Abstract
BackgroundDiatoms usually dominate phytoplankton blooms in open oceans, exhibiting extremely high population densities. Although the iron uptake rate of diatoms largely determines the magnitude and longevity of diatom blooms, the underlying mechanisms regulating iron uptake remain unclear.ResultsThe transcription of two iron uptake proteins, ISIP2a and ISIP1, in the marine diatom Phaeodactylum tricornutum was enhanced with increasing cell density, whereas the cellular iron content showed the opposite trend. When compared with the wild-type strain, knockdown of ISIP2a resulted in 43% decrease in cellular iron content, implying the involvement of ISIP2a in iron uptake under high-cell density conditions. Incubation of the diatom cells with sonicated cell lysate conditioned by different cell densities did not affect ISIP2a and ISIP1 expression, ruling out regulation via chemical cues. In contrast, ISIP2a and ISIP1 transcription were strongly induced by red light. Besides, chlorophyll fluorescence excited from the blue light was also positively correlated with population density. Subsequently, a “sandwich” illumination incubator was designed to filter out stray light and ensure that the inner layer cells only receive the emitted chlorophyll fluorescence from outer layers, and the results showed that the increase in outer cell density significantly elevated ISIP2a and ISIP1 transcription in inner layer cells. In situ evidence from Tara oceans also showed positively correlated between diatom ISIP transcripts and chlorophyll content.ConclusionsThis study shows that chlorophyll fluorescence derived from neighboring cells is able to upregulate ISIP2a and ISIP1 expression to facilitate iron assimilation under high-cell density. These results provide novel insights into biotic signal sensing in phytoplankton, which can help to elucidate the underlying mechanisms of marine diatom blooms.
Highlights
Diatoms usually dominate phytoplankton blooms in open oceans, exhibiting extremely high population densities
Physiological response of Phaeodactylum tricornutum to high-cell density The effects of different cell densities on the intracellular iron concentration in P. tricornutum were examined under white light (WL) and dark conditions using ICP-OES
Expression of iron uptake system in P. tricornutum is correlated with cell density The results of the present study showed that P. tricornutum can sense high-cell density and induce ISIP2a and ISIP1 expression to accelerate iron absorption, which is unrestricted by environmental iron concentrations
Summary
Diatoms usually dominate phytoplankton blooms in open oceans, exhibiting extremely high population densities. During natural phytoplankton blooms when extracellular iron is sufficient, the phytoplankton population density mostly reaches 106 cells/L [6,7,8], sometimes even as high as 3×108 cells/L in the HNLC regions, such as the South Pacific Ocean, North Pacific Ocean, and Indian Ocean et al [9]. Such explosive increase in a cell population can result in increased inter- and intraspecies competition for iron. As the two most predominant proteins in the iron uptake system, ISIP2a and ISIP1 play important roles in the primary response to iron deficiency, resulting from multiple environmental factors
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