Specific Projects:

Our laboratory group is interested in the ecology and physiology of marine planktonic protists (microalgae and protozoa), especially dinoflagellates, diatoms and ciliates.   Our research is focused in three areas. 

The first research area is mixotrophy. Mixotrophy is the use of alternate modes of nutrition, such as feeding and photosynthesis.  Some “protozoa” are photosynthetic due to the presence of algal endosymbionts or retention of functional organelles from algal prey.  Surprisingly, some of these protozoa retain functional nuclei from their algal prey to help run their photosynthetic machinery. Many of the dinoflagellates responsible for red tides are mixotrophs and although they are “phytoplankton they also prey on other cells.

Karlodinium veneficum (= K. micrum)	Myrionecta rubra (= Mesodinium rubrum)	Prorocentrum minimum

The second is the ecology of  harmful algae. We have found that several species of Pseudo-nitzschia (a potentially toxic diatom) are present in Maryland estuaries and that some produce the neurotoxin domoic acid.  We are currently investigating the effects of two dinoflagellates, Karlodinium veneficum and Prorocentrum minimum, on early life history stages of oysters. We have found that K. veneficum, which is a common harmful alga in the Chesapeake Bay area, can cause high mortality of oyster embryos and larvae in laboratory experiments.

The third is grazing by micro-zooplankton (primarily ciliates and heterotrophic dinoflagellates) on phytoplankton and the role of grazing in preventing or limiting blooms. We are particularly interested in how changes in plankton community structure due to eutrophication or climate change influence top-down control of algal blooms.

Myrionecta rubra, a planktonic ciliate that is photsynthetic, under transmitted light (left) and epiflourescence microscopy (right). The orange fluorescence is due to cryptophyte plastids.
	Kalodinium feeding sequence

Abstracts of recent papers

Matthew D. Johnson, David Oldach, Charles F. Delwiche and Diane K. Stoecker

Retention of transcriptionally active cryptophyte nuclei by the ciliate Myrionecta rubra
Nature 445, 426-428 (25 January 2007)

It is well documented that organelles can be retained and used by predatory organisms, but in most cases such sequestrations are limited to plastids of algal prey. Furthermore, sequestrations of prey organelles are typically highly ephemeral as a result of the inability of the organelle to remain functional in the absence of numerous nuclear-encoded genes involved in its regulation, division and function. The marine photosynthetic ciliate Myrionecta rubra (Lohmann 1908) Jankowski 1976 (the same as Mesodinium rubrum) is known to possess organelles of cryptophyte origin, which has led to debate concerning their status as permanent symbiotic or temporary sequestered fixtures. Recently, M. rubra has been shown to steal plastids (that is, chloroplasts) from the cryptomonad, Geminigera cryophila, and prey nuclei were observed to accumulate after feeding. Here we show that cryptophyte nuclei in M. rubra are retained for up to 30 days, are transcriptionally active and service plastids derived from multiple cryptophyte cells. Expression of a cryptophyte nuclear-encoded gene involved in plastid function declined in M. rubra as the sequestered nuclei disappeared from the population. Cytokinesis, plastid performance and their replication are dependent on recurrent stealing of cryptophyte nuclei. Karyoklepty (from Greek karydi, kernel; kleftis, thief) represents a previously unknown evolutionary strategy for acquiring biochemical potential.

Jason E. Adolf, Diane K. Stoecker, Lawrence W. Harding.

The balance of autotrophy and heterotrophy during mixotrophic growth of Karlodinium micrum (Dinophyceae).
Journal of Plankton Research, Volume 28, Number 8 (August 2006), pp. 737-751

We studied autotrophic and heterotrophic C metabolism during mixotrophic growth of Karlodinium micrum (Leadbeter et Dodge) Larsen (Dinophyceae) on prey Storeatula major (Cryptophyceae). Our goal was to determine the balance of autotrophy and heterotrophy that supports mixotrophic growth in K. micrum. Assimilation of inorganic 14C and 14C -labeled prey was used to separate the quantity and quality (i.e., lipid, polysaccharide and protein) of C obtained by autotrophy and heterotrophy, respectively. Growth rates (µ) of mixotrophic K. micrum were 0.52–0.75 div.·day-1, equal to or greater than the maximum autotrophic growth rate (0.55 div.·day-1) of K. micrum. Autotrophy represented 27–69% of gross C uptake during mixotrophic growth. Cellular photosynthetic performance (PPcell, pg C cell-1·day-1) was 24–52% lower during mixotrophic growth than during autotrophic growth of K. micrum. Mixotrophic K. micrum assimilated 16% less photosynthate as protein compared to autotrophic K. micrum, while protein was the major net assimilation product (52%) from ingested prey C. Growth efficiency (%GE) of mixotrophic cultures, based on both autotrophic and heterotrophic C sources, averaged 36 ± 2.9%, slightly lower than the 40–50% GE typical of purely autotrophic K. micrum, but higher C gains associated with heterotrophic feeding more than compensated for the decrease in %GE in mixotrophic K. micrum. We conclude that mixotrophic growth of K. micrum is dominated by heterotrophic metabolism, although photosynthesis continues at a lowered rate. This is consistent with a shift toward secondary production in plankton assemblages dominated by mixotrophically growing K. micrum.

Reaugh ML, Roman MR, Stoecker DK (in press)

Changes in plankton community structure and function in response to variable freshwater flow in two tributaries of the Chesapeake Bay, USA. Estuaries and Coasts.

The biomass of phytoplankton, microzooplankton, copepods, and gelatinous zooplankton were measured in two tributaries of the Chesapeake Bay during the springs of consecutive dry (below average freshwater flow), wet (above average freshwater flow) and average freshwater flow years.  The potential for copepod control of microzooplankton biomass in the dry and wet years was evaluated by comparing the estimated grazing rates of microzooplankton by the dominant copepod species (Acartia spp. and Eurytemora affinis ) to microzooplankton growth rates and by calculating the percent of daily microzooplankton standing stock removed through copepod grazing. There were significant increases in phytoplankton and copepod biomass but not for microzooplankton biomass in the wet year as compared to the dry year.  The ctenophore, Mnemiposis leidyi, was present during the dry year but was absent during the sampling period of the wet and average freshwater flow years. Grazing pressure on microzooplankton was greatest in the wet year, with Acartia spp. and Eurytemora affinis ingesting 0.21-2.64 mg of microzooplankton C copepod-1 d-1 and removing up to 60% of the microzooplankton standing stock per day.  In the dry year, these copepod species ingested 0.10 – 0.73 mg of microzooplankton C copepod-1 d-1 with a maximum daily removal of approximately 3% of the microzooplankton standing stock.  Potential copepod grazing pressure was significantly less than microzooplankton growth in the dry year, but was equivalent to microzooplankton growth in the wet year, implying strong top-down control of the microzooplankton community in the wet year.  These results suggest that increased grazing control of microzooplankton populations by more copepods in the wet year released top-down control of phytoplankton.  Reduced microzooplankton grazing, in conjunction with increased nutrient availability, resulted in the large increases in phytoplankton biomass in the wet year.  Thus, increased freshwater flow has the potential to influence trophic cascades and the partitioning of plankton production in estuarine systems. The presence of strong top-down control of microzooplankton community biomass in wet years may be a mechanism by which phytoplankton blooms are able to develop and persist.

Stoecker DK, Adolf JE, Place AR, Glibert PM, Meritt D. (submitted)

Effects of the dinoflagellates Karlodinium veneficum and Prorocentrum minimum on early life history stages of the Eastern Oyster, Crassostrea virginica.

The bloom-forming dinoflagellates, Prorocentrum minimum and Karlodinium veneficum, can have detrimental effects on marine life, including shellfish, but little is known about their effects on early life history stages of bivalves. In the Chesapeake Bay region, blooms of these dinoflagellates over-lap with the spawning season of the eastern oyster, Crassostrea virginica. In laboratory experiments, we compared the effects of P. minimum and K. veneficum on the survival and development of embryos and larvae of the eastern oyster. At ~10,000 cells ml-1, P. minimum (strain PM-1) did not have a negative  affect on embryos and larvae in 2 day exposures. The yield of D-hinge larvae was > than  in control treatments. At ~2 X10,000 cells ml-1(~biomass of P. minimum treatment) K. veneficum (strain CCMP 1974) caused significant mortality to oyster embryos within 1 day and almost no embryos developed into D-hinge larvae. This effect was not alleviated by the provision of an alternate food source (Isochrysis). Significant mortality was observed when larvae were exposed to K. veneficum at concentrations as low as 2.5 X 1,000 cells ml-1 (~40 pg ml-1 of karlotoxin). A two day exposure of larvae to 5 X 1,000 cells ml-1of K. veneficum reduced yield of mature larvae by ~66%. The K. veneficum cultures that we used in our experiments were relatively low in toxin content, more toxic strains could be expected to cause mortality at lower cell concentrations. Survival and maturation of larvae may be reduced when spawns of the eastern oyster coincide with high bloom densities of K. veneficum.