Nutritional Flexibility of Mixotrophic Dinoflagellates in Lower Chesapeake Bay
Tributaries
Margaret R. Mulholland1, R. E. Morse, P. W. Bernhardt1 and A. M. Watson1
1
Department of Ocean, Earth and Atmospheric Sciences, Old Dominion University,
Norfolk, Virginia, USA
In certain environments, mixotrophic species may have a competitive advantage
over strictly autotrophic species that can only fix carbon during the day through
photosynthesis or are limited by light. Many harmful algal bloom species are
mixotrophic. The Elizabeth and Lafayette Rivers, sub-tributaries of the lower Chesapeake
Bay, experience seasonal dinoflagellate blooms including a variety of mixotrophic
dinoflagellates: Heterocapsa triquetra in the late winter, Prorocentrum minimum in the
spring and Akashiwo sanguinea in the early summer and Scrippsiella trochoidea and
Cochlodinium polykrikoides later in the summer and fall. We conducted a multi-year
study during which we measured uptake of various inorganic and organic nitrogen (N)
and carbon (C) compounds during dinoflagellate blooms over the course of several years
and during multiple seasons when different species were dominant. We determined that
these species have the capacity to take up a variety of N compounds, including inorganic
and organic N, as well as organic C. Both organic and inorganic N and C contribute
substantially to their growth, with uptake of organic compounds often exceeding that of
inorganic compounds. Results from two years of field work suggest that rates of these
processes vary on seasonal and diel time scales and as blooms initiate and develop and
that there is no single nutrient form that triggers blooms. The ability to assimilate
dissolved organic carbon may put these species in direct competition with bacteria.
However, many of these organisms appear to compete effectively with bacteria for
organic compounds, including amino acids used to estimate bacterial productivity, on
relevant timescales.