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Rhizophydium pollonis on Pine pollen

The zoospore of the fungus encysts on the wall of the pollen grain and penetrates to the interior.   Inside the pollen the fungus produces a branching system of very fine hyphae (rhizoids) for absorbing nutrients.   Outside the pollen grain the fungus produces a spherical thallus that, at maturity, becomes a zoosporangium and gives rise to a new population of  uniflagellate zoospores that escape through a small perforation in the zoosporangial wall (red arrow) and swim off to attack new prey.   The empty zoosporangia appear as blister-like outgrowths over the surface of the pollen grains.

The amount of nitrogen and other nutrientst hat become available during the release of pollen grains by coniferous trees is seldom fully appreciated.  In rough terms it is equal to the amount of nitrogen fixed by nitrogen-fixing bacteria in the forest soils!    Thus, it is a major source of  nutrient  or a supplementary source of nitrogen for a number of  microscopic forest inhabitants.  Remember in most forest soils Nitrogen and/or Phosphorus are often unavailable.  i.e. the limiting factors to growth.   Len Hutchison and I showed that many so-called 'saprophytic' wood decay fungi could sense the presence of pine pollen in their vicinity, locate the PGs with directional hyphae, then invade and colonize the PGs as nutritional supplements.  If I can find one of the slides I'll put it here. Sometime!