Grazing on <i>Synechococcus spp.</i> by the Red-Tide Dinoflagellate <i>Karenia brevis</i>: Implications for Bloom Dynamics in the Gulf of Mexico

<p><em>Karenia brevis</em>, the toxic dinoflagellate responsible for massive red tides in the Gulf of Mexico (GOM), causes fish kills, shellfish poisoning, and acute respiratory irritation in humans. Bloom initiation and maintenance have been linked to the physical environment as well as various nutrient input mechanisms. To date, efforts to quantify nitrogen (N) sources fueling <em>K. brevis</em> blooms in the GOM have not included mixotrophic grazing although many dinoflagellates, including <em>K. brevis</em>, are known to be capable of mixotrophy. This dissertation reports field and laboratory results demonstrating that natural bloom populations and <em>K. brevis</em>isolates from the West Florida Shelf (WFS) can ingest a WFS <em>Synechococcus</em> isolate. Maximum <em>K. brevis</em> ingestion rates were measured within the first 2 to 6 hours in laboratory incubations augmented with <em>Synechococcus</em> prey and rates ranged from 7.2 to 48.0 <em>Synechococcus K. brevis</em>^-1 hr^-1. I calculated a lower feeding threshold of 1.86 × 10⁴ <em>Synechococcus</em> ml^-1 , which is the prey concentration necessary for <em>K. brevis </em>to ingest this prey organism.</p> <p>To determine whether dissolved N or light affected ingestion rates for <em>Karenia brevis</em> on <em>Synechococcus</em>, grazing was measured in N-replete and -deplete cultures and during the day and night when incubation lights were on or off, respectively. Ingestion rates ranged from 2.7 to 7.2 <em>Synechococcus K. brevis</em>^-1 hr^-1 and there were no significant differences in ingestion rates between treatments. I calculate that the N-specific uptake rates from <em>Synechococcus</em> prey were on the order of 10^-2 to 10¹μmol N l^-1 hr^-1. I also demonstrate for the first time that <em>K. brevis</em> is able to ingest <em>Prochlorococcus </em>(27.3 ± 8.3 <em>Prochlorococcus K. brevis</em> ^-1 hr^-1) and heterotrophic bacteria (0.1 - 3.1 bacteria <em>K. brevis</em>^-1 hr^-1), although the latter are likely underestimates as I tried to minimize contamination by heterotrophic bacteria in <em>K. brevis</em> cultures.</p> <p><em>Karenia brevis</em> ingestion rates on live and heat-killed <em>Synechococcus</em> were not statistically different, 23.4 ± 18.1 and 21.38 ± 12.6 <em>Synechococcus K. brevis</em>^-1 hr^-1, respectively. This allowed me to examine prey uptake versus photosynthetic or amino acid C uptake in the same incubation bottles where grazing was measured. C-specific uptake from <em>Synechococcus </em>ingestion ranged from 11.2 to 38.8 pmol C <em>K. brevis</em> -1 hr-1, which was 7.5 to 22.4 times greater than photosynthetic C uptake in parallel incubations.</p> <p>Ingestion rates by <em>Karenia brevis</em> on <em>Synechococcus </em>measured during cruises to the WFS during three blooms were 0.04 to 15.5 <em>Synechococcus K. brevis</em>^-1 hr ^-1, which falls within the range found in laboratory studies. The highest ingestion rates by <em>K. brevis</em> on the WFS were measured in 2009 despite low ambient concentrations of <em>Synechococcus.</em> N-specific uptake from <em>Synechococcus</em> ranged from 0.05 to 13.86 μmol N l^-1 hr^-1 during laboratory and field experiments. Grazing on <em>Synechococcus</em>, as well as other possible picoplanktonic prey, can contribute substantially to the N budget for <em>K. brevis</em> growth in the GOM, which has been reported between 0.056 to 0. 267 μmol N l^-1 d^-1 for moderately sized (10⁵ cells l^-1) blooms growing autotrophically.</p>

Name thesis:degree_name

Doctor of Philosophy (PhD)

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Ocean & Earth Sciences

Year dc:date.available

2012

Author dc:creator

• Procise, Leo A.

Contributors dc:contributor

• Margaret R. Mulholland
• Alexander Bochdansky
• Harold G. Marshall
• Diane K. Stoecker

• Bloom dynamics
• Grazing
• Gulf of Mexico
• Karenia brevis
• Mixofrophy
• Red-tide dinoflagellate
• Synechococcus
• Ecology and Evolutionary Biology
• Microbiology
• Oceanography

Statement dc:rights

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Identifier

9781267890535

OAI identifier oai:identifier

oai:digitalcommons.odu.edu:oeas_etds-1066