Results
Resilience of the oxic
state
The trait diversity effect on the resilience of the oxic state depended
on which functional group varied in traits (Fig. 3 a-f, red lines).
Resilience increased with increasing trait variation in the group that
dominated the oxic state (cyanobacteria; Fig. 3a, b). In contrast,
increasing diversity in the two suppressed groups either reduced
resilience of the oxic state (sulfate-reducing bacteria; Fig. 3c, d) or
had no effect on resilience (phototrophic sulfur bacteria; Fig. 3e, f).
Simultaneous variation in more than one functional group reduced or did
not change the diversity effect of individual functional groups (Fig. 3
g-o; red lines). Simultaneous trait variation in cyanobacteria and
sulfate-reducing bacteria (Fig. 3g) led to smaller diversity effects
than variation in only one of these two groups; the negative diversity
effect of sulfate-reducing bacteria outweighed the positive diversity
effect of cyanobacteria when trait variation was low, but the effect of
cyanobacteria diversity prevailed at high trait variation (Fig. 3b, d,
h). Diversity in the phototrophic sulfur bacteria did not change the
diversity effects of other functional groups (Fig. 3 i-o).
Resilience of the anoxic
state
The diversity effect on the resilience of the anoxic state also differed
among groups (Fig. 3 a-f, blue lines). The two groups that dominated the
anoxic state had contrasting diversity effects on its resilience:
increasing diversity in the sulfate-reducing bacteria increased the
resilience of the anoxic state (Fig. 3 c, d), whereas increasing
diversity in the phototrophic sulfur bacteria decreased resilience (Fig.
3e, f). Diversity in the suppressed group (cyanobacteria) had no effect
on resilience when diversity was low but slightly reduced resilience
when diversity was high (Fig. 3 a, b).
When more than one functional group varied in traits, their effects were
either additive (cyanobacteria and sulfate-reducing bacteria) or one
group erased the diversity effect of another group (Fig. 3 g-o,
Supplementary Report Section 12). Specifically, variation in
sulfate-reducing bacteria erased the diversity effect of phototrophic
sulfur bacteria (Fig. 3 l, m), and variation in phototrophic sulfur
bacteria erased the (small) diversity effect of cyanobacteria (Fig. 3 i,
k).
Dynamics of strains and
substrates
Trait variation led to compositional turnover along the oxygen
diffusivity gradient when a functional group was on the collapse
trajectory (see Box 1 for explanation of terms). Less tolerant strains
were replaced by more tolerant strains as the concentration of the
inhibiting substrate increased (Fig. 4). On the trajectory of decreasing
oxygen diffusivity, the ecosystem was initially dominated by the
cyanobacteria strain with lowest sulfide tolerance and highest maximum
growth rate (Fig. 4a). As oxygen diffusivity declined, more tolerant
strains of cyanobacteria replaced the fastest growing strain until the
strain with highest sulfide tolerance dominated. Subsequently, the
cyanobacteria collapsed, probably due to their reduced capacity to
suppress the sulfur bacteria, and simultaneously the ecosystem shifted
from oxic to anoxic (Fig. 4 d, e). Prior to the tipping point, the most
tolerant strains of the two sulfur bacteria groups slightly increased in
abundance, in particular in the sulfate-reducing bacteria (Fig. 4b).
However, once the system shifted to anoxic, the sulfur bacteria strains
with lowest tolerance and highest maximum growth rate dominated (Fig. 4
b, c).
Strain dynamics were similar on the trajectory of increasing oxygen
diffusivity (Fig. 4). Replacement of less tolerant by more tolerant
strains in both groups of sulfur bacteria was followed by the collapse
of the phototrophic sulfur bacteria. Then sulfate-reducing bacteria
collapsed, simultaneously with the shift from anoxic to oxic and the
rise of the least tolerant cyanobacteria strain. In contrast to the
trajectory of decreasing oxygen diffusivity, the switch from least to
most tolerant strains occurred over a comparatively broad range of
oxygen diffusivity.
The strain dynamics deviated from this pattern when there was medium to
high variation in only the phototrophic sulfur bacteria. In this case,
all three functional groups coexisted at low levels of oxygen
diffusivity likely because high maximum growth rates in the phototrophic
sulfur bacteria led to lower sulfide concentrations (Supplementary
Report Section 10.2). The same pattern occurred for simultaneous
variation in phototrophic sulfur bacteria and cyanobacteria, albeit only
on the trajectory of increasing oxygen diffusivity (Supplementary Report
Section 10.2).
Mechanisms of the functional diversity
effects
At low levels of trait variation, the diversity effects were driven
entirely by the most tolerant strains. At higher levels of trait
variation, however, strains with low tolerance (and high maximum growth
rate) also contributed to the diversity effects in five of the seven
combinations (Supplementary Report Section 11). For example, in the
sulfate-reducing bacteria, absence of strains with high maximum growth
rates led to reduced production of sulfide and therefore to coexistence
with cyanobacteria at low oxygen diffusivity. That is, high maximum
growth rates often had ecosystem-level consequences.