Richardot M, Debroas D, Thouvenot A, et al.
Influence of
cladoceran grazing activity on dissolved organic matter, enzymatic hydrolysis
and bacterial growth
J PLANKTON RES 23 (11): 1249-1261 NOV 2001
Abstract:
To assess the influence of grazing by cladocerans on dissolved organic matter
(DOM), glycolytic and proteolytic activities and bacterial growth were measured
by in situ incubation of lake water from the epilimnion of an oligotrophic
reservoir in three different treatments: in absence of zooplankton, and in
presence of zooplankton (natural abundance and concentrated four-fold). These
experiments were conducted at two periods in the succession of plankton
populations (May and June 1998), that differed in the quality of the prey
ingested (Eudorina sp. compared to Cryptomonas sp. and Rhodomonas sp.)
and their grazing intensity (31.8 +/- 2.2 mug C l(-1) day(-1) compared to 10.2
+/- 0.5 mug C l(-1) day(-1)). A systematic increase in bacterial biomass was
measured in the treatments containing the highest zooplankton concentrations.
The DOM concentrations produced in situ showed few significant differences
between the three treatments, but the assimilation of DOM was higher in the
presence of zooplankton than in their absence. These results show that the
influence of cladocerans on the DOM was more of a qualitative than a quantitative
nature. The protein compounds derived from the grazing activities of metazoans
seem to be a major nutrient source for growth for bacteria (r = 0.81, P <
0.05). In this study, the highest hydrolytic activities were recorded in the
presence of high concentrations of metazoan zooplankton. However, the processes
that regulated these activities differed between the two experimental dates
(repression compared to enzyme stimulation). Grazing activities could lead to
an increase in phytoplanktonic excretion during the growth phase, and therefore
the production of low molecular weight compounds that are easily assimilated by
the bacterial plankton.
Coleman AW
Biogeography
and speciation in the Pandorina/Volvulina (Chlorophyta) superclade
J PHYCOL 37 (5): 836-851 OCT 2001
Abstract:
Mating affinity, nuclear rDNA internal transcribed spacer (ITS) sequence, and
geographic distribution of more than 100 isolates of Pandorina,Volvulina, and
Yamagishiella were determined. Comparative analysis of ITS sequences reaffirmed
the unity of Pandorina morum and its separation from all other species of
Volvocaceae except those of Volvulina. This latter genus, represented by four
species, appears to represent only morphological variants of several P. morum
subclades. The P. morum clade (including also additional species such as P.
smithii and P. colemaniae) encompasses an evolutionary span, as determined by
comparison of ITS, greater than either of the multispecies genera Gonium or Eudorina.
There are at least 30 Pandorina/Volvulina syngens, sexually isolated groups, so
far as can be determined, among the current collection of strains. In addition,
as in other volvocacaean genera, two clones are homothallic, capable of forming
zygotes within a genetic clone. The existence of so many syngens suggests that
considerable evolutionary diversification of the genes controlling gamete
compatibility and intercross survival has occurred, unaccompanied by
significant morphological change. Within each syngen, genetic distance
increases with geographic distance between collection sites. At least half of
the isolates studied must have been introduced northward since the Pleistocene.
Although we probably know more about characters in this group of algae than any
other algal group, in part because they are so easy to culture, we are still
largely ignorant of what circumscribes their niche in nature. The study of all
these organisms, distributed throughout the world presumably by the activities
of shorebirds, emphasizes the evolutionary role of mating genes in the
inevitable formation of new genetically isolated subclades in these eukaryotes
and provides initial data on their rate of appearance versus their rate of
distribution over the earth.
Nozaki H, Krienitz L
Morphology
and phylogeny of Eudorina
minodii (Chodat) Nozaki et Krienitz, comb. nov (Volvocales, Chlorophyta) from
Germany
EUR J PHYCOL 36 (1): 23-28 FEB 2001
Abstract:
Morphology, sexual reproduction and phylogeny of a colonial green alga
collected from