al, simply by measuring them solely from early morning until late in that (or a subsequent) afternoon.…”

“…Such CO 2 fertilization of marine nitrogen--fixation could potentially provide a negative feedback on anthropogenic CO 2 emissions [2,5,6].…”

“…2017) 7 . Nitrogen--fixation was measured and normalized the same way in all experiments shown; previously published data 2,5,6 was recalculated using the same 4:1 ethylene:N 2 conversion ratio used in Hong et al (2017) 7 .…”