Mesodinium rubrum (Lohmann 1908) Jankowski 1976 (= Myrionecta rubra) is a common photosynthetic marine planktonic ciliate which can form coastal red-tides. It may represent a 'species complex' and since Darwin's voyage on the Beagle, it has been of great cytological, physiological and evolutionary interest. It is considered to be functionally a phytoplankter because it was thought to have lost the capacity to feed and possesses a highly modified algal endosymbiont. Whether M. rubrum is the result of a permanent endosymbiosis or a transient association between a ciliate and an alga is controversial. We conducted 'feeding' experiments to determine how exposure to a cryptophyte alga affects M. rubrum. Here we show that although M. rubrum lacks a cytostome (oral cavity), it ingests cryptophytes and steals their organelles, and may not maintain a permanent endosymbiont. M. rubrum does not fall into recognized cellular or functional categories, but may be a chimaera partially supported by organelle robbery.

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The population dynamics of interior ice microalgae were investigated at a snow-free site on annual land-fast sea ice in McMurdo Sound, Antarctica, during the austral spring and summer of 1995-96. A dynamic successional sequence was observed with life history transformations playing an important role. During late November and early December (austral spring), cryo-and halotolerant dinoflagellates and chrysophytes bloomed in brine channels within the upper ice. At this time, competition and grazing pressure are low because of the inability of most marine species to grow under the extreme environmental conditions found in the upper ice during the austral spring. In November and December, dinoflagellates, chrysophytes, and prasinophytes contributed an average of 66%, 44%, and Ͻ1% of the phytoflagellate biomass, respectively. Both the dinoflagellates and the chrysophytes encysted in December, with cyst formation most intense just prior to surface melt and flushing of the ice. The cysts appear to be an adaptation for survival and dispersal in the plankton during ice decay and/or overwintering in the sea ice. In January (austral summer), when ice temperatures were similar to those in the water column, pennate diatoms replaced flagellates as the photosynthetic dominants in the upper sea ice. The upper land-fast sea ice undergoes dramatic seasonal changes in light availability, temperature, brine salinity, and inorganic nutrient availability. Ephemeral blooms of cyst-forming phytoflagellates exploit this habitat in the austral spring, when both inorganic nutrients and light are available but temperatures ϽϪ2Њ C and brine salinities elevated.

Extreme environmental conditions have been thought to limit algal growth in the upper sea-ice. In McMurdo Sound, Antarctica, chrysophyte statocysts (stomatocysts) and dinoflagellate hypnozygotes (resting cysts) overwinter in $first-and secondyear land-fast sea-ice exposed to tmperatures of -20" C or lower. I n early November, when temperatures in the upper ice are <-8" C and bnne salinities are >126 p s~, dinoflagellate cysts activate and shortly thereafter excyst. During early November, chlysophyte statocysts also begin to excyst. Net daily przmary production occurs in the sea-ice bnne at temperatures as low as -7.1 O C, at brine salinities as high as 129 psu, and at average photon flux densities as low as 5 pmol photons.m-z.s-i. Dinoflagellate densities were >lo6 vegetative cells.L-' of ice while temperatures in the upper ice were between -6.8 and -5.8" C and brine salinities were -100 psu. Chrysophyte densities reached > 106.L-' of ice by ear4 December. High densities of physiologically active clyo-and halotolerant algae can occur in the upper landfast sea-ice under extreme conditions of temperature and salinity.