Micromonas

Abstract: Small cells dominate photosynthetic biomass and primary production in many marine ecosystems. Traditionally, picoplankton refers to cells < or =2 microm. Here we extend the size range of the organisms considered to 3 microm, a threshold often used operationally in field studies. While the prokaryotic component of picophytoplankton is dominated by two genera, Prochlorococcus and Synechococcus, the eukaryotic fraction is much more diverse. Since the discovery of the ubiquitous Micromonas pusilla in the early 1950s, just over 70 species that can be <3 microm have been described. In fact, most algal classes contain such species. Less than a decade ago, culture-independent approaches (in particular, cloning and sequencing, denaturing gradient gel electrophoresis, FISH) have demonstrated that the diversity of eukaryotic picoplankton is much more extensive than could be assumed from described taxa alone. These approaches revealed the importance of certain classes such as the Prasinophyceae but also unearthed novel divisions such as the recently described picobiliphytes. In the last couple of years, the first genomes of photosynthetic picoplankton have become available, providing key information on their physiological capabilities. In this paper, we discuss the range of methods that can be used to assess small phytoplankton diversity, present the species described to date, review the existing molecular data obtained on field populations, and end up by looking at the promises offered by genomics.

Abstract: Our pigment analyses from a year-long study in the coastal Beaufort Sea in the western Canadian Arctic showed the continuous prevalence of eukaryotic picoplankton in the green algal class Prasinophyceae. Microscopic analyses revealed that the most abundant photosynthetic cell types were Micromonas-like picoprasinophytes that persisted throughout winter darkness and then maintained steady exponential growth from late winter to early summer. A Micromonas (CCMP2099) isolated from an Arctic polynya (North Water Polynya between Ellesmere Island and Greenland), an ice-free section, grew optimally at 6°C–8°C, with light saturation at or below 10 μmol photons·m−2·s−1 at 0°C. The 18S rDNA analyses of this isolate and environmental DNA clone libraries from diverse sites across the Arctic Basin indicate that this single psychrophilic Micromonas ecotype has a pan-Arctic distribution. The 18S rDNA from two other picoprasinophyte genera was also found in our pan-Arctic clone libraries: Bathycoccus and Mantoniella. The Arctic Micromonas differed from genotypes elsewhere in the World Ocean, implying that the Arctic Basin is a marine microbial province containing endemic species, consistent with the biogeography of its macroorganisms. The prevalence of obligate low-temperature, shade-adapted species in the phytoplankton indicates that the lower food web of the Arctic Ocean is vulnerable to ongoing climate change in the region.

Abstract: Small eukaryotic species (<1 mm) are thought to behave as prokaryotes in that, lacking geographical barriers to their dispersal due to their tiny size, they are ubiquitous. Accordingly, the absence of geographical insulation would imply the existence of a relatively small number of microeukaryotic species. To test these ideas, we sequenced and compared several nuclear, mitochondrial, and chloroplast genes from the isolates of a marine picoeukaryotic alga (approximately 2 microm), Micromonas pusilla, collected worldwide. Independent and combined phylogenetic analyses demonstrate that this traditional single morphospecies actually comprises several independent lineages, some of which are shown to be ubiquitous in oceans. However, while some lineages group closely related strains, others form distant clusters, revealing the existence of cryptic species. Moreover, molecular dating using a relaxed clock suggests that their first diversification may have started as early as during the Late Cretaceous (approximately 65 MYA), implying that "M. pusilla" is the oldest group of cryptic species known to date. Our results illustrate that global dispersal of a picoeukaryote is possible in oceans, but this does not imply a reduced species number. On the contrary, we show that the morphospecies concept is untenable because it overlooks a large genetic and species diversity and may lead to incorrect biological assumptions.

Abstract: As part of the SHEBA/JOIS drift experiment, we continually analysed abundance and biomass of autotrophic and heterotrophic microbes in the upper 120 m of the water column of the ice-covered Central Arctic Ocean from November 1997 through August 1998. Microbial biomass was concentrated in the upper 60 m of the water column. There were low but persistent stocks of heterotrophic and autotrophic microbes during the winter months. Phytoplankton biomass began increasing when winter snow melted from the ice-pack in early June, after which there was a progressive decline of nitrate and silicate in the euphotic zone. We observed three distinct blooms over the summer. The initial bloom consisted of diatoms and phytoflagellates, mainly 2 μm-sized Micromonas sp.; the two subsequent blooms were dominated by the flagellated (non-colonial) Phaeocystis sp. The carbon:chlorophyll ratio of the phytoplankton was 31±11. Stocks of bacteria and heterotrophic protists approximately doubled during the growing season, increasing in tandem with increase in phytoplankton biomass. Increase in cell abundances of bacteria and of the phytoflagellate Micromonas over 40–50 d periods during the initial bloom period yielded estimates of realised growth rate of 0.025 d −1 for bacteria and of 0.11 d −1 for Micromonas . Heterotrophic protists included flagellates, ciliates, and dinoflagellates, with biomass divided nearly evenly between nanoplankton (Hnano, 0–20 μm) and microplankton (Hmicro, 20–200 μm) size classes.