COMMENT
Limnol. Uceunn~e. 42( I f. 1997, 198-205
0 1997, by tk American Swtety of Limnofogy
and ~~~nogmphy,
Inc.
Phagotrophy in phototrophs
tophyta, Dinophyta, Euglenophyta, and Chlorarachniophyta
(Table 1). As in the first round, the plastids of these organisms are incapable of independent existence because many
of the genes needed for their functioning have been transferred (generally) to the nucleus of the “new” host (see Allen and Raven 1996).
A third round of endosymbiosis involves dinoflagellates
(plastids almost always from the second round of endosymbiosis) with various marine protists and metazoa (Table 1).
Here there is no loss, or transfer, of genes from the endosymbiont, at least to such an extent as to preclude the freeliving occurrence of the endosymbiont. Indeed, the endosymbionts are generally freshly obtained from the environment by phagotrophy in each generation of the animal host,
although in a few cases there is transovarian transmission of
endosymbionts (Smith and Douglas 1987). Similar symbioses occur (mainly in freshwaters) involving once more protists and invertebrates as the host but with green algae (Chlorophyta) as the endosymbiont. Here there always seems to
be the need for reinfection in each host generation (Smith
and Douglas 1987). These green algal symbioses are effectively in the second round of endosymbioses because the
green algal endosymbionts are the result of primary endosymbiosis (see Table 1). Smith and Douglas (1987) suggested that such endosymbioses with (multicellular) invertebrate hosts are unlikely to lead to the animals acquiring
“real” transmitted plastids incapable of independent growth
if the algal endosymbionts cannot reach germ cells.
A further means by which eucaryotes acquire the capacity
for photosynthesis arises from the retention of photosynthetically functional plastids from ingested algal food material (see Clark 1992). This process of kleptoplastidy involves the acquisition of plastids containing only - 10% of
the genes needed for their synthesis and repair by a host
whose nucleus lacks the complementary 90% of the genes
needed for plastid continuity. Accordingly, such associations
are best not regarded as symbioses (Law and Lewis 1983).
Phagotrophy underlies all of these endosymbioses, ranging from a continuing requirement for reinfection in each
generation when there is no transovarian transmission to
“once for all” phagotrophy, which gave rise to plastids in
the green and red algae at least 7 X lo* and perhaps 2 X lo9
yr ago (Butterfield et al. 1990; Han and Runnegar 1992;
Shixing and Huineng 1995). Interesting recent work has
shown that some of the earliest possibly primitively mitochondria-less (but see Palmer 1997) eucaryotes have a number of photosynthetic genes, at least some of which are expressed in terms of the occurrence of membraneous structures containing photosynthetic polypeptides and chlorophyll
(Hackstein and Mackenstedt 1995). Furthermore, some
genes of mitochondria-contained eucaryotes were derived
Phagotrophy was (and is!) a crucial element in the evolution of photosynthetic eucaryotes. The photosynthetic
function in all eucaryotes can be traced to one or more endosymbiotic events involving the ingestion and retention of
(unicellular) photosynthetic organisms. In some cases (chloroplasts of green and red algae) this loss of autonomy of the
ingested procaryote occurs by transfer of many genes essential for functioning as a plastid to the host nucleus and the
loss of many others. At the other end of the time scale is
the temporary (for a month or two) retention of functioning
chloroplasts from food items by ascoglossan (sacoglossan)
gastropods. The universal involvement of phagotrophy in the
evolution of photosynthetic eucaryotes means that phagotrophy is considered a primitive character and its absence a
derived character in these organisms. Starting from an elaboration of these basic phenomena, I consider two questions.
The first relates to the ecosystem role and evolutionary significance of phagotrophy by 0, evolvers: Is there a common
biogeochemical thread that links all (or most) phagotrophic
O? evolvers? The second question addresses the evolutionary
role of phagotrophy in 0, evolvers: What are the evolutionary costs that may offset the benefits of phagotrophy in O,evolving phototrophs (or phototrophy in phagotrophs)?
This article does not consider the important topic of saprotrophy (i.e. the uptake of low-M, organic C compounds)
in phagotrophic (or nonphagotrophic)
phototrophs (see
Droop 1974; Sanders 1991; Raven 1993b; Chisholm et al.
1996). Saprotrophy as well as phagotrophy by phototrophs
should strictly be embraced by the commonly used term
although this term is frequently reserved for
“mixotrophy,”
the combination of phototrophy and phagotrophy considered
here.
Evolutionary
role of phagotrophy
in phototrophs
The involvement of phagotrophy in producing photosynthetic eucaryotes is via endosymbiosis, whereby cyanobacteria originally ingested as food were retained as photosynthetic endosymbionts. Subsequently, genes needed for independent existence of the cyanobacterium were lost, whereas most of those needed for photosynthetic functioning of
the cyanobacterium were transferred to the host nucleus.
These processes account for the plastids of Chlorophyta (and
higher plants) and Rhodophyta (Bhattacharya and Medlin
1995; van den Hoek et al. 1995) (Table 1). Other eucaryotes
obtained their plastids by a second round of endosymbiosis
involving the ingestion of plastid-containing
unicellular
members of the Chlorophyta or Rhodophyta (Bhattacharya
and Medlin 1995; van den Hoek et al. 1995), giving rise to
photosynthetic capacity in the Heterokonta (containing diatoms, chrysophytes, and brown algae), Cryptophyta, Hap198
Comment
199
Table 1. The number of sequential endosymbioses involved in the evolution of photosynthetic eucaryotes. For clarity the occurrence of,
for example green algal endosymbionts in dinoflagellates, have been ignored in constructing this diagram. References: Bhattacharya and
Medlin 1995; Smith and Douglas 1987; van den Hoek et al. 1995; Palmer and Delwiche 1996.
No endosymbiosis
One endosymbiosis
Cyanobacteria
Plastids of Chlorophyta
(and higher plants)
Two endosymbioses
Three endosymbioses
1
Chlorophyta in protists and
invertebrates
Plastids of Euglenophyta
Plastids of Chlororachniophyta
Plastids of Rhodophyta
Plastids
Plastids
Plastids
Plastids
Endosymbiont incapable of
independent existence
from a &purple bacterium that formed a transitory endosymbiosis that has not left an extant organelle (Henze et al.
1995). A major conclusion at this point is that the earliest
phototrophic eucaryotes in any of the major clades of eucaryotic phototrophs were phagotrophic. This means that
phagotrophy in photosynthetic eucaryotes is a primitive
character and its absence is a derived character. The selective
significance of retaining phototrophic food items as photosynthetic endobionts by originally nonphotosynthetic phagotrophs presumably relates to a relative shortage of energy
and C as compared to N, P, Fe, and other sources, which can
be obtained by digestion of the ingested food items.
Ecosystem role and evolutionary
phagotrophy in phototrophs
significance of
Nearly all O,-evolving photosynthesis worldwide is carried out by organisms that are either procaryotic [cyanobacteria including the chloroxybacteria (=Prochlorophyta)]
or
are primary or secondary endosymbioses involving host
cells containing plastids, with no expressed capacity for phagotrophy. Among the eucaryotes, the nonphagotrophic (in
the sense of lacking routine intracellular digestion of prey
items) major taxa of 0, evolvers include diatoms, brown
algae, red algae, green algae, and higher plants, i.e. ~98%
of the -275,000 described species of 0, evolvers (John
1994). A similar fraction of global net primary productivity
is carried out by these nonphagotrophic 0, evolvers. Where
intracellular phagotrophy occurs in these organisms (e.g. in
those flowering plants with intracellular N,-fixing symbioses
involving rhizobia or Frankia), the intracellular procaryotes
are not directly major food sources or O,-evolving phototrophs (see Raven 1993a). Assuming that half of the chrysophytes, cryptophytes, haptophytes, and dinophytes are
phagotrophic phototrophs, and that there are 2,000 species
of submerged aquatic bryophytes and vascular plants of
which only Utricularia is a phagotrophic (extracellular digestion) phototroph, only 5% of known species of aquatic
0, evolvers are phagotrophic phototrophs with genetically
integrated plastids (John 1994; Sculthorpe 1967).
of
of
of
of
Heterokonta
Haptophyta
Cryptophyta
Dinophyta
Dinophyta in protists and
invertebrates
L
Endosymbiont capable of
independent existence
Phagotrophy is thus maintained by a minority of those
eucaryotic 0, evolvers with genetically integrated photosynthetic apparatus, i.e. certain members of the Chloromonadophyceae, Chrysophyceae, Cryptophyta, Dinophyta, and
Haptophyta (see Nygaard and Tobieson 1993) (Table 2).
Phagotrophy also occurs in those eucaryotes that acquire intact algal cells or plastids at least once in each generation
and use them in inorganic C fixation, e.g. a range of protists,
poriferans, platyhelminths, coelenterates, and molluscs (see
Smith and Douglas 1987) (Table 2). In all of these cases,
there is intracellular (i.e. in a food vacuole) digestion associated with the acquisition of organic C, N, P, and Fe from
particulate food. This intracellular digestion contrasts with
the other example of phagotrophy in aquatic eucaryotic photosynthetic 0, evolvers, i.e. the carnivorous flowering plant
Utricularia (Table 2). Here the digestion is within the organism (inside multicellular bladders) but is extracellular,
i.e. as in the guts of arthropods and vertebrates.
These phagotrophic phototrophs occur in a variety of
freshwater and seawater habitats (Table 2). Many of the ciliate, flagellate, foraminifera, radiolaria, and algal phagotrophic phototrophs are planktonic, but some are haptophytes
(sensu Luther 1949). The invertebrates harboring plastids
and algal cells are generally haptophytes, but some are pleustophytes (both terms sensu Luther 1949). Aquatic species of
Utricularia are also pleustophytes, but wetland and terrestrial members are rhizophytes (sensu Luther 1949), rooted
in sediment or soil via their bladder-bearing underground
stems. With regard to the biogeochemical role of the phagotrophic phototrophs, information available on the role of
phagotrophy in C and energy nutrition by phototrophs is
limited to a few species. Nevertheless, these studies indicate
a great variation in the potential for C gain by photosynthetic
fixation relative to phagotrophy, suggesting that different
phagotrophic phototrophs have different biogeochemical
roles. This conclusion arises from data such as those provided by Caron et al. (1990, 1993) on two freshwater members of the Chrysophyceae, Poterioochromonas malhamensis and Dinobryon cylindricum.
Poterioochromonas malhamensis has a low photosynthet-
200
Comment
Table 2. Phylogenetic and ecological diversity of phagotrophy
in aquatic O,-evolving phototrophs. Nature of photosynthetic components obtained by primary, secondary, or tertiary endosymbiosis
and other than plastids that are genetically integrated into the eucaryotic cell: kp-plastid
from a food alga (kleptoplastidy); calgal cell; w-plastid derived (in recent evolutionary history) from
a food item. Salinity of habitat: fw-freshwater;
SW-seawater. Life
form of organism (sensu Luther 1949): pla-planktophyte
(small,
in water body); ple-pleustophyte
(large, in water body or resting
on substrate); ha-haptophyte
(attached to substrate particle that is
large relative to size of organism); rhi-rhizophyte
(with attachment
organs in substratum of small particle size relative to the organism).
Information presented in this table is from Albert et al. 1992; Bennett et al. 1990; Car-on et al. 1990, 1993; Clark 1992; Jacobsen and
Anderson 1996; Jones et al. 1994; Juniper et al. 1989; Law and
Lewis 1983; Marin and Ros 1992; Nygaard and Tobieson 1993;
Raven 1981, 1993a; Smith and Douglas 1987; Tranvik et al. 1989;
and Wilcox and Wedemayer 1985, 1991.
Taxon
Flagellate and ciliate
protists lacking genetically
integrated photosynthetic
apparatus
Rhodophyta
Chlorophyta
Euglenophyta
Chlorachniophyta
Heterokonta
Chrysophyceae
Phaeophyceae
Bacillariophyceae
Cryptophyta
Haptophyta
Dinophyta
Foraminifera
Radiolaria
Porifera
Cnidaria
Turbellaria
Molluscs
Magnoliophyta
Occurrence
of phagotrophy in
photosynthetic
members
Ecological
status of
phagotrophic
photosynthetic
members
+ (kp; c>
-
fw; SW; pla; ha
+ (WI
fw. ple
+ (WI
-
fw; pla
+ (WI
+ (WI
fw; SW; pla
fw; SW; pla
fw; SW; pla
SW; ple
SW; pla
fw; SW; ha
fw, SW,ha, ple
SW; ple
fw; SW; ha; ple
fw; ple (rhi)
+ (w; kp)
+
+
+
+
+
+
+
cc>
cc>
Cc)
cc>
cc>
(kp; c>
(WI
ic capacity and a low specific growth rate when relying on
light and inorganic elemental supplies (C, N, P) (Myers and
Graham 1956; Lewitus and Caron 1991). Chemoorganotrophic nutrition by saprotrophic use of glucose or by bacterivory gives much higher growth rates that are independent
of light, NH4+, and H,PO,-. Of the ingested bacterial C, 45%
is incorporated; because the C : N ratio of Poterioochromonas is higher than that of its bacterial food, the percentage
retention of ingested N is less than that of C. These data
suggest that phagotrophy is a major pathway of C nutrition
in P. malhamensis, at least when cultured in media rich in
organic C (see Jones et al. 1996 and discussion below).
Dinobryon cylindricum, by contrast, is an obligate phototroph that cannot survive in continuous darkness even
when supplemented with high concentrations of bacteria.
However, growth in the light in the absence of bacteria was
slow, and bacterivory was needed for rapid, sustained
growth in the light. Despite this, photosynthesis supplies
175% of the C in Dinobryon cells growing at light saturation with bacteria, and the major role of bacterivory seems
to be in supplying major nutrients (N, P) or growth factors,
or to allow survival in low light or short photoperiods (Caron
et al. 1993). A similar small role for bacterivory in growth
at high light in Dinobryon divergens was indicated by Veen
(1991). Growth at light saturation was not stimulated by added bacteria, and, at high light, <lo% of the C assimilated
was derived from bacteria, with the rest coming from photosynthesis. Veen (199 1) showed that bacterivory played major roles in P and Fe nutrition in D. divergens, and the work
of Auclair (1995) suggested that cyanobacterivory is a major
Fe source for phagotrophic chrysophytes in Canadian lakes.
In summary, the major role of phagotrophy (or, if adequate dissolved usable organic C is present, saprotrophy) in
the C and energy nutrition of P. malhamensis contrasts with
the minor role of phagotrophy in C and energy nutrition (at
least in high light) of Dinobryon spp. In the obligately phototrophic Dinobryon spp., bacterivory seems to be involved
mainly in N, P and Fe nutrition at saturating light and in C
nutrition at low light.
The detailed elemental analyses available to interpret the
role(s) of phagotrophy in freshwater chrysophytes are not
generally available for other phagotrophic phototrophs. In
general, the C and energy budgets of these organisms (or
symbiotic associations of organisms) have been investigated
in more detail than have the N, P, or Fe budgets. It is clear
that phagotrophic photosynthetic organisms retain the capacity to take up N, P and Fe by saprotrophic (molecule by
molecule) processes (see Muscatine et al. 1989; Sanders
199 1). More significantly, symbioses involving photosynthetic microorganisms within phagotrophic animal hosts also
have high-affinity influx systems for N (NH,‘; sometimes
NO,-)- and P (H,P0,-/HP0,2-)-containing
solutes (see Raven 1993a). These animal plasmalemma transport systems
are not expressed in, and may not be genetically encoded
by, the closest asymbiotic relatives of these symbiotic animals. By whatever means the animal plasmalemma acquired
these transporters (see below), it seems that there is a selective advantage in having phototroph-like rather than phagotroph-like transport of N and P at the plasmalemma [i.e. the
capacity for net in&x from low external concentrations rather than the invariable outward direction of any net flux in
nonphotosynthetic (or chemosynthetic?) phagotrophs].
The role of phagotrophy by phototrophs in biogeochemical cycles clearly involves consideration of several factors.
If we consider only C, N, P and Fe, the requirement for
these elements depends on the composition of the phagotrophic phototroph with (for C) additional requirements for
organic C to cover the energy costs of growth and maintenance processes. This means that even with some excretion
of N, P, and Fe, the ratio of gross influx of C : N : P : Fe is
biased toward C relative to the C : N : P : Fe of the organism.
Such analyses of the need for gross C : N : P: Fe influx
ratios are relatively robust. The availability of inorganic C,
N, P and Fe is difficult to evaluate a priori. Even the use of
the total external concentrations to predict the possible ex-
Comment
tent of growth is vitiated if any of the molecular species of
the inorganic C, N, P, or Fe are unavailable for growth, even
over very long timespans. This lack of availability applies
to the NO,- and NO,- components of inorganic combined
N for phototrophs that cannot use NO,- or NO,-, as well as
to certain (insoluble) Fe fractions. However, absolute unavailability is not (granted sufficient time and tolerance of
high pH) the case for the HCO,- and C0,2p components of
inorganic C in organisms that cannot catalyze the external
conversion of HC0,-/C0,2to CO,, the species used by
Rubisco, or bring about the active influx of inorganic C species.
When the rate rather than the extent of growth with
steady-state concentrations of inorganic resources is to be
analyzed, the dependence of the rates of uptake and assimilation of the various resources relative to the need for these
resources and to their external concentrations must be considered. For nutrient solutes at low concentrations, we need
to consider the V, : K,,, ratio relative to the gross need for the
nutrient, where V, is in mol g-l dw s-l, K, is in mol m-“,
and the gross need is the specific nutrient acquisition rate
based on specific growth rate and cell quota in mol g-l dw
s-l. The flexibility in cell quota can confuse this analysis,
but the principle can certainly be applied.
Generally, such kinetic analyses suggest that inorganic N,
P or Fe rather than C is likely to limit the growth rate (as
well as the extent of growth) of phototrophs in most mesoto oligotrophic waters (Raven 1994; Raven and Johnston
1991). Any inputs of soluble organic C would exacerbate
this difference. If we assume saturating light, phagotrophy
would be more relevant to the supply of N, P, and Fe rather
than of C and energy. This suggestion is consistent with the
fact that the C : N, C : P and C : Fe ratios of such prey items
as chemoorganotrophic bacteria are lower than those of photolithotrophs or, more relevantly, phagotrophic phototrophs
(e.g. chrysophytes and their prey; Caron et al. 1990, 1993).
Because Utricularia is a phylogenetically and structurally
very different organism, it has higher C : N, C : P, and C : Fe
values than does its prey items (crustaceans, insect larvae).
Phagotrophy in Utricularia may be of significance as a C
and energy source, especially since this plant seems to be
limited to free CO, as its inorganic C source and cannot use
exogenous HCO,- (Raven 198 1).
When light is not saturating for photosynthesis, phagotrophy could be (and in some cases has been shown to be)
important in energy (reduced C) supply to phagotrophic photolithotrophs (Car-on et al. 1990, 1993). Here the previously
mentioned C : N, C : P, and C : Fe ratio differences between
those of the gross assimilation needs of phagotrophic phototrophs and those of prey items would give the phagotrophic photolithotroph an excess of N, P and Fe under these
growth conditions. However, this excess would not be as
great as that for a nonphotosynthetic phagotroph that had a
similar elemental composition and was using the same particulate food source.
These elemental composition and energetic considerations
may be overridden by requirements by the phagotrophic
phototroph for specific organic products of metabolism by
the prey. For example, the freshwater chrysophyte Uroglena
201
americana needs phospholipids from prey items (Kimura
and Ishida 1989).
Considerations of the ratios of gross resources required
for growth of phagotrophic phototrophs, as well as the ratios
of rates at which they are supplied from the environment
(other than by prey items), suggest that phagotrophic phototrophs should be favored in more oligotrophic habitats
with regard to inorganic N, P, and Fe. This advantage for
phagotrophic phototrophs is particularly the case if they have
relatively high capacities for photosynthesis, i.e. comparable
to those of related nonphagotrophs. For species such as P.
malhamensis, which has very low capacities for photosynthesis relative to those of phagotrophic gain of C and energy,
different needs are seen-phagotrophy
here is essential for
prolonged growth, not just a means of increasing growth rate
in habitats deficient in energy, N, P, or Fe. However, Jones
et al. (1996) pointed out that most of the recent work on
Poterioochromonas has used a strain that has been cultured
since 1948 in the organic C-rich Ochromonas medium, and
that a more photoautotrophic mode of nutrition has been
selected for in another strain grown since 195 1 in diatom
medium with no added organic C. It is not entirely clear if
the relative abundance of phagotrophic phototrophs in habitats of different trophic status supports their predicted (at
least for those with a high photosynthetic capacity) greater
significance in oligotrophic habitats (see Andersson et al.
1989; Bennett et al. 1990; Nygaard and Tobieson 1993). Further investigations are needed for this important point.
Evolutionary costs that may offset the benefits of
phagotrophy in phototrophs
Having seen potential advantages (benefits) of phagotrophic photolithotrophy in at least certain environments, it
must be remembered that there are costs of phagotrophic
phototrophy. The most obvious one is that of synthesizing
both the photosynthetic and the phagotrophic apparatus.
These costs can be expressed in the same units as the benefits. Just as the photosynthetic (and other inorganic nutrient
assimilation) or phagotrophic apparati have benefits in terms
of joules of energy or moles of C, N, P, or Fe acquired per
unit time per organism in a given environment, the cost of
synthesis (and maintenance) of these apparati can be expressed as joules of energy or moles of C, N, P or Fe acquired per unit time per organism in the same environment
permitting a certain growth rate. Clearly the sum of costs
for a cell with both photosynthetic (plus other inorganic nutrient acquisition) and phagotrophic machinery is greater
than for a cell with either apparatus (of equal size) alone.
The energy, C, N, P and Fe of the photosynthetic apparatus and of the machinery for uptake and assimilation of
inorganic nutrients other than C can be readily costed (Raven
1984; cf. Rothhaupt 1996a). The cost of synthesis of the
phagotrophic apparatus (elements of which are present in
essentially all eucaryotic cells) is less readily estimated.
However, Raven (1995) has estimated that while the photosynthetic apparatus (plus the machinery for uptake and assimilation of other inorganic nutrients) can account for up
to 50% of the energy, C, N, (P), and Fe costs of cell syn-
202
Comment
thesis, and comprises a corresponding fraction of the biomass, the corresponding figure for the phagotrophic apparatus is < 10%. This means that per cell with a given quantity
of structural and catalytic machinery downstream of the
phagotrophic or photosynthetic/inorganic
nutrient acquisition machinery, the photosynthetic organism has an additional -50% cost of synthesis and the phagotrophic organism a - 10% extra cost. Assuming optimal allocation of resources, this implies a corresponding reduction in resourcesaturated or resource-limited specific growth rates relative to
a hypothetical organism needing neither the phagotrophic or
phototrophic apparati (Raven 1984, 1995). Such differences
in specific growth rate for organisms of similar sizes as a
function of phototrophic or chemoorganotrophic nutrition
has been noted for microorganisms (Banse 1982; Raven
1986, 1995), with no significant difference found between
phagotrophic and saprotrophic chemoorganotrophs. When
phagotrophic phototrophs were considered, those (e.g. Dinobryon) with photosynthetic capacities similar to those of
nonphagotrophic photolithotrophic chrysophytes of a similar
size had photolithotroph-like specific growth rates. However,
Poterioochromonas, which has minimal <photosynthetic apparatus as a fraction of biomass, had specific growth rates
closer to obligately chemoorganotrophic
(phagotrophic)
chrysophytes of a similar size (Raven 1995). The conclusion
is that production of an organism with a normal photolithatrophic fraction of the cell occupied by photosynthetic apparatus imposes a restriction on maximum specific growth
rate relative to a comparable chemoorganotroph. However,
the phagotrophic apparatus in a normal photolithotroph does
not produce a measurable restriction in maximum growth
rate, nor does a minimal photosynthetic apparatus in a
phagotroph such as Poterioochromonas.
The emphasis on measurable in the foregoing discussion
is important. The precision of the measurements of specific
growth rates in these comparative measurements might not
be adequate to detect a 10% difference in specific growth
rate, although any such differences may be significant in
natural selection (cf. Rothhaupt 1996a, b).
The implicit assumption in the preceding discussion is an
invariable allocation of resources to the phototrophic or the
phagotrophic apparatus. At least as far as the photosynthetic
machinery is concerned, considerable variation is possible,
e.g. in light-grown vs. dark-grown cells of Poterioochromonas; in this case, the variation is from - 10% to - 1% of
biomass. These values are much less than the 50% cited
above for thoroughgoing photolithotrophs, reflecting the
small phototrophic effort in Poterioochromonas (Myers and
Graham 1956; Sanders et al. 1990). P. malhamensis is at the
extreme among phagotrophic phototrophs in terms of the
small fraction of biomass occupied by the photosynthetic
apparatus and the phenotypic variability of this fraction. Other phagotrophic phototrophs have a larger fraction of their
biomass occupied by the photosynthetic apparatus and show
less phenotypic variability in ratios of maximum to minimum fraction of biomass occupied by the photosynthetic apparatus (although the variation in absolute fraction of biomass occupied by the photosynthetic apparatus may be
greater than in P. malhamensis). With regard to the phagotrophic apparatus, it is likely that this is much more “all-or-
none,” at least in protists. By all-or-none is meant that the
minimum fraction of biomass that must be allocated to produce a functional phagotropic apparatus is relatively constant
at -5-10%. This contrasts with the great variation (i.e. l50% of the biomass) occupied by the photosynthetic apparatus, with the smallest fraction found in P. malhamensis.
The contrast between the phagotrophic and photosynthetic
apparati relates to the 10 orders of magnitude or more that
separate the C content of a CO, molecule or the energy content of a photosynthetically usable photon and the C and
energy content of the smallest phototroph, and the <3 orders
of magnitude that separate the biomass of food items and
the biomass of phagotrophic phototrophs.
An example where the percentage of biomass allocated to
the phagotrophic apparatus can vary is the aquatic species
of Utricularia. Here the individual traps on a plant are the
functional equivalent of individual cells with their single
phagotrophic apparatus in protistan phagotrophic phototrophs. The allocation to traps in the aquatic species of Utricularia can amount to 50% of the biomass, with a considerable variation in volume (25-fold) of individual traps on the
same plant (Friday 1991, 1992). Contradicting initial expectations, Knight and Frost (1991) showed that Utricularia
produced more traps when the nutrient concentration in lake
water was higher. They suggested that the plants are using
dissolved nutrients as a proxy for prey availability.
A further possible cost of phagotrophy, at least as far as
phagotrophs with intracellular digestion are concerned, is
that volume regulation of the cells must be by active solute
fluxes at the plasmalemma (in seawater) or active water efflux (in freshwater) rather than by cell walls (see Raven
1982, 1984). Although certain endosymbioses in higher
plants (e.g. diazotrophy involving rhizobia or Frankia) involve entry of the microsymbionts into a walled host cell,
this involves complicity (shaped by natural selection) on the
part of the bacteria that is not necessarily present when they
are prey items for phagotrophs. Accordingly, intracellular
digestion by phagotrophic phototrophs implies volume regulation other than by cell walls, which may be more energyefficient at high growth rates but not at low growth rates
relative to volume regulation by using cell walls. Of course,
one could argue that phagotrophy in unicells is usually combined with motility or production of feeding currents by cilia
or flagella, which is in any case inconsistent with turgorresisting cell walls.
More measurements are clearly needed to establish the
costs and benefits of phagotrophy in photolithotrophs (and
vice versa) as well as their implications for ecology and
evolution. An excellent start has been made by Rothhaupt
(1996b) in his laboratory experiments involving a mixotrophic chrysophyte and obligately phagotrophic and phototrophic competitors.
A final evolutionary point concerns the extent to which
the partners in the symbiosis have the potential to transmit
their genes into future generations. Raven (1993a) considered this question for a number of photosynthetic symbioses
and concluded that essentially all of the symbioses can transmit the genes of both partners to the next generation. Raven
also considered the extent to which phagotrophy could increase the overall population and its growth rate (fitness) of
Comment
prey items that become symbiotic vs. those that do not become symbiotic. The analysis suggested that symbiosis can
indeed increase the fitness of the phototrophic partner, perhaps even when compared with a nongrazed free-living phototroph in similar oligotrophic environments. I do not, of
course, dissent from the view of Law and Lewis (1983) that
kleptoplastidy is best not regarded as symbioses since the
plastids have no genetic future once isolated from the alga
unless they are endocytosed by a phagotroph with the complementary genes to those in the plastids present in the nucleus.
Evolutionary significance of phagotrophy for nutrient
acquisition by phototrophs
203
caryotic phototrophs. Such transporters would have dealt
with the digestion products of phagotrophed particles (e.g.
phosphate, amino acids, nucleotides, nucleosides, purines,
pyrimidines, sugars) and probably some NH,+. There would
have been no need for NO,- transport in food vacuoles. In
addition to this qualitative lack of certain transporters, there
would have been quantitative differences, e.g. lower affinity
of the transporters dealing with high concentrations of substrate in the food vacuoles than is the case for plasmalemma
transporters dealing with lower substrate concentrations.
Molecular genetic analysis of the transporters could help to
show which of these possible ancestors of the transporters
actually occurred.
Conclusions
These cost-benefit discussions bring us back to the topic
of the evolution of phagotrophic phototrophs. We have seen
that the earliest O,-evolving eucaryote was a phagotroph. To
this extent, the suggestion of Ishida and Kimura (1986)that phagotrophy (with intracellular digestion) in phototrophs is a primitive character and its absence (e.g. in Chlorophyta, Viridiplantae) is derived-is acceptable. In general,
the extant eucaryotic 0, evolvers have lost phagotrophy.
Similarly, many protista and metazoa have lost, or never had,
plastids.
Phagotrophy has been regained by Utricularia (albeit
without intracellular digestion), but kleptoplastidy and intracellular symbiosis involving microalgae has restored, or provided for the first time, photosynthetic capacity for several
protista and metazoa with intracellular digestion. Raven
(1993a) discussed the extent to which the phagotrophic feeding mechanisms of ancestors of symbiotic O,-evolving protista and metazoa are preadapted for phototrophy, especially
in terms of surface area per unit volume. Raven (1981)
pointed out that the photosynthetic capacity of (haptophytic)
symbiotic photosynthetic organisms (e.g. corals) on an area
basis is higher than that of (nonsymbiotic) haptophytic seaweeds. This difference in capacities was attributed to the
need for a large area per unit volume for uptake of inorganic
N, P and Fe in the seaweeds as opposed to symbiotic photosynthetic organisms with their supplementary, phagotrophic mode of N, P and Fe nutrition.
A concomitant of phototrophy in eucaryotes is that there
are high-affinity
nutrient [NH,‘, NO,-, H2P0,~/HP0,Z~;
Fe2+/Fe’+ chelate; frequently CO, (HCO,-)] transport (influx) systems of the plasmalemma. These systems could
have arisen by transfer of the site of expression from cyanobacterial plasmalemma to host plasmalemma, with transfer of the site of coding from the cyanobacterial to the host
genome. Similar transfers could occur when secondary and
tertiary symbioses take place (Table l), although with symbioses in which the phototrophic partner retains its capacity
for independent existence (e.g. all Chlorella and Symbioninium endosymbioses) any such transfer must leave a copy
(or copies) of the relevant genes in the photobiont (see Raven 1993a).
An alternative source for some of these transporters is
from the nutrient uptake mechanism expressed at the food
vacuole membranes of the phagotrophic ancestors of the eu-
All eucaryotic phototrophs arose (and continue to arise)
by phagotrophy at the single cell level; phagotrophy among
eucaryotic phototrophs is the primitive condition, whereas
the absence of phagotrophy is the derived condition. Phagotrophy of phototrophic cells, with very little digestion but
subsequent loss of genes needed for independent life of the
phototrophs and transfer of no others to the nucleus, led to
plastids in eucaryotes. Phagotrophic acquisition of photosynthetically competent cells or plastids from food items,
without genetic integration into the phagotroph, continues
today. Phagotrophy of phototrophs by eucaryotes lacking a
genetically integrated photosynthetic apparatus, with retention of the photosynthetic capacity of the food item, was
(and is) presumably a response to a restricted supply of C
and energy relative to that of other nutrients by digestion of
the phagotrophic food items. Phagotrophy by phototrophic
eucaryotes, with digestion of the food items, is apparently
mainly a response to a relative shortage of N, P, Fe, etc.
from monodispense sources compared to that of C and energy (except for very weakly photosynthetic organisms such
as P. malhamensis, where phagotrophy is a major source of
C and energy). However, ecological evidence bearing on this
point is limited. Phagotrophy of phototrophs by phototrophic
eucaryotes lacking a genetically integrated photosynthetic
apparatus is the means of obtaining (and replacing) the photosynthetic cells or plastids. Application of cost-benefit analysis to the considerations suggests selective advantage in
certain environments for phagotrophy by phototrophs in organisms with integrated photosynthetic endosymbionts and
for both components of symbioses involving microphototrophs and phagotrophs, but not for the plastids involved in
kleptoplastidy. An important point about the cost-benefit
analysis is the much greater fraction (up to 50%) of biomass
occupied by the photosynthetic apparatus than is the case for
the phagotrophic apparatus (< 10%). Further testing is needed of these cost-benefit analyses by laboratory studies, including competition experiments comparing photosynthetic
phagotroph performance with that of obligate phototrophs
and obligate phagotrophs. Phagotrophy by phototrophs
seems to have a significance for biogeochemical cycling out
of proportion to the fraction of phototroph species that perform phagotrophy.
204
Comment
J. A. Raven
Department of Biological Sciences
University of Dundee
Dundee DDl 4HN, U.K.
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