Distinct Responses of the Nitrogen-Fixing Marine Cyanobacterium Trichodesmium to a Thermally Variable Environment as a Function of Phosphorus Availability.

Pingping Qu,David A Hutchins,Xinwei Wang,Joshua D Kling,Fei-Xue Fu,Megan Huh

doi:10.3389/fmicb.2019.01282

Pingping Qu, David A Hutchins + Show 4 more

Open Access

https://doi.org/10.3389/fmicb.2019.01282

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Abstract

Surface temperature in the ocean is projected to be elevated and more variable in the future, which will interact with other environmental changes like reduced nutrient supplies. To explore these multiple stressor relationships, we tested the influence of thermal variation on the key marine diazotrophic cyanobacterium Trichodesmium erythraeum GBRTRLI101 as a function of the limiting nutrient phosphorus (P). Two constant temperature treatments represented current winter (22°C) and summer (30°C) mean values. Three variable temperature treatments fluctuated around the constant control values: Mean 22°C, either ± 2°C or ± 4°C; and mean 30°C ± 2°C. Each thermal treatment was grown under both P-replete (10 μmol/L) and P-limiting conditions (0.2 μmol/L). Effects of thermal variability on Trichodesmium were mainly found in the two winter variable temperature treatments (22°C ± 2°C or ± 4°C). P availability affected growth and physiology in all treatments and had significant interactions with temperature. P-replete cultures had higher growth and nitrogen and carbon fixation rates in the 22°C constant control, than in the corresponding variable treatments. However, physiological rates were not different in the P-replete constant and variable treatments at 30°C. In contrast, in P-limited cultures an advantage of constant temperature over variable temperature was not apparent. Phosphorus use efficiencies (PUE, mol N or C fixed h-1 mol cellular P-1) for nitrogen and carbon fixation were significantly elevated under P-limited conditions, and increased with temperature from 22 to 30°C, implying a potential advantage in a future warmer, P-limited environment. Taken together, these results imply that future increasing temperature and greater thermal variability could have significant feedback interactions with the projected intensification of P-limitation of marine N2-fixing cyanobacteria.

Highlights

All environmental factors that affect the growth and fitness of organisms vary over time, and these biological impacts are projected to intensify as environmental variability increases under global climate change scenarios (Burroughs, 2007; IPCC, 2012; Thornton et al, 2014; Boyd et al, 2016)
We explored the influence of thermal variation on the growth and physiology of the diazotrophic cyanobacterium Trichodesmium erythraeum GBRTRLI101
Our results suggest that the future trends of Trichodesmium GBRTRLI101 growth in the winter Great Barrier Reef (GBR) area will depend on the combination of temperature increases and intensified thermal variability

Summary

Introduction

All environmental factors that affect the growth and fitness of organisms vary over time, and these biological impacts are projected to intensify as environmental variability increases under global climate change scenarios (Burroughs, 2007; IPCC, 2012; Thornton et al, 2014; Boyd et al, 2016). In a more stratified warming ocean, an expected shallower upper mixed layer will become more vulnerable to disturbance and more sensitive to varying air-sea heat fluxes and solar radiation (Behrenfeld et al, 2006; Häder et al, 2011; Boyd et al, 2016). Global warming effects have been addressed in voluminous studies, but until recently the influence of changing thermal variability on ecosystems has received relatively little attention. Bernhardt et al (2018) proposed a non-linear averaging model based on the principle of Jensen’s inequality, suggesting that temperature fluctuations exceeding the thermal optimum can depress temperature growth response curves compared to the constant temperature condition

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