Mixotrophy and the Antarctic

A review of: Moorthi, S. et al. (2009) Mixotrophy: a widespread and important ecological strategy for planktonic and sea-ice nanoflagellates in the Ross Sea, Antarctica. Aquatic Microbial Ecology. 54: 259-277

Mixotrophy combines both phototrophic and heterotrophic processes to acquire carbon and energy. Mixotrophic organisms have been increasingly identified and investigated in recent years establishing itself as an alternative to the widely acknowledged phototrophic and heterotrophic divisions of planktonic protists.

Mixotrophic characteristics can be widely observed in algal taxa. Mixotrophic algae are vital primary producers and consumers. They contribute to 79% of total bacterivory and represent half of the total phototrophic nanoflagellates in oceanic and freshwater environments. The balance between phototrophy and heterotrophy mixotrophic organisms can fluctuate significantly. However population growth can be maintained photosynthetically in the absence of particulate food and in low light conditions where food is available. Each nutritional strategy can be influenced by nutrient and prey concentrations, abundances of solely phototrophic or heterotrophic competitors as well as intensity of light.

Further understanding is needed in determining the role of mixotrophic algae in carbon flow of microbial food webs. This has been hindered by methodological problems in the past. In this study, the abundance of mixotrophic nanoflagellates (MNF) was compared to parallel assessments of phototrophic nanoflagellate (PNF) and heterotrophic nanoflagellate (HNF) assemblages. This was conducted to identify the degree of MNF activity in a region of the Southern Ocean.

MNF were enumerated in the sea ice and plankton of the Ross Sea, Antarctica between October and December (austral spring). This was achieved by using fluorescently labelled bacteria (FLB) as food tracers in order to quantify MNF and establish their prevalence in regards to chloroplastidic and bacteriovorous nanoflagellates. Samples were taken from microbial assemblages in the water column (surface water, under the ice and within the ice). Counts were then conducted on PNF, HNF, diatoms, bacteria and FLB. MNF were determined as auto-fluorescent cells containing 1 or more ingested FLB. PNF and HNF were distinguished by the presence or absence of chlorophyll auto-fluorescence.

MNF abundance encompassed 8 – 42% of bacteriovorous nanoflagellates in the water column and 3-25% in the ice cores. They further represented up to 10% of the chloroplastidic nanoflagellates during the bloom of Phaeocystis antarctica (unicellular, eukaryotic algae). In ice core samples, MNF consisted of 5-10% chloroplastidic nanoflagellates.

Highest levels of MNF were observed in surface water samples and in plankton assemblages located beneath the ice. This advocates MNF as proficient bacterial grazers in this locality. Ingestion rates were not calculated in this study. However, the high abundance of mixotrophic bacteriovores observed proposes that mixotrophs significantly contribute to bacterial mortality rates. This is supported by mixotroph grazing rates that are comparative to or surpass that of HNF.

Mixotrophy is advocated as an important alternative dietary strategy in this expanse because of the presence of MNF in every sea and ice sample studied. Previous research that noted high abundances of MNF at low light intensities suggests that mixotrophy could be an alternative strategy during austral winter in polar surface waters as well as under the sea ice year round. This strategy could be an ecological advantage in marine environments less favourable to the growth of protists which are solely photosynthetic.

Mixotrophic organisms inhabiting the sea ice and beneath the sea ice are suggested to primarily utilise heterotrophic processes to sustain themselves. This is advocated to be attributed to the very low levels of light in this environment. However the obvious theory that an increased abundance of mixotrophic organisms should be found at low light intensity sites, was not robustly upheld in the study. This can be identified as larger proportions of mixotrophs were observed in open water. Nevertheless, it is likely that nutrient concentrations and prey abundance will have influenced these observations.

This is the first investigation to identify and describe the abundance and distribution of mixotrophic flagellates in the Southern Ocean. It seems to me that the contribution of mixotrophs in the Polar Regions is an open debate and further research is needed to distinguish their importance in food webs by investigating the factors that sustain them.