TY  - JOUR
A1  - Laybourn-Parry, J.
A1  - Roberts, E. C.
A1  - Bell, Elanor M.
T1  - Mixotrophy as a survival strategy in Antarctic lakes
N2  - Mixotrophy is a widespread phenomenon among planktonic protists. It involves the combination of autotrophy and heterotrophy in varying degrees. Many phytoflagellate species ingest bacteria as a means of obtaining nutrients for photosynthesis or for supplementing their carbon budget under light limitation. Ciliates either sequester the plastids of their algal prey or harbour endosymbiotic algae. In the saline lakes of the Vestfold Hills and in Lakes Hoare and Fryxell in the McMurdo Dry Valleys the dominant phytoflagellates ingest bacteria, and there is evidence to suggest that during the winter months they lack chlorophyll and may become entirely heterotrophic. In Lake Fryxell phagotrophic pyhtoflagellates (cryptophytes) made a significant impact on bacterial production, removing up to 13% of the bacterial biomass day-1. These cryptophytes suffered predation from Plagiocampa (a ciliate), which appears to harbour them for a significant period before digesting them. We suspect that this may be equivalent to an intermediate stage in the evolution of mixotrophy. A significant number of the planktonic ciliates in Antarctic lakes were mixotrophic. The final evolutionary end point is the situation seen in Mesodinium rubrum, which now relies entirely on its cryptophycean endosymbiont and no longer ingests food. Mesodinium is the dominant ciliate in many of the saline lakes of the Vestfold Hills, which are of marine origin. It can reach abundances in excess of 60,000l-1 in Ace Lake, This ciliate is a ubiquitous member of the marine plankton worldwide and has successfully adapted to the lacustrine environment in Antarctica. The evidence suggests that among the survival strategies seen in Antarctic lake plankton, mixotrophy plays and important role among a number of the dominant protozoan species.
Y1  - 2000
ER  - 
TY  - JOUR
A1  - Laybourn-Parry, J.
A1  - Bell, Elanor M.
A1  - Roberts, E. C.
T1  - Growth of Protozoa in Antarctic lakes
N2  - The growth rates of heterotrophic nanoflagellates (HNAN), mixotrophic cryptophytes, dinoflagellates and ciliates in field assemblages from Ace Lake in the Vestfold Hills (eastern Antarctica) and Lakes Fryxell and Hoare (McMurdo Dry Valleys, western Antarctica), were determined during the austral summers of 1996/1997 and 1997/1998. The response of the nanoflagellates to temperature differed between lakes in eastern and western Antarctica. In Ace Lake the available bacterial food resources had little impact on growth rate, while temperature imposed an impact, whereas in Lake Hoare increased bacterial food resources elicited an increase in growth rate. However, the incorporation of published data from across Antarctica showed that temperature had the greater effect, but that growth is probably controlled by a suite of factors not solely related to bacterial food resources and temperature. Dinoflagellates had relatively high specific growth rates (0.0057-0.384 h(-1)), which were comparable to Antarctic lake ciliates and to dinoflagellates from warmer, lower latitude locations. Temperature did not appear to impose any significant impact on growth rates. Mixotrophic cryptophytes in Lake Hoare had lower specific growth rates than HNAN (0.0029-0.0059 h(-1) and 0.0056-0.0127 h(-1), respectively). They showed a marked seasonal variation in growth rate, which was probably related to photosynthetically active radiation under the ice at different depths in the water column. Ciliates' growth rates showed no relationship between food supply and mean cell volume, but did show a response to temperature. Specific growth rates ranged between 0.0033 and 0.150 h(-1) for heterotrophic ciliates, 0.0143 h(-1) for a mixotrophic Plagiocampa species and 0.0075 h(-1) for the entirely autotrophic ciliate, Mesodinium rubrum. The data indicated that the scope for growth among planktonic Protozoa living in oligotrophic, cold extreme lake ecosystems is limited. These organisms are likely to suffer prolonged physiological stress, which may account for the highly variable growth rates seen within and between Antarctic lakes.
Y1  - 2000
ER  - 
TY  - JOUR
A1  - Bell, Elanor M.
A1  - Laybourn-Parry, J.
T1  - Mixotrophy in the Antarctic phytoflagellate, Pyramimonas gelidicola (Chlorophyta: Prasinophyta)
N2  - Grazing by the planktonic, phytoflagellate, Pyramimonas gelidicola McFadden (Chlorophyta: Prasinophyta), and heterotrophic nanoflagellates (HNAN) in meromictic, saline Ace Lake in the Vestfold Hills, Eastern Antarctica, was investigated in the austral summers of 1997 and 1999. Up to 47% of the P. gelidicola population ingested fluorescently labelled prey (FLP). Ingestion rates varied with depth. In January 1997 and November 1999, maximum P. gelidicola ingestion rates of 6.95 and 0.79 FLP;cell-1;h-1, respectively, were measured at the chemocline (6-8 m) where a deep chlorophyll maximum composed of phototrophic nanoflagellates (PNAN DCM), predominantly P. gelidicola, persisted all year. During the summers of 1997 and 1999, the grazing P. gelidicola community removed between 0.4 and approximately 16% of in situ bacterial biomass, equivalent to between 4 and >100% of in situ bacterial production. Due to their higher abundance, the community clearance rates of HNAN in Ace Lake generally exceeded those of P. gelidicola but HNAN removed approximately only 3 to 4% of bacterial biomass, equivalent to between 28 and 32% of bacterial production. P. gelidicola growth rates were highest at the PNAN DCM concomitant with the highest ingestion rates. It is estimated that during the summer P. gelidicola can derive up to 30% of their daily carbon requirements from bacterivory at the PNAN DCM. This study confirms mixotrophy as an important strategy by which planktonic organisms can survive in extreme, polar, lacustrine ecosystems.
Y1  - 2003
ER  - 
TY  - JOUR
A1  - Bell, Elanor M.
A1  - Laybourn-Parry, J.
T1  - The plankton community of a young, eutrophic, Antarctic saline lake
N2  - A shallow, saline lake (Rookery Lake) close to the sea and surrounded by a penguin rookery was investigated during the austral spring and summer of 1996/1997. The proximity to the sea means that the lake is likely to have been formed recently during isostatic uplift. Inputs of carbon and nutrients from the penguin rookery have rendered Rookery Lake eutrophic compared with other brackish and saline lakes in the Vestfold Hills. Chlorophyll a concentration, bacterioplankton, heterotrophic nanoflagellate and phototrophic nanoflagellate abundances were all significantly higher than in other non-enriched lakes. The high productivity created seasonal anoxia during winter and spring below ice cover. The ciliate community resembled the marine community, and was dissimilar to that seen in older saline lakes within the Vestfold Hills. Thus Rockery Lake provides valuable evidence of the impact of natural eutrophication on an Antarctic lake, as well as of the evolution of the typical microbial community which dominates the older lakes of the Vestfold Hills.
Y1  - 1999
ER  - 
TY  - JOUR
A1  - Bell, Elanor M.
A1  - Laybourn-Parry, J.
T1  - Annual plankton dynamics in an Antarctic saline lake
N2  - 1. The plankton dynamics of Ace Lake, a saline, meromictic basin in the Vestfold Hills, eastern Antarctica was studied between December 1995 and February 1997. 2. The lake supported two distinct plankton communities; an aerobic microbial community in the upper oxygenated mixolimnion and an anaerobic microbial community in the lower anoxic monimolimnion. 3. Phytoplankton development was limited by nitrogen availability. Soluble reactive phosphorus was never limiting. Chlorophyll a concentrations in the mixolimnion ranged between 0.3 and 4.4 mu g L-1 during the study period and a deep chlorophyll maximum persisted throughout the year below the chemo/oxycline. 4. Bacterioplankton abundance showed considerable seasonal variation related to light and substrate availability. Autotrophic bacterial abundance ranged between 0.02 and 8.94 x 10(8) L-1 and heterotrophic bacterial abundance between 1.26 and 72.8 x 10(8) L-1 throughout the water column. 5. The mixolimnion phytoplankton was dominated by phytoflagellates, in particular Pyramimonas gelidicola. P. gelidicola remained active for most of the year by virtue of its mixotrophic behaviour. Photosynthetic dinoflagellates occurred during the austral summer, but the entire population encysted for the winter. 6. Two communities of heterotrophic flagellates were apparent; a community living in the upper monimolimnion and a community living in the aerobic mixolimnion. Both exhibited different seasonal dynamics. 7. The ciliate community was dominated by the autotroph Mesodinium rubrum. The abundance of M. rubrum peaked in summer. A proportion of the population encysted during winter. Only one other ciliate, Euplotes sp., occurred regularly. 8. Two species of Metazoa occurred in the mixolimnion; a calanoid copepod (Paralabidocera antarctica) and a rotifer (Notholca sp.). However, there was no evidence of grazing pressure on the microbial community. In common with most other Antarctic lakes, Ace Lake appears to be driven by 'bottom-up' forces.
Y1  - 1999
ER  -