Frede Thingstad

VIRUSES AND THE MICROBIAL LOOP

The abundance of viral-like particles in marine ecosystems ranges from <104 ml−1 to >108 ml−1. Their distribution in time and space parallels that of other biological parameters such as bacterial abundance and chlorophyll a. There is a lack of consensus between methods used to assess viral activity, i.e., rate of change in viral abundance (increase or decrease). The highest rates, 10–100 days−1, are observed in experiments with short sampling intervals (0.2–2 h), while lower rates, on the order of 1 day−1, are observed in experiments with longer sampling intervals (days). Few studies have been carried out, but viruses appear, at least in some cases, to have a significant impact on carbon and nutrient flow in microbial food webs. Viruses have also been demonstrated to exert a species specific control of both bacteria and phytoplankton populations in natural waters.

Microbial degradation of Phaeocystis material in the water column

Abstract Observational evidence shows that the large amounts of mucilaginous substances produced by blooms of Phaeocystis colonies largely resist rapid microbial degradation in surface waters of most Phaeocystis -dominated ecosystems. In this paper the biodegradability of Phaeocystis colony-derived material is analysed with respect to current knowledge and novel data on the chemical nature of Phaeocystis material in relationship with specific bacterial enzymatic activities. Particular emphasis is given to the chemical nature of Phaeocystis colony matrix which constitutes more than 80% of total colony biomass at maximum development. This analysis gives evidence of the potential biodegradability of this mucilaginous material made of nutrient-deprived polysaccharides. Other factors controlling microbial degradation as the production of antibacterial substances by Phaeocystis colonies, cold temperature and lack of inorganic nitrogen and phosphate are further considered. It is concluded that nutrient limitation currently observed at the senescent stage of Phaeocystis blooms might well explain the low biodegradability of Phaeocystis material. However the lack of bacteria attached to colonies during the exponential phase of Phaeocystis bloom development are not clearly understood and needs further investigations.

The Role of Mixotrophy in Pelagic Environments

This paper reviews the occurrence and ecological importance of mixotrophic flagellates and ciliates in pelagic environments, particularly in marine ecosystems. Mixotrophy is here defined as the combination of photoautotrophic and heterotrophic nutrition in a single individual, often used in the restricted sense of combining photosynthesis and phagotrophy. Mixotrophic protists represent an alternative strategy that allows a shortcut between the traditional food web and the microbial loop. A large number of reports have been published on the ecological importance of mixotrophic flagellates in freshwater, yet only a few studies have been carried out in seawater. In contrast, most of the knowledge of mixotrophic ciliates comes from marine environments. Results from field studies have demonstrated that both mixotrophic flagellates and ciliates are commonly found in many marine environments, and mixotrophic flagellates can dominate the biomass of photoautotrophs and be responsible for the entire grazing of bacteria or protists. Results from laboratory experiments on factors controlling the degree of photoautotrophy/phagotrophy in flagellates are presented. Finally, we present a hypothesis for a growth strategy of bacterivorous mixotrophic flagellates.

The elemental composition of bacteria: A signature of growth conditions?

Abstract X-ray microanalysis combined with Transmission Electron Microscopy (TEM) has been employed to measure cell quotas of P:C, N:C and O:C of individual cells from cultures and natural microbial communities. The relevance of such data for evaluation of nutrient availability and growth is discussed according to the Droop model. Using minimum subsistent quotas of 0.031 (P:C) and 0.17 (N:C) and μmax of 1 h−1 we have been able to identify some of the criteria for nutrient limited growth, but energy/carbon limitation will interfere with interpretation of these data. An additional signature for growth limitation may be the O:C ratio for which preliminary data indicate values of >0.40 for growing cells and

Photo-induced toxicity of north sea crude oils toward bacterial activity

Abstract Toxicity of photolysis products from Ekofisk and Statfjord crude oils toward natural assemblages of marine bacteria was investigated. Incorporation of 14C-glucose, 14C-amino acids or 3H-thymidine were used as indicators of microbial activity. While none of these measurements revealed any significant effects of non-illuminated oil, a toxic effect was found after a few hours illumination of an oil slick.

Mg2+ as an indicator of nutritional status in marine bacteria.

Cells maintain an osmotic pressure essential for growth and division, using organic compatible solutes and inorganic ions. Mg2+, which is the most abundant divalent cation in living cells, has not been considered an osmotically important solute. Here we show that under carbon limitation or dormancy native marine bacterial communities have a high cellular concentration of Mg2+ (370–940 mM) and a low cellular concentration of Na+ (50–170 mM). With input of organic carbon, the average cellular concentration of Mg2+ decreased 6–12-fold, whereas that of Na+ increased ca 3–4-fold. The concentration of chlorine, which was in the range of 330–1200 mM, and was the only inorganic counterion of quantitative significance, balanced and followed changes in the concentration of Mg2++Na+. In an osmotically stable environment, like seawater, any major shift in bacterial osmolyte composition should be related to shifts in growth conditions, and replacing organic compatible solutes with inorganic solutes is presumably a favorable strategy when growing in carbon-limited condition. A high concentration of Mg2+ in cells may also serve to protect and stabilize macromolecules during periods of non-growth and dormancy. Our results suggest that Mg2+ has a major role as osmolyte in marine bacteria, and that the [Mg2+]/[Na+] ratio is related to its physiological condition and nutritional status. Bacterial degradation is a main sink for dissolved organic carbon in the ocean, and understanding the mechanisms limiting bacterial activity is therefore essential for understanding the oceanic C-cycle. The [Mg2+]/[Na+]-ratio in cells may provide a physiological proxy for the transitions between C-limited and mineral nutrient-limited bacterial growth in the oceans surface layer.