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J.A. Raven
benthic plants which are primarily or secondarily haptophytic, and planophytes which are primarily or secondarily planophytic (Table 15.1). Raven
(1981, 1984a) suggests that the planophyte/haptophyte/rhizophyte distinction
is very significant in terms of the extent to which that part of the surface
area of the plant which is involved in photosynthesis is also involved in
the acquisition of resources other than carbon and energy, and thus in the
extent to which the area involved in photosynthesis is optimized for the
photosynthetic process. Raven (1981, 1984a) argues that rhizophytes, with
their ability to obtain much of their resources other than inorganic carbon
and photons via their rhizoids and roots (they rarely obtain most of their
inorganic carbon via their roots: Raven et al. 1988), have a photosynthetic
surface which is more optimized for photosynthesis per se than are planophytes and haptophytes, in which the large fraction of the total surface used
for photosynthesis is also the only surface area available for acquisition of
dissolved inorganic nitrogen, phosphorus, iron, etc. Raven (1981, 1984a,b,
1992a, 1993a) also notes that Utricularia, certain flagellates and algainvertebrate symbioses (planophytic and haptophytic) which are phagotrophic as well as photosynthetic, and thus obtain nitrogen, phosphorus, and
iron in particulate form, share with rhizophytes the possibility of optimizing
their photosynthetic surface for this function alone. The necessary respiratory loss of carbon as CO2 in converting particulate organic carbon into
phagotroph carbon, and the frequently lower carbon: nitrogen, carbon:
phosphorus, and carbon: iron ratios in food particles than in the phagotroph,
make particle intake a less quantitatively significant source of carbon than of
nitrogen, phosphorus or iron for phagotrophic algal-invertebrate symbioses,
the phagotrophic flagellates, and Utricularia (Raven 1981, 1984a, 1992a,
1993a).
15.4 Contribution of Aquatic Plants to Global Net Primary Productivity
Table 15.2 shows that aquatic photolithotrophs contribute less to global net
primary productivity than would be expected on a pro rata basis from the
fraction of the earth's surface which they occupy. Marine phytoplankton,
which as planophytes are the only photolithotrophic life-form able to grow
where water depth exceeds a few hundred meters (Littler et al. 1985, 1986),
and thus are the sole photolithotrophs growing on more than half of the
earth's surface, and the main life-form accounting for the low global aquatic
productivity. Raven (1981, 1984a, 1986a,b) argues that the small size of the
open-ocean planophytes (i.e., planktophytes) relates to the large surface
area per unit volume of smaller cells and its influence on nutrient and
photon absorption in the frequently nutrient-limited and deep-mixed, and
hence photon-limited, environment as well as to its influence in reducing
J.A. Raven
benthic plants which are primarily or secondarily haptophytic, and planophytes which are primarily or secondarily planophytic (Table 15.1). Raven
(1981, 1984a) suggests that the planophyte/haptophyte/rhizophyte distinction
is very significant in terms of the extent to which that part of the surface
area of the plant which is involved in photosynthesis is also involved in
the acquisition of resources other than carbon and energy, and thus in the
extent to which the area involved in photosynthesis is optimized for the
photosynthetic process. Raven (1981, 1984a) argues that rhizophytes, with
their ability to obtain much of their resources other than inorganic carbon
and photons via their rhizoids and roots (they rarely obtain most of their
inorganic carbon via their roots: Raven et al. 1988), have a photosynthetic
surface which is more optimized for photosynthesis per se than are planophytes and haptophytes, in which the large fraction of the total surface used
for photosynthesis is also the only surface area available for acquisition of
dissolved inorganic nitrogen, phosphorus, iron, etc. Raven (1981, 1984a,b,
1992a, 1993a) also notes that Utricularia, certain flagellates and algainvertebrate symbioses (planophytic and haptophytic) which are phagotrophic as well as photosynthetic, and thus obtain nitrogen, phosphorus, and
iron in particulate form, share with rhizophytes the possibility of optimizing
their photosynthetic surface for this function alone. The necessary respiratory loss of carbon as CO2 in converting particulate organic carbon into
phagotroph carbon, and the frequently lower carbon: nitrogen, carbon:
phosphorus, and carbon: iron ratios in food particles than in the phagotroph,
make particle intake a less quantitatively significant source of carbon than of
nitrogen, phosphorus or iron for phagotrophic algal-invertebrate symbioses,
the phagotrophic flagellates, and Utricularia (Raven 1981, 1984a, 1992a,
1993a).
15.4 Contribution of Aquatic Plants to Global Net Primary Productivity
Table 15.2 shows that aquatic photolithotrophs contribute less to global net
primary productivity than would be expected on a pro rata basis from the
fraction of the earth's surface which they occupy. Marine phytoplankton,
which as planophytes are the only photolithotrophic life-form able to grow
where water depth exceeds a few hundred meters (Littler et al. 1985, 1986),
and thus are the sole photolithotrophs growing on more than half of the
earth's surface, and the main life-form accounting for the low global aquatic
productivity. Raven (1981, 1984a, 1986a,b) argues that the small size of the
open-ocean planophytes (i.e., planktophytes) relates to the large surface
area per unit volume of smaller cells and its influence on nutrient and
photon absorption in the frequently nutrient-limited and deep-mixed, and
hence photon-limited, environment as well as to its influence in reducing