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Salinity decline promotes growth and harmful blooms of a toxic alga by diverting carbon flow

global climate change,Harmful Algal Blooms (HABs)

Global climate change intensifies the water cycle and makes freshest waters become

fresher and vice-versa. But how this change impacts phytoplankton in coastal, par-

ticularly harmful algal blooms (HABs), remains poorly understood. Here, we moni-

tored a coastal bay for a decade and found a significant correlation between salinity

decline and the increase of Karenia mikimotoi blooms. To examine the physiological

linkage between salinity decreases and K. mikimotoi blooms, we compare chemical,

physiological and multi-omic profiles of this species in laboratory cultures under high

(33) and low (25) salinities. Under low salinity, photosynthetic efficiency and capacity

as well as growth rate and cellular protein content were significantly higher than that

under high salinity. More strikingly, the omics data show that low salinity activated

the glyoxylate shunt to bypass the decarboxylation reaction in the tricarboxylic acid

cycle, hence redirecting carbon from CO2 release to biosynthesis. Furthermore, the

enhanced glyoxylate cycle could promote hydrogen peroxide metabolism, consist-

ent with the detected decrease in reactive oxygen species. These findings suggest

that salinity declines can reprogram metabolism to enhance cell proliferation, thus

promoting bloom formation in HAB species like K. mikimotoi, which has important

ecological implications for future climate-driven salinity declines in the coastal ocean

with respect to HAB outbreaks.