Molecular underpinnings and biogeochemical consequences of enhanced diatom growth in a warming Southern Ocean

Abstract

The Southern Ocean (SO) harbors some of the most intense phytoplankton blooms on Earth. Changes in temperature and iron availability are expected to alter the intensity of SO phytoplankton blooms, but little is known about how these changes will influence community composition and downstream biogeochemical processes. We performed light-saturated experimental manipulations on surface ocean microbial communities from McMurdo Sound in the Ross Sea to examine the effects of increased iron availability (+2 nM) and warming (+3 and +6 °C) on nutrient uptake, as well as the growth and transcriptional responses of two dominant diatoms, Fragilariopsis and Pseudo-nitzschia. We found that community nutrient uptake and primary productivity were elevated under both warming conditions without iron addition (relative to ambient −0.5 °C). This effect was greater than additive under concurrent iron addition and warming. Pseudo-nitzschia became more abundant under warming without added iron (especially at 6 °C), while Fragilariopsis only became more abundant under warming in the iron-added treatments. We attribute the apparent advantage Pseudo-nitzschia shows under warming to up-regulation of iron-conserving photosynthetic processes, utilization of iron-economic nitrogen assimilation mechanisms, and increased iron uptake and storage. These data identify important molecular and physiological differences between dominant diatom groups and add to the growing body of evidence for Pseudo-nitzschia’s increasingly important role in warming SO ecosystems. This study also suggests that temperature-driven shifts in SO phytoplankton assemblages may increase utilization of the vast pool of excess nutrients in iron-limited SO surface waters and thereby influence global nutrient distribution and carbon cycling.

Transcriptomic-Guided Phosphonate Utilization Analysis Unveils Evidence of Clathrin-Mediated Endocytosis and Phospholipid Synthesis in the Model Diatom, Phaeodactylum tricornutum

“…4c and Table S4b ). By searching through the Southern Ocean (SO) diatom assemblage transcriptome database ( 49 ), abundant transcript expressions of four selected genes were also identified ( Table S4c ) in diatom species shown in Fig. 4a .…”

Molecular physiology of Antarctic diatom natural assemblages reveals multiple strategies contributing to their ecological success

“…Cryptophytes may not be as transcriptionally active as diatoms or dinoflagellates, which could account for their low relative transcript abundances, or there may be differences in mRNA recovery or representative database sequences (57) . Inferred low cryptophyte cell abundances may also be the result of niche exclusion by dinoflagellates and diatoms as these two groups generally grow faster and can inhabit a wider range of temperatures and salinities (38,41).…”

Southern Ocean Iron Limitation of Primary Production between Past Knowledge and Future Projections

“…Temperature is the other environmental factor that should be considered prohibitive for phytoplankton to thrive in the SO. Temperature increments, for example, even if to a lesser extent, have been shown to have a possible synergistic effect with Fe on Antarctic phytoplankton [81][82][83]. Bloom-forming Pseudonitzschia species from the Ross Sea showed significantly increased growth and high managing capabilities for cellular Fe demand under warmer low-Fe conditions [83].…”