Abstract
Many species of harmful algae transition between a motile, vegetative stage in the water column and a non-motile, resting stage in the sediments. Physiological and behavioral traits expressed during benthic-pelagic transition potentially regulate the timing, location and persistence of blooms. The roles of key physiological and behavioral traits involved in resting cell emergence and bloom formation were examined in two geographically distinct strains of the harmful alga, Heterosigma akashiwo. Physiological measures of cell viability, division and population growth, and cell fatty acid content were made using flow cytometry and gas chromatography – mass spectrometry techniques as cells transitioned between the benthic resting stage and the vegetative pelagic stage. Video-based tracking was used to quantify cell-level swimming behaviors. Data show increased temperature and light triggered rapid emergence from the resting stage and initiated cell swimming. Algal strains varied in important physiological and behavioral traits, including survivorship during life-stage transitions, population growth rates and swimming velocities. Collectively, these traits function as “population growth strategies” that can influence bloom formation. Many resting cells regained the up-swimming capacity necessary to cross an environmentally relevant halocline and the ability to aggregate in near-surface waters within hours after vegetative growth supporting conditions were restored. Using a heuristic model, we illustrate how strain-specific population growth strategies can govern the timescales over which H. akashiwo blooms form. Our findings highlight the need for identification and quantification of strain-specific physiological and behavioral traits to improve mechanistic understanding of bloom formation and successful bloom prediction.
Highlights
Harmful algal blooms (HABs) occur when algal cells produce toxins or accumulate to densities sufficient to be deleterious to other organisms, causing damage to aquatic ecosystems and/or risks to public health [1,2,3,4]
Quantifying physiological and behavioral characteristics that influence algal cells’ rates of benthic emergence and vertical fluxes to the upper water column is critical for understanding the basic timescales over which H. akashiwo harmful algal blooms form
Greater cell survivorship and activation rates suggest that CCMP 452 was more successful at transitioning into and out of the benthic resting stage than UWC 13.03
Summary
Harmful algal blooms (HABs) occur when algal cells produce toxins or accumulate to densities sufficient to be deleterious to other organisms, causing damage to aquatic ecosystems and/or risks to public health [1,2,3,4]. Some HABs are thought to initiate when benthic cells return to the vegetative state and rapidly repopulate the water column [8,9] This process typically requires benthic cells to increase metabolic activity, to emerge from the sediments and ascend toward the surface of the water column, and to undergo rapid cell division to form population densities characteristic of blooms. Despite this potentially causal role in bloom dynamics, life stage transitions are among the least understood aspects of HAB dynamics
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