Seminar WEDNESDAY at UF - hosted by Soil and Water Science Department

Speaker:    Dr. Christian Steinberg, Institute of Biology, Humboldt Universitat zu Berlin
Location: McCarty A G186
Date:  3/26/2008
Time: 3-4pm

Title: Mild stress by dissolved humic substances is essential for freshwater life

Abstract  (also attached)

Despite their ubiquitous abundance in all freshwaters, the role of humic substances HS is

still inadequately understood. These substances have been considered too large to be taken

up by freshwater organisms. On the contrary, we present evidence that dissolved HS are

indeed taken up and interact with freshwater organisms. We show that dissolved HS exert a

mild chemical stress upon aquatic organisms in many ways; they induce molecular chaperones

(stress shock proteins: HSP70 as well as crystalline ones, which are known to play a

key role in lifespan extension), induce and modulate biotransformation enzymes. Furthermore,

they produce an oxidative stress. HS modulate the multixenobiotic resistance activity

and probably other membrane-bound exporters. This property may lead to an increased

bioaccumulation of xenobiotic chemicals. The talk will focus on three topics.

Feminizing potential: HS can modulate the numbers of offspring in a nematode and feminize

fish (Fig. 1) and amphibians. In a recent study, we asked whether this potential applies

to humic substances in general. With quantitative PCR of the genes of the nematode

Caenorhabditis elegans, we showed that all humic substances studied induced the vitellogenin

gene induction, (vitellogenin is the most prominent yolk protein), hence, this finding

indicates a general feminizing property of humic substances even to invertebrates. The estrogenic

pathway is one of the potential modes of action.

Fish life in acidic freshwaters: In very soft, humic freshwaters, such as the Amazonian Rio

Negro, Brazil, HS stimulate the uptake of essential ions, such as Na and Ca, at extremely

low pH (3.5-4.0) and prevent the ionoregulatory disturbance induced by acid waters,

thereby enabling fish to survive in these environments. We re-visit recent papers on fish adaptation

in the Rio Negro and re-interpret the results on the basis of the recent finding of

gene control in the nematode Caenorhabditis elegans. In the nematode, several genes are

up-regulated. This may also account for the maintenance of fish life in the Rio Negro.

Lifespan extension: Exposure to HSs exerts mild chemical stress on the exposed organisms

and deprives them of energy; however, the compost dweller C. elegans actively looks

for such environments. With life-table and DNA gene filter studies we show that HSs may

cause longevity and multiple stress resistance. With C. elegans, a lifespan extension was

induced with certain HS qualities. Intrigued by this result, we studied the potential of lifespan

extension with other animals, for instance the water flea, Daphnia magna. Upon exposure to

HS, the usually parthenogenetic females (reproduction without males) started to produce

males. Subsequently, males and females were separately exposed to HS. Surprisingly, the

lifespans of males was extended, but not that of females, rather their lifespan was reduced.

We do not yet understand the ecological consequences, and possible advantages and disadvantages

of this.

In sum, dissolved HS interact with freshwater organisms in a variety of ways in unenriched

humic lakes. In addition to the well known effects of HS on light regime, for example, and

the direct and indirect supply with carbon (energy), other interactions may be much more

subtle. We are just at the beginning of understanding that these interactions between dissolved

HS and freshwater organisms indicate HS features as an ecological driving force.