Ace Lake: three decades of research on a meromictic, Antarctic lake

Ace Lake (Vestfold Hills, Antarctica) has been investigated since the 1970s. Its close proximity to Davis Station has allowed year-long, as well as summer only, investigations. Ace Lake is a saline meromictic (permanently stratified) lake with strong physical and chemical gradients. The lake is one of the most studied lakes in continental Antarctica. Here we review the current knowledge of the history, the physical and chemical environment, community structure and functional dynamics of the mixolimnion, littoral benthic algal mats, the lower anoxic monimolimnion and the sediment within the monimolimnion. In common with other continental meromictic Antarctic lakes, Ace Lake possesses a truncated food web dominated by prokaryote and eukaryote microorganisms in the upper aerobic mixolimnion, and an anaerobic prokaryote community in the monimolimnion, where methanogenic Archaea, sulphate-reducing and sulphur-oxidizing bacteria occur. These communities are functional in winter at subzero temperatures, when mixotrophy plays an important role in survival in dominant photosynthetic eukaryotic microorganisms in the mixolimnion. The productivity of Ace Lake is comparable to other saline lakes in the Vestfold Hills, but higher than that seen in the more southerly McMurdo Dry Valley lakes. Finally we identify gaps in the current knowledge and avenues that demand further investigation, including comparisons with analogous lakes in the North Polar region

The carbohydrates of Phaeocystis and their degradation in the microbial food web

The ubiquity and high productivity associated with blooms of colonial Phaeocystis makes it an important contributor to the global carbon cycle. During blooms organic matter that is rich in carbohydrates is produced. We distinguish five different pools of carbohydrates produced by Phaeocystis. Like all plants and algal cells, both solitary and colonial cells produce (1) structural carbohydrates, (hetero) polysaccharides that are mainly part of the cell wall, (2) mono- and oligosaccharides, which are present as intermediates in the synthesis and catabolism of cell components, and (3) intracellular storage glucan. Colonial cells of Phaeocystis excrete (4) mucopolysaccharides, heteropolysaccharides that are the main constituent of the mucous colony matrix and (5) dissolved organic matter (DOM) rich in carbohydrates, which is mainly excreted by colonial cells. In this review the characteristics of these pools are discussed and quantitative data are summarized. During the exponential growth phase, the ratio of carbohydrate-carbon (C) to particulate organic carbon (POC) is approximately 0.1. When nutrients are limited, Phaeocystis blooms reach a stationary growth phase, during which excess energy is stored as carbohydrates. This so-called overflow metabolism increases the ratio of carbohydrate-C to POC to 0.4–0.6 during the stationary phase, leading to an increase in the C/N and C/P ratios of Phaeocystis organic matter. Overflow metabolism can be channeled towards both glucan and mucopolysaccharides. Summarizing the available data reveals that during the stationary phase of a bloom glucan contributes 0–51% to POC, whereas mucopolysaccharides contribute 5–60%. At the end of a bloom, lysis of Phaeocystis cells and deterioration of colonies leads to a massive release of DOM rich in glucan and mucopolysaccharides. Laboratory studies have revealed that this organic matter is potentially readily degradable by heterotrophic bacteria. However, observations in the field of accumulation of DOM and foam indicate that microbial degradation is hampered. The high C/N and C/P ratios of Phaeocystis organic matter may lead to nutrient limitation of microbial degradation, thereby prolonging degradation times. Over time polysaccharides tend to self-assemble into hydrogels. This may have a profound effect on carbon cycling, since hydrogels provide a vehicle to move DOM up the size spectrum to sizes subject to sedimentation. In addition, it changes the physical nature and microscale structure of the organic matter encountered by bacteria which may affect the degradation potential of the Phaeocystis organic matter.